In vitro, reconstituted HiPIP precursor preHoloHiPIP was targeted to inverted membrane vesicles from E.. Keywords: ATPdependence; high potential iron–sulfur protein HiPIP; in vitro foldi
Trang 1Membrane targeting of a folded and cofactor-containing protein
Thomas Bru¨ser1, Takahiro Yano2, Daniel C Brune3and Fevzi Daldal1,2
1 Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018, USA; 2 Johnson Research Foundation,
Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104-6059, USA;
3 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA
Targeting of proteins to and translocation across the
membranes is a fundamental biological process in all
organisms In bacteria, the twin arginine translocation
(Tat) system can transport folded proteins Here, we
demonstrate in vivo that the high potential iron-sulfur
protein (HiPIP) from Allochromatium vinosum is
trans-located into the periplasmic space by the Tat system of
Escherichia coli In vitro, reconstituted HiPIP precursor
(preHoloHiPIP) was targeted to inverted membrane
vesicles from E coli by a process requiring ATPwhen
the Tat substrate was properly folded During membrane
targeting, the protein retained its cofactor, indicating that
it was targeted in a folded state Membrane targeting did not require a twin arginine motif and known Tat system components On the basis of these findings, we propose that a pathway exists for the insertion of folded cofactor-containing proteins such as HiPIP into the bacterial cytoplasmic membrane
Keywords: ATPdependence; high potential iron–sulfur protein (HiPIP); in vitro folding; membrane targeting; twin arginine translocation
Bacteria translocate proteins across the cytoplasmic
mem-brane by two main pathways, the general secretory (Sec)
and the twin arginine translocation (Tat) systems [1,2] In
the past, most studies on protein targeting have focused on
translocation or membrane integration of unfolded protein
substrates by the Sec machinery, and many components
have been identified that play specific roles in Sec-dependent
targeting pathways [3] On the other hand, the Tat system
has been shown to translocate folded proteins powered by
the transmembrane proton gradient [4] So far, only four
components, TatA, TatB, TatC and TatE, have been
identified in Escherichia coli Three of the corresponding
genes, tatABC, are organized in an operon together with
tatD, which encodes a nuclease that is probably unrelated to
the Tat system [5] TatE is a structural and functional
homolog of TatA and encoded at a different locus [6] TatA
and TatB together can form a double-layered ring structure
and are suggested to constitute the translocation pore [7]
The precise role of TatC remains to be determined, but it is already known that this component can form a functional unit with TatB [8] It is currently believed that TatA or TatE, together with TatB and TatC, can carry out most of the required functions, such as binding of Tat substrates, recognition of the folded state, formation of a translocation pore, usage of the DpH for translocation, or prevention of ion leakage [2]
Protein substrates for both the Sec and Tat systems are synthesized with similar N-terminal signal peptides, com-posed of a hydrophilic and positively charged n-region, followed by a hydrophobic h-region and then often by a c-region which determines a cleavage site (Fig 1) Sub-strates of the bacterial Tat system contain longer signal peptides which include a conserved (S/T)RRXFLK motif in their n-region [9,10] and a significantly less hydropho-bic h-region [11] In addition, Tat signal peptides often contain charged amino-acid residues in their c-region, which are not common in Sec-typical signal peptides [2,12,13] Folding of Tat substrates before their Tat-dependent translocation in E coli has been demonstrated in vivo in several cases, including the green fluorescent protein and hydrogenase [14–16] Furthermore, cytoplasmic matur-ation systems that induce protein folding such as iron-sulfur cluster assembly pathways can also act in conjunction with translocation [17] Moreover, natural Tat substrates that acquire a folded and often cofactor-containing state before their translocation appear not be secreted by the Sec system [2] On the other hand, Sanders et al [18] have shown that typical Sec substrates such as c-type cytochromes can be translocated via the Tat system only when they are synthesized with Tat signal sequences and if they receive in the cytoplasm their heme cofactor allowing their folding
Recently, a functional in vitro Tat system has been established using in vitro translated and cofactor-free Tat
Correspondence to T Bru¨ser, Institut fu¨r Mikrobiologie,
Universita¨t Halle, Kurt-Mothes-Str 3, 06120 Halle, Germany.
Fax: + 49 345 5527010, Tel.: + 49 345 5526360,
E-mail: t.brueser@mikrobiologie.uni-halle.de
Abbreviations: Tat, twin arginine translocation; HiPIP, high potential
iron-sulfur protein; INV, inverted membrane vesicle; MalE,
maltose-binding protein; DDM, dodecyl maltoside.
Note: The prefixes Holo, Mal and Apo are used herein solely for the
description of HiPIP, the cofactor content and folded state of which
has been investigated in vitro On the other hand, the prefixes pre
and mat are used to distinguish precursor and mature proteins.
For example, the precursor of the HiPIP holoprotein is termed
preHoloHiPIP, whereas a HiPIP precursor of unknown folded state
is termed preHiPIP.
(Received 10 October 2002, revised 19 January 2003,
accepted 27 January 2003)
Trang 2substrates [19,20] However, the mechanism of targeting and
translocation of folded redox proteins by the Tat system
remains largely unknown In this study, we opted to use a
fully folded and cofactor-containing Tat substrate As the
folded state is of importance for the Tat system in vivo, we
expected that this approach might lead to new insights We
have chosen as a model substrate the high-potential
iron-sulfur protein (HiPIP) from Allochromatium vinosum, which
is a monomeric 9-kDa periplasmic protein containing one
[4Fe)4S]2+/3+cluster bound via four cysteines [21,22] As
biosynthesis of iron-sulfur clusters is thought to take place
in the cytoplasm, it is thought that HiPIP is folded and
loaded with an iron-sulfur cluster before its translocation
into the periplasmic space [17] The signal peptide of HiPIP
has all the characteristics of Tat substrates [23] (Fig 1), and
its structure and folding properties are well characterized
[24]
In this work, we first demonstrated that the
hetero-logously expressed HiPIP is translocated into the
periplas-mic space in E coli, and that its translocation requires the
tatABCgenes as well as the RR motif of the signal sequence
Next, we successfully reconstituted the [4Fe)4S] cluster into
the HiPIP precursor (preHoloHiPIP) in vitro and used this
folded protein for in vitro targeting experiments We found
that freshly prepared preHoloHiPIP can be targeted
efficiently to inverted cytoplasmic membrane vesicles
(INVs) from E coli, that this process requires ATP
hydrolysis and an ATP-regeneration system, and that the
membrane-associated preHoloHiPIP undergoes a
conform-ational change without losing its [4Fe)4S] cofactor
Fur-thermore, this in vitro targeting reaction requires neither the
known Tat components (TatABCE) nor the RR motif in
the signal peptide We also observed that, on extended
storage, preHoloHiPIP could be converted into a form
capable of integrating into the membrane without ATP,
probably because of conformational changes induced by the
loss of its cofactor The overall findings suggest that a
pathway exists for the membrane insertion of folded and
cofactor-containing proteins Such a pathway may play a
role in the biogenesis of membrane-bound redox proteins,
or it may precede recognition and translocation by the Tat
system in vivo
Materials and methods
Genetic methods TatABC genes (including the tatA promoter region) were amplified with Pfu polymerase (Stratagene) from genomic DNA from E coli MC4100 using the primers 5¢-AGTCGTGGATCCAAGATCAGGTCGGTATT-3¢ and 5¢-TGCGCGGCGAGCTCAATAATCGCTTC-3¢ The P CR product was cleaved with BamHI and SacI at primer-generated cleavage sites and cloned into the corresponding sites of pRK415, resulting in pRK-tatABC The XbaI– BamHI fragment of pCVH1 [23], which contains hip and its promoter region, was cloned into the corresponding sites of pRK-tatABC, resulting in pRK-tatABC-hip For construc-tion of the hip expression vector pRK-hip, the tatABC-containing fragment from pRK-tatABC-hip was removed
by restriction with SacI–BamHI and self-ligation of the remaining vector The RR in the signal peptide of preHiPIP was mutated to KK using the primer couples 5¢-AAGAGC AAGAAAGACGCTGTCAAAGTGATG-3¢/5¢-TCCGG ATATAGTTCCTCCT-3¢ and 5¢-ACGTTACTGGTTTC ACATTC-3¢/5¢-AGCGTCTTTCTTGCTCTTGCTGATT GGCTT-3¢ to generate two overlapping PCR fragments with pEXH5 as template [13] These two PCR fragments were subjected to a second round of PCR to generate the RRfiKK-mutant fragment, which was then cleaved by NdeI and HindIII and cloned into the corresponding sites of pET22b+ (Novagen), resulting in pEXH15 used for inclu-sion body formation of the mutant protein For the in vivo analyses, the RRfiKK exchange was achieved using the primers 5¢-CATCACTTTGACAGCGTCTTTCTTGCTC TTGCTGATTGGCTTATCG-3¢ and 5¢-CGATAAGCCA ATCAGCAAGAGCAAGAAAGACGCTGTCAAAGTG ATG-3¢ using the QuikChange kit (Stratagene) with pCVH1 as a template The RRfiKK exchanges were confirmed by sequencing
In vivo analysis of preHiPIP translocation
E colistrain MC4100 and its derivatives B1LK0 (DtatC) [25] and DADE (DtatABCDE) [5] were generously provided
by T Palmer (University of East Anglia, Norwich, UK) and grown on Luria–Bertani medium under aerobic conditions,
or on Luria–Bertani medium supplemented with 0.4% NaNO3and 0.5% glycerol under anaerobic growth condi-tions hip was expressed from its own promoter using either pRK-hip or pRK-tatABC-hip, which also contains tatABC under the control of the tat promoter, and which was used for complementation of HiPIP translocation in Tat mu-tants Periplasmic fractions were prepared using 50 mL anaerobically grown cell cultures, as described elsewhere [26] Immunoblot analysis was carried out as described previously [13]
Preparation of fully folded precursor HiPIP (preHoloHiPIP)
For preparation of preApoHiPIP inclusion bodies, a 1-L
E coli BL21 DE3 culture carrying pEXH5 [13] was grown in Luria–Bertani medium with high aeration, and hip expression was induced for 3 h with 1 m isopropyl
Fig 1 The signal peptide of A vinosum HiPIP contains all known
determinants specific of the Tat translocation pathway The Tat signal
peptide of A vinosum HiPIP (upper sequence) is compared with the
Sec-typical signal peptide sequence of the outer membrane protein A
(OmpA) from E coli Note that, compared with Sec signal peptide
sequences, the Tat signal peptide sequence is generally longer, it has a
twin arginine motif (underlined bold) within a conserved pattern
(bold), an extended hydrophilic N-terminus (n-region), a longer
uncharged region with moderate hydrophobicity (h-region), and often
a charged residue near the cleavage site (c-region followed by an
arrow).
Trang 3thio-b-D-galactoside at D600¼ 1 Harvested cells were
washed once in 50 mL 100 mM Tris/HCl, pH 8.0,
resus-pended in 30 mL of the same buffer, and broken by two
passages through a French pressure cell operating at
138 MPa Inclusion bodies and cell debris were sedimented
and washed twice by centrifugation (20 min, 25 000 g,
4C), dissolved in 20 mL ice-cold 50 mM Tris/HCl
(pH 8.0)/2 mMdithiothreitol/8Murea, and cell debris was
separated by centrifugation (30 min, 30 000 g, 4C) The
supernatant (inclusion body solution) was shock-frozen in
liquid nitrogen in 1 mL aliquots and stored at)80 C
For in vitro folding, preApoHiPIP was first allowed to
assemble iron at room temperature in a reaction mixture
containing 43 lM preApoHiPIP, 220 lM Fe(NH4)SO4,
2 mM dithiothreitol and 5M urea in a total volume of
15 mL After 5 min of incubation, 1.25 mM Na2S was
added and folding was continued for 20 min The solution
was then applied to a 2-mL DEAE-Sephacryl (Pharmacia)
column equilibrated with 20 mMTris/HCl, pH 9.0 Folded
and cofactor-containing preHoloHiPIP passed through the
column and was subsequently dialyzed against STM buffer
(250 mMsucrose, 5 mMTris/HCl pH 8.0, 5 mM MgSO4)
PreHoloHiPIP prepared in this way was stable for about
1 week on ice Iron content of HiPIP was determined using
the bathophenanthrolinedisulfonate method [27] UV/vis
spectra were recorded using an Hitachi U3210
spectro-photometer
Preparation of inverted cytoplasmic membrane vesicles
Cells (6 g wet weight) were resuspended in 40 mL 10 mM
Tris/acetate, pH 7.6, containing 20% sucrose, 0.1 mM
EDTA, and 1 mMdithiothreitol, incubated for 10 min at
room temperature, and sedimented at 5000 g for 20 min at
4C The pellet was resuspended in 40 mL ice-cold 5 mM
MgSO4 and incubated on ice for 20 min, followed by
centrifugation at 5000 g for 20 min at 4C The pellet was
resuspended in 50 mM Tris/acetate, pH 7.6, containing
250 mM sucrose, 1 mM dithiothreitol, and 50 lgÆmL)1
DNase I, and passed through a French pressure cell
operating at 27.6 MPa to produce INVs [28] The solution
was then centrifuged for 10 min at 5000 g, and the
supernatant was centrifuged again at 150 000 g for 2 h at
4C The membrane pellet was resuspended in 2 mL STM
buffer supplemented with 1 mMdithiothreitol Aggregated
material was removed by a final centrifugation at 15 000 g,
and the clear supernatant was divided into aliquots and
frozen in liquid nitrogen
Cofactor tracing and membrane-targeting assays
The [4Fe)4S]2+/3+cofactor was radioactively labeled by
including 50 lCi55FeCl3in the iron-assembly step of the
folding protocol described above PreHoloHiPIP was
targeted to INVs in a mixture containing 600 pmol
preHoloHiPIP ( 40 000 c.p.m.), 26 lg INV protein,
250 mMsucrose, 5 mMMgSO4, 5 mMATP, 60 mM
phos-phocreatine, 100 lgÆmL)1creatine kinase, 5 mgÆmL)1BSA,
1 mM dithiothreitol, and 15 mM Tris/HCl, pH 7.5 The
reaction was started after 1 min preincubation by addition
of INVs, carried out for the indicated amounts of time at
37C, and terminated by rapid vacuum filtration through
0.22-lm pore size GV-type membranes (Millipore) The filtered INVs were immediately washed with 3 mL STM/
200 mMNaCl/50 mMMgSO4, and55Fe bound to INVs was monitored by determination of the radioactivity thus retained by liquid scintillation counting For immunoblots, filter membranes were extensively washed with 100 lL SDS/PAGE sample buffer, and 10 lL were used for SDS/ PAGE and blotting as described elsewhere [13] [35 S]Met-labeled maltose-binding protein (MalE) was produced by
in vitrotranslation with rabbit reticulocyte lysate (Promega protocol) from RNA obtained by in vitro transcription of HindIII-digested pBAR107N [29] The MalE used herein is
a C-terminally truncated form which cannot fold and therefore has been found to be more suitable for Sec-dependent in vitro translocation than full-length MalE [29] MalE was targeted for 40 min at 37C to INVs in a mixture containing 1 lL MalE and 50 lg INV protein under conditions identical with the targeting assay of preHoloHiPIP described above The assay mixture was then incubated on ice with or without thermolysin (200 lgÆmL)1,
1 h), followed by trichloroacetate precipitation, SDS/PAGE analysis, and analysis of the radioactive protein bands by use of the phosphoimager system and the quantification program IMAGEQUANT (Molecular Dynamics) When indicated, the targeting reaction was carried out in the presence of gramicidine (10 lM) or cyanide m-chlorophenyl-hydrazone (CCCP, 100 lM), or ATPwas replaced by NADH
Other biochemical methods Mature HiPIP (matHoloHiPIP) was purified from photo-heterotrophically grown A vinosum Cells (10 g) were broken in 20 mM Tris/HCl, pH 8.5, by two passages through a French pressure cell operating at 138 MPa After low-speed and ultracentrifugation steps, the supernatant containing the soluble proteins was loaded onto a 100-mL DEAE-Sephacel column equilibrated with the same buffer After washing of the column, matHoloHiPIP was eluted by changing the buffer to 20 mM Tris/HCl, pH 7.0 HiPIP-containing fractions were dialyzed against 20 mMTris/HCl,
pH 8.5, and further purified using a 1-mL Mono Q FPLC column and a 40-mL gradient of 0–200 mMNaCl in 20 mM Tris/HCl, pH 8.5 MatHoloHiPIP was homogeneous as judged by Coomassie-stained SDS/PAGE gels
SDS/PAGE analysis was carried out with 15% T Laemmli gels, and protein was determined by the Lowry method [30,31] For N-terminal amino-acid sequence determination of Coomassie-stained, Immobilon filter blot-ted proteins, Edman degradation was carried out using a Proton 2090E gas-phase protein sequencer (Beckman, Fullerton, CA, USA) equipped with an online Hewlett– Packard 1090L HPLC [32] For affinity purification,
150 nmol preHoloHiPIP was coupled to a 2-mL Aminolink column matrix (Pierce, Rockford, IL, USA) Membranes from 6 g cells of the E coli strain DADE lacking TatA-BCDE were resuspended in STM buffer, and solubilized for
1 h by stirring at 4C and addition of dodecyl maltoside (DDM) to a final concentration of 1% Solubilized mem-branes were centrifuged (145 000 g, for 1 h) and the supernatant was diluted with STM buffer to 0.2% DDM The Aminolink-preHiPIP column (operated by gravity
Trang 4flow) was equilibrated to STM/0.2% DDM, and solubilized
membranes were loaded, followed by a wash with 10
column volumes (20 mL) of STM/0.2% DDM and elution
with 10 mL STM/0.2% DDM/2 mM ATP The column
was then washed with 10 mL STM/0.2% DDM/200 mM
NaCl, followed by a final wash with 10 mL STM/0.2%
DDM/500 mM NaCl in order to detect any additional
preHoloHiPIP-binding protein not eluted by ATP
EPR measurements
X-band (9.4 GHz) EPR spectra were recorded by a Bruker
ESP300E spectrometer using an Oxford Instruments
ESR-9 helium flow cryostat to control desired sample
temperature HiPIP preparations were oxidized with 5 mM
ferricyanide Final EPR spectra were obtained after
subtracting a spectrum of the buffer containing 5 mM
ferricyanide measured under the same conditions EPR
conditions used are described in detail in the legends to the
individual figures
Chemicals
55FeCl3 and [35S]Met were obtained from Perkin–Elmer
Life Sciences DNA-modifying enzymes were from
Bio-Labs, and in vitro transcription and translation kits from
Promega All other chemicals and enzymes were from
Sigma or from Fisher Scientific and were of the highest
available purity
Results
A vinosum HiPIP is a Tat substrate in E coli
The gene hip from A vinosum encoding HiPIP was
expressed from its own promoter in various E coli strains
and their subcellular fractions were prepared Western-blot
analyses indicated that HiPIP was translocated into the
periplasm under anaerobic growth conditions (Fig 2) In
wild-type cells carrying the plasmid pRK-hip, all processed
HiPIP was detected in the periplasmic fraction (Fig 2A,
lanes 1–3) In contrast, no HiPIP could be detected in the
periplasm of tat mutants deficient in TatC or TatABCDE,
indicating that translocation of HiPIP to the periplasm does
not occur (Fig 2A, lanes 4–9) Consequently, HiPIP
precursor accumulated in the cytoplasm fraction of the
mutant strains Translocation of HiPIP could be restored in
the tatABCDE mutant by providing only the tatABC genes
in trans (Fig 2A, lanes 10 and 11), demonstrating that the
lack of translocation of HiPIP was due to the absence of the
tatABCgenes To further establish that HiPIP is indeed a
substrate of the Tat translocon in E coli, the twin arginine
residues in its signal peptide were exchanged with lysines
This kind of substitution has been reported to block
Tat-dependent translocation of other Tat substrates in E coli
[9] When theRRfiKK signal sequence mutant of HiPIP was
analyzed, we observed that the translocation was blocked,
and that the precursor as well as a degradation product with
the size of mature HiPIP accumulated inside the cytoplasm
in large amounts (Fig 2B) These findings confirmed that
the twin arginine motif in the signal peptide of A vinosum
preHiPIP is required for its Tat-dependent translocation in
E coli, and established A vinosum HiPIP as a bona fide Tat substrate
HiPIP precursor can be foldedin vitro to native conformation
Highly purified HiPIP precursor apoprotein (preApo-HiPIP) was obtained from inclusion bodies and folded to its native conformation in vitro as described in Materials and Methods Reconstituted HiPIP precursor (preHolo-HiPIP) contained 3.9 Fe atoms per protein and showed the typical optical absorption spectrum of purified mature HiPIP (matHoloHiPIP) with maxima at 283 nm and
388 nm and an absorbance ratio A282/A388 of 2.6 ± 0.1 [13,33] Further, EPR spectra of reconstituted preHolo-HiPIP exhibited the well-characterized matHolopreHolo-HiPIP signature with gx, gyand gztensor values of 2.037, 2.045 and 2.122 and a gavvalue of 2.068 (Fig 3) The data indicate that preHoloHiPIP was folded fully under the conditions used, as shown previously for mature HiPIP [24] As the dimerization of HiPIP induces well-characterized hetero-geneities in the gzregion [34], a close examination of our data indicated that in vitro folded preHoloHiPIP is mono-meric in solution Treatments of preHoloHiPIP with proteinase K, thermolysin, trypsin or chymotrypsin as well
as with combinations of these proteases generated a mature
Fig 2 Translocation of A vinosum HiPIP depends on the Tat system
in E coli Detection of A vinosum HiPIP precursor (pre) and mature HiPIP (mat) by Western blotting in cell fractions from E coli wild-type (MC4100) and Tat-deficient strains expressing hip Strains and plas-mids are indicated above corresponding lanes, as follows: wt, wild type
E coli MC4100 in (A), or XL1-BLUE in (B) and (C); DtatC, B1LK0; DtatABCDE, DADE strains The plasmid pRK-tatABC-hip contained both the E coli tatABC and A vinosum hip genes, expressed inde-pendently (see Materials and methods for description of this plasmid).
In each lane, protein corresponding to 375 lL bacterial culture were loaded, and the applied cell fractions are indicated: c, cytoplasm; m, membrane; p, periplasm (A) Restoration of HiPIP translocation in a DtatABCDE deletion strain complemented with the tatABC genes carried by pRK-tatABC-hip; (B) effect of replacing the conserved arginine residues R(10)R(11) with K(10)K(11) residues of hip on the translocation of HiPIP protein (C) Coomassie-stained gel from a representative cell fractionation Molecular mass markers are indicated
on the left.
Trang 5form of HiPIP, suggesting that only its 4-kDa signal peptide
could be truncated by protease treatment without losing or
perturbing the EPR characteristics of its [4Fe)4S] cluster
(data not shown), and indicated that the iron-sulfur
cluster-binding domain of preHoloHiPIP is highly resistant to
proteases The cleavage site of protease-treated
preHolo-HiPIP was determined by N-terminal amino-acid
sequen-cing Proteinase K cleaved at position)3, )2, )1, 0 and +1
relative to the natural signal peptide cleavage site, with 80%
cleavage at position)2 Thermolysin treatment on the other
hand resulted in a clean single cut of the signal peptide at
position )3 This indicated that these proteases indeed
cleaved off the signal peptide from in vitro folded
pre-HoloHiPIP without affecting the remainder of the protein
Therefore, preHoloHiPIP treated with proteinase K or
thermolysin was regarded as mature matHoloHiPIP
ATP-dependent targeting of HiPIP precursor into
inverted membrane vesicles fromE coli
In vitro folded and [4Fe)4S] cluster-containing
preHolo-HiPIP was next incubated with INVs under the conditions
described in Materials and Methods In the presence of ATP
and an ATP-regenerating system consisting of
phosphocre-atine and crephosphocre-atine kinase, we observed that a large amount
of preHoloHiPIP accumulated in the membranes (Fig 4A)
To quantify this membrane-targeting reaction, and also
to monitor the fate of the cofactor during this process,
preHoloHiPIP was radiolabeled by including55FeCl3into the folding procedure, and time-dependent55Fe accumula-tion was monitored The data indicated that only a small amount of55Fe could be associated with the membrane in the absence of ATP, and addition of ATP enhanced it by about 5- to 20-fold (Figs 4B and 5) From55Fe label tracing
Fig 3 preHoloHiPIP reconstituted in vitro has native spectroscopic
properties EPR spectra of in vitro reconstituted preHoloHiPIP are
compared with those of mature HiPIP as purified from A vinosum The
EPR spectroscopy conditions were as follows: modulation frequency,
100 kHz; modulation amplitude, 10.145 G; time constant, 163.84 ms;
conversion time, 163.84 ms (see Materials and methods for more
details) Note that the reconstituted preHoloHiPIP exhibits a
symmet-rical peak in the g z region (see insert), indicating its monomeric state [34].
Fig 4 ATP-dependent targeting of preHoloHiPIP to INVs from
E coli (A) Immunoblot analysis using anti-HiPIP serum after tar-geting INVs from 15 min targeting assays were filtered and resus-pended in 100 lL SDS/PAGE sample buffer for analysis, and samples were separated by SDS/PAGE (15% T) and blotted on nitrocellulose for HiPIP detection using polyclonal antibodies Lane 1, HiPIP pre-cursor standard (solubilized inclusion bodies, 0.25 lg); lanes 2–5, HiPIP after targeting to INVs in the absence (lanes 2 and 3) or presence (lanes 4 and 5) of ATP; lane 6, mature HiPIP purified from A vinosum ( 0.25 lg, see Materials and methods), used as a control The posi-tions of precursor (pre) and mature (mat) HiPIP bands are indicated Note that without INVs no significant preHoloHiPIP was retained on the filter, and that ATPhad no effect when INVs were absent (not shown) (B) 55 Fe-labeled preHoloHiPIP was targeted to E coli INVs Targeting was terminated using a rapid filtration assay, and filter retained 55 Fe radioactivity was monitored by liquid scintillation counting (see Materials and methods) Assay mixtures contained besides the standard mix (see Materials and methods) 5 m M ATP/
10 l M gramicidin (r), 5 m M ATP(j); no ATP(negative control) (e);
5 m M NADH (h) Each data point is the mean of three independent incubation/filtration assays, and error bars show the standard devia-tions observed between samples The presence of a DpH is indicated and generated by either addition of ATPor NADH The formation of
a DpH under the assay conditions used was confirmed by fluorescence quenching assays performed separately (data not shown).
Trang 6kinetics, the specific activity of ATP-dependent membrane insertion was estimated to be 33 ± 10 pmol targeted preHiPIPÆmin)1Æ(mg INV protein))1 Up to 18 pmol ( 0.3 lg) preHoloHiPIP could be targeted to 26 lg mem-branes, indicating that the membranes were efficiently loaded with preHoloHiPIP An ATP-regenerating system was required for targeting, indicating that ATPis hydrolyzed in the assay Additional experiments indicated that hydrolysis
of ATPis required for this process, as neither the non-hydrolyzable ATPanalogue p[NH]ppA nor AMPcould substitute for ATP(S Trautmann and T Bru¨ser, unpub-lished results) The existence of a transmembrane proton gradient, as generated by either reverse action of ATP synthase or NADH-dependent electron transport, was not required for membrane targeting, and it rather affected negatively the kinetics (Fig 4B) Moreover, with a similar assay, matHoloHiPIP could not be targeted to INVs, indicating that the signal peptide is required for the targeting process (Fig 5A)
Next, to probe whether the ATP-dependent targeting of preHoloHiPIP to the membrane required the twin arginine motif in the signal peptide or the known Tat components, membrane targeting with an in vitro folded R(10)R(11)fi K(10)K(11) signal sequence mutant (preHoloHiPIP-KK) and targeting to INVs derived from various Tat-deficient mutant strains were tested The data indicated that preHoloHiPIP-KK was accepted as an efficient substrate for ATP-dependent membrane insertion, and that deletion
of tatC or tatABCDE did not significantly affect the ATP-dependent targeting of preHoloHiPIP to INVs (Fig 5B,C) Therefore, the ATP-dependent membrane-targeting reac-tion does not appear to require the twin arginine motif
in vitro, nor does it depend on any of the as yet known Tat components To examine the possibility that the ATP dependence results from an involvement of SecA in the targeting process, we compared the azide sensitivity of preHoloHiPIP targeting with that of the targeting of the
Fig 5 Requirements for membrane targeting of HiPIP (A) 55Fe tracing data obtained using standard targeting assays (see Materials and methods) with matHoloHiPIP and preHoloHiPIP to INVs To produce matHoloHiPIP, preHoloHiPIP reconstituted in vitro was digested for 60 min on ice with 100 lgÆmL)1proteinase K The reac-tion was stopped by addireac-tion of phenylmethanesulfonyl fluoride (10 m M , 10 min, 0 C) in dimethyl sulfoxide It was confirmed that this phenylmethanesulfonyl fluoride treatment resulted in complete inac-tivation of proteinase K For assays with the unprocessed substrate, preHoloHiPIP was incubated in parallel on ice without protease, and treated thereafter with phenylmethanesulfonyl fluoride Note that the absence of the N-terminal signal peptide in the matHoloHiPIP results
in targeting deficiency even in the presence of ATP (B)55Fe tracing data obtained using standard targeting assays with either wild-type preHoloHiPIP (RR) or its RRfiKK signal peptide mutant (KK) derivative (C) 55 Fe tracing data obtained using standard targeting assays with preHoloHiPIP and INVs prepared from wild-type E coli (wt ¼ MC4100), a DtatC mutant (B1LK0) and a DtatABCDE mutant (DADE) Incubation times (min) and ATPaddition are also indicated All values are given as a percentage of ATP-dependent targeting of preHoloHiPIP observed using wild-type E coli INVs, and error bars indicate the standard deviation observed under the assay conditions used.
Trang 7model Sec substrate MalE In protease-protection assays as
described in Materials and methods, we observed an
inhibition of 37 ± 5% of MalE targeting, whereas
mem-brane targeting of preHoloHiPIP was inhibited by
12.5 ± 7.5% under the same conditions
Characterization of membrane-targeted preHoloHiPIP
To determine whether or not the targeted preHoloHiPIP
retained the [4Fe)4S] cluster, EPR spectroscopy was used
After ATP-dependent preHoloHiPIP targeting, membrane
fractions exhibited EPR signals that were not detectable
when ATPwas omitted from the assay (Fig 6, upper two
traces) The difference of both spectra gave a typical
HiPIP spectrum (Fig 6, lower trace) Moreover,
mem-brane-associated preHoloHiPIP could be degraded by
thermolysin or proteinase K treatment (Fig 7A, lane 1)
These data indicate that, during ATP-dependent
mem-brane targeting, preHoloHiPIP retained its iron-sulfur
cofactor, and that the cofactor-binding domain was
exposed on the INV membrane surface, and not
trans-located across the membrane Apparently, this protease
sensitivity of targeted preHoloHiPIP was induced by INV
binding (Fig 7A, lanes 1/5), as nontargeted
preHolo-HiPIP could be digested only to mature size by various
proteases (Fig 7A,B, lanes 5 and 6) This suggested to us
that membrane targeting alters the conformation of
preHoloHiPIP, thereby increasing its protease sensitivity
Interestingly, when membrane-associated preHoloHiPIP
was treated with protease, a small peptide of similar size
to that of the signal peptide of HiPIP remained protected,
and could be detected with polyclonal antibodies raised
against the precursor of HiPIP (Fig 7A, lane 1) The
detection of the protease-protected HiPIP fragment in
conjunction with the salt-wash-resistant association of the
precursor with the membranes suggests a
membrane-insertion process As the C-terminus of preHoloHiPIP
binds the cofactor that is retained during membrane
targeting (Fig 6), this membrane-insertion process
appears to be mediated by the N-terminus of this protein
Thus, association of the preHoloHiPIP with the
mem-brane apparently reflects two distinct processes: (a) the
insertion of the N-terminus into the membrane, and (b) a
change in the preHoloHiPIP conformation
Change in preHoloHiPIP conformation requires ATP
and not insertion of the signal peptide
When older preparations (over 1 week) of in vitro
reconstituted preHoloHiPIP were used instead of fresh
preparations, we found that ATPdependence of
target-ing disappeared (Fig 7B, lane 4) These aged
prepara-tions exhibited significantly altered optical spectroscopic
properties such that the [4Fe)4S] cluster absorbance
significantly decreased and the absorption maximum in
the UV area was shifted from 283 nm to 275 nm,
indicating that up to 40% of the preparation shifted to a
modified conformation, lacking its cofactor (called
MalHiPIP; Fig 7C) Nonetheless, even when this
pre-MalHiPIP was targeted to the membrane, a peptide of
the size of the signal peptide became protease-protected,
as in the case of preHoloHiPIP (Fig 7B, lanes 1 and 2)
These observations suggest that the membrane insertion per sedoes not require ATP, but rather ATP is needed for structural conversion of preHoloHiPIP into a less tightly folded, protease-digestible form capable of membrane insertion Attempts to purify this ATPase by affinity with preHoloHiPIP covalently attached to an Aminolink matrix resulted in isolation of a protein identified as DnaK by N-terminal amino-acid sequencing However, preliminary data obtained using DnaK-deficient mutants suggest that DnaK is not essential for the translocation of HiPIP in vivo, and it is also not the ATPase responsible for the above targeting process in vitro
Fig 6 Membrane-associated HiPIP precursor contains its high-poten-tial iron-sulfur cofactor Membranes from several independent stand-ard targeting reactions with or without ATPwere pooled and analyzed
by EPR spectroscopy (see Materials and methods) Total INVs (130 lg) from five standard assays were used to obtain each spectrum The spectrum obtained with samples that contained ATPshowed that preHoloHiPIP was targeted to INVs and the [(+ATP) – (–ATP)] difference spectrum revealed a typical EPR signature that is charac-teristic of HoloHiPIP The EPR spectroscopy conditions were as described in Fig 3, and spectra were averaged from 10 scans.
Trang 8Evidence for membrane targeting of HiPIPin vivo
The in vitro data described above raised the possibility that
preHiPIP could also be targeted to the membrane in vivo
However, membranes of wild-type E coli expressing hip
from its own promoter did not contain readily detectable
preHiPIP(Fig 2) As membrane-targeted preHoloHiPIPis highly protease sensitive, we considered that preHiPIP might be rapidly degraded in membranes Thus we tested membrane targeting of preHiPIP-KK in an E coli strain that expressed hip from the stronger T7 promoter (Fig 8)
In such a strain, preHiPIP-KK was readily detected in membranes that had been washed twice (once in low-salt and once in high-salt buffer) to ensure high purity Moreover, degradation products of HiPIP were also detected, consistent with the high protease sensitivity of membrane-targeted HiPIP precursor Thus, these results suggest that membrane targeting of HiPIP also occurs
in vivo
Discussion
Recent studies suggest that Tat substrates need to fold in the cytoplasm before translocation to their final destinations For example, the Tat substrate glucose–fructose oxido-reductase from Zymomonas mobilis requires correct folding and cofactor binding for efficient translocation [35] In addition, it has been demonstrated that translocation of c-type cytochromes via the Tat system requires cytoplasmic attachment of its cofactor, which induces folding [18] It is also known that HiPIP can fold and assemble its cofactor in the cytoplasm [36] Therefore, we chose HiPIP from
A vinosum, as a small and well-characterized [4Fe)4S] cluster-containing protein with a signal sequence exhibiting the known characteristics of typical Tat substrates (Fig 1) Using the E coli system as the best-characterized bacterial Tat system, we then established the Tat dependence of HiPIP translocation in vivo to pave the way for experiments
in vitro
Fig 8 Detection of the preHiPIP in membranes of E coli expressing the RRfiKK signal peptide mutant derivative of HiPIP Western-blot analysis of membrane fractions from E coli strains BL21 DE3 car-rying plasmids pEXH15 (KK) or pEXH5 (RR) Membranes were prepared from crude extract after low-speed centrifugation (30 min,
23 000 g, 4 C), ultracentrifugation (143 000 g, 2 h, 4 C), a first wash
in low-salt buffer (20 m M Tris/HCl, pH 8.0, followed by ultracentri-fugation), and a second wash and sonication in high-salt buffer (200 m M NaCl, 50 m M MgSO 4 , 5 m M Tris/HCl, pH 8.0, 250 m M
sucrose, followed by ultracentrifugation) and resuspension in 5 m M
MgSO 4 /5 m M Tris/HCl (pH 8.0)/250 m M sucrose for Western-blot analysis Each lane corresponds to material obtained from 100 lL
E coli culture.
Fig 7 Only correctly folded preHoloHiPIP requires ATP for
mem-brane targeting In (A) the ATPdependence of memmem-brane insertion of
preHoloHiPIP is shown by immunoblot analysis Lanes 1–4 show the
analysis of filtered and washed INVs after preHoloHiPIP targeting.
The presence or absence of ATPin the assay mixture is indicated The
material analyzed in lanes 1 and 2 was further subjected to protease
treatment [200 lgÆmL)1thermolysin (TL) for 40 min on ice] In
par-allel assays, the effect of thermolysin treatment on soluble
pre-HoloHiPIP in STM buffer was tested (lanes 5 and 6) In (B) the same
analysis was carried out with preMalHiPIP Lanes 1–4 show the
analysis of filtered and washed INVs after preMalHiPIP targeting.
The presence or absence of ATPin the assay mixture is indicated The
material analyzed in lanes 1 and 2 was further subjected to protease
treatment [200 lgÆmL)1proteinase K (PK) for 40 min on ice] In
parallel assays, the effect of proteinase K treatment on soluble
preMalHiPIP in STM buffer was tested (lanes 5 and 6) (C)
Com-parison of UV-visible spectra of preHoloHiPIP and preMalHiPIP
( 15 l M ) in STM buffer Abbreviations: pre, precursor; mat, mature
protein; ppf, protease-protected fragment.
Trang 9HiPIP is abona fide Tat substrate in E coli
As expected, in vivo translocation of HiPIP required the
tatABCgene products as well as the twin arginine motif in
its signal peptide (Fig 2) When the twin arginines in the
signal peptide are replaced by lysines, the translocation is
completely blocked, indicating that HiPIP is a substrate of
the Tat system [9] Some accumulating precursor is
degraded to mature size, probably as the result of
cytoplasmic folding before degradation (compare with
Fig 7A, lanes 5,6) There are known cases in which a Tat
substrate signal sequence from one bacterial species is not
accepted by the Tat system of another species One such
example is the glucose–fructose oxidoreductase from
Z mobilis, which is translocated by the E coli Tat system
only when its signal peptide is substituted by a signal
peptide from the E coli Tat substrate TorA [37] Our results
indicate that A vinosum HiPIP behaves as an efficient Tat
substrate in E coli Thus, the E coli Tat system is not
restricted to proteins with endogenous Tat signal sequences,
and no general incompatibility between a Tat system and a
heterologous substrate is apparent in this case The use of
heterologous but natural Tat substrates has the advantage
that results are more likely to be related to general
properties of the Tat system, as substrate-specific targeting
factors can be excluded In particular, HiPIP is an excellent
tool for studies on the translocation of folded proteins,
because its structure is known, it is small, it has only one
cofactor, it is monomeric, and it is soluble Its functionality
as an E coli Tat substrate was the basis for the following
studies
Folded HiPIP can be targeted to theE coli membranes
We found that the preHoloHiPIP obtained by in vitro
refolding, starting with inclusion bodies and reconstituting
the [4Fe)4S] cluster, could be efficiently targeted in vitro to
inverted membrane vesicles in the presence of ATPand an ATP-regeneration system (Fig 4) The membrane-targeted preHoloHiPIP could not be washed from the vesicles with high-salt buffers Furthermore, a 4-kDa fragment of preHoloHiPIP became protease-protected on targeting (Fig 7A,B), suggesting that the targeting of preHoloHiPIP
is a membrane-insertion process We believe that the membrane-inserted peptide corresponds to the N-terminus
of preHoloHiPIP for the following reasons: (a) the N-terminal signal sequence is required for membrane insertion (Fig 5A) because matHoloHiPIP cannot be targeted to INVs; (b) the size of the protease-protected fragment is that of the signal sequence; (c) the C-terminus of HiPIP binds the cofactor and has a globular folded structure, and thus is not available for membrane insertion The exact topology of the membrane-inserted N-terminus
of preHoloHiPIP remains to be determined (Fig 9) Membrane targeting of preHoloHiPIP appears to be a highly efficient process that requires ATP However, this ATPdependence of membrane targeting vanishes when preHoloHiPIP loses its cofactor on prolonged storage, i.e
aging, that results in malfolding or partial unfolding (preMalHiPIP, Fig 7B) Moreover, from studies on HiPIP structure flexibility and folding, it is known that only the N-terminal half of mature HiPIP can be unfolded without the loss of cofactor [38] These facts suggest that ATPis not required for the membrane-insertion process per se, but it may rather serve to convert the protein structure, probably its N-terminus, into an insertion-compatible conformation (Fig 9) The increase in protease sensitivity on membrane insertion indicates that the mature part of HiPIP remains on the cytoplasmic side of the membrane, and that its conformation is affected by membrane insertion Searching for the ATPase responsible for membrane targeting of preHoloHiPIP, we purified the ATP-dependent chaperone DnaK from the membrane fraction by its affinity for preHoloHiPIPand release by ATP(see Materials and
Fig 9 Model for membrane targeting of
HiPIP precursor It is proposed that
mem-brane targeting of native preHoloHiPIP
requires an ATP-dependent step as indicated
on the left, whereas aged and presumably
partially unfolded preMalHiPIP can be
inser-ted into the membrane without any ATP
requirement as shown on the right The latter
ATP-independent step may involve additional
specific protein(s) of currently unknown
nature (not shown) Two alternatives for
membrane topology of the N-terminus of
membrane-inserted preHoloHiPIP (with
N-terminal outside or inside shown as
con-tinuous or dotted lines, respectively) are also
indicated The TatABC-independent
mem-brane insertion observed in vitro is suggested
to precede the TatABC-dependent
transloca-tion observed in vivo, which is indicated with
discontinuous arrows and a question mark.
The fate of unfolded protein is unknown See
the text for more details.
Trang 10methods) However, preliminary experiments using DnaK–
mutant strains indicate that both in vivo translocation and
in vitromembrane targeting still occur in the absence of this
protein Thus, although DnaK is a cytoplasmic protein, up
to 25% of which may be membrane-associated [39],
apparently it is not the ATPase observed during membrane
targeting of preHoloHiPIP Why DnaK recognizes
pre-HoloHiPIP so efficiently in an ATP-dependent manner is at
present unclear, but, considering that this chaperone can
also bind specifically to other Tat signal peptides [40], its role
could be to ensure complete folding of Tat substrates before
membrane insertion
How could membrane insertion take place?
Proteins may insert into the membrane in either a
sponta-neous or a catalyzed mode Our observation that a
membrane potential can slow down the membrane-insertion
process (Fig 4B) suggests that a positively charged segment
of preHoloHiPIP may be transferred across the membrane
during this process If such a charge translocation across a
hydrophobic membrane takes place, then the process is likely
to require a protein factor for catalysis Currently known
insertases, SecA and YidC, are thought to accept unfolded
substrates [41], and their involvement in Tat substrate
targeting has previously been ruled out in some cases
[42,43] It is unlikely that the ATPase SecA accounts for
the membrane-insertion process described here, as no ATP
dependence is observed in the case of preMalHiPIP (see
Fig 7B) Further evidence for the independence of the
targeting reaction from SecA was obtained by including
3 mM sodium azide in the assay, a well-known SecA
inhibitor At this concentration of azide, we observed only
about 10–15% inhibition of the targeting process, whereas
the targeting of the model Sec substrate MalE was about
37% inhibited The higher azide sensitivity of Sec-dependent
translocation and the ATPindependence of preMalHiPIP
targeting argue against the involvement of SecA in the
targeting of preHoloHiPIP, suggesting that the ATP
dependence may be due to another factor, which is required
in the case of the correctly folded substrate only However,
SecA inhibition by azide is very leaky in vitro and therefore
we do not rule out at this stage that SecA may be involved in
the ATP-dependent targeting step of folded HiPIP Thus, the
molecular nature of the protein factor(s) required for
preHoloHiPIP membrane insertion remains to be
deter-mined
Does membrane targeting occur before
the translocation?
The fact that the twin arginine motif and the known
Tat-system components are not required for membrane
inser-tion of preHoloHiPIP indicates that the inserinser-tion process
seen here is not confined to Tat substrates If Tat substrates
such as preHoloHiPIP are among the natural substrates of
this membrane-targeting pathway, then Tat-substrate
recognition and translocation by specific Tat-system
compo-nents must occur after membrane insertion, and what we
observe in vitro is the accumulation of the targeting
intermediate in the membrane This hypothesis has to be
taken into consideration, because (a) HiPIP is a Tat
substrate in vivo (Fig 2), (b) membrane targeting is adapted
to the folded state of the protein with an ATP-consuming step (Fig 7), (c) Tat-independent membrane targeting of Tat substrates in vivo has been documented previously [8,44], and (d) translocation after membrane targeting has been described for the thylakoidal system [45] As no in vitro translocation of folded and cofactor-containing natural Tat substrates could be demonstrated so far, we do not exclude the alternative that the observed membrane targeting may not be coupled to translocation In this case, the data would suggest that there exists a pathway for the biogenesis of membrane proteins, which are allowed to fold before membrane targeting
In summary, we found that preHoloHiPIP can be inserted into the cytoplasmic membrane and that only correctly folded preHoloHiPIPrequires ATPfor this process To our knowledge, this is the first description of membrane targeting
of a [4Fe)4S] cluster-containing folded protein This mem-brane-targeting pathway may be of importance for the biogenesis of membrane-bound redox proteins or for the targeting of folded Tat substrates The molecular basis of this process is currently under investigation
Acknowledgements
We are grateful to Tracy Palmer for providing us with various Tat deletion mutants and to Ute Lindenstrauß for excellent technical assistance We are indebted to Jan R Andreesen, Donna M Gordon,
Bo Hou, Ralf Bernd Klo¨sgen, Debkumar Pain, Mecky Pohlschro¨der, Philip Rea and Carsten Sanders for many valuable discussions and help This work was supported by grants DOE 91ER20052 and NIH GM38237 to F D and by grant BMBF-LPD 9901/8-14 from the German Academy of Natural Scientists Leopoldina to T B.
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