Abbreviations b2m, b2-microglobulin; CE, capillary eletrophoresis; cK58-b2m, b2-microglobulin cleaved after lysine-58; dK58-b2m, b2-microglobulin with lysine-58 deleted; DRA, dialysis-re
Trang 1the main conformational features of the native protein
but are more conformationally heterogeneous and
unstable at physiological temperature
Maria C Mimmi1, Thomas J D Jørgensen2, Fabio Pettirossi1, Alessandra Corazza1, Paolo Viglino1, Gennaro Esposito1, Ersilia De Lorenzi3, Sofia Giorgetti4, Mette Pries5, Dorthe B Corlin6,
Mogens H Nissen5and Niels H H Heegaard6
1 Dipartimento di Scienze e Tecnologie Biomediche and MATI Centre of Excellence, University of Udine, Italy
2 Department of Biochemistry and Molecular Biology, University of Southern Denmark, Odense, Denmark
3 Department of Pharmaceutical Chemistry, School of Pharmacy, University of Pavia, Italy
4 Department of Biochemistry, School of Pharmacy, University of Pavia, and Biotechnology Laboratories, IRCCS Policlinico San Matteo, Pavia, Italy
5 Institute of Medical Anatomy, University of Copenhagen, Denmark
6 Department of Autoimmunology, Statens Serum Institut, Copenhagen, Denmark
Keywords
amyloidosis; cleaved b 2 -microglobulin;
human b 2 -microglobulin; NMR; protein
conformation
Correspondence
N Heegaard, Department of
Autoimmunology, Statens Serum Institut
81 ⁄ 536, Artillerivej 5, DK-2300 Copenhagen
S, Denmark
Fax: +45 32683876
Tel: +45 32683378
E-mail: nhe@ssi.dk
(Received 31 January 2006, accepted 31
March 2006)
doi:10.1111/j.1742-4658.2006.05254.x
Cleavage of the small amyloidogenic protein b2-microglobulin after
lysine-58 renders it more prone to unfolding and aggregation This is important for dialysis-related b2-microglobulin amyloidosis, since elevated levels of cleaved b2-microglobulin may be found in the circulation of dialysis patients However, the solution structures of these cleaved b2-microglobulin variants have not yet been assessed using single-residue techniques We here use such methods to examine b2-microglobulin cleaved after lysine-58 and the further processed variant (found in vivo) from which lysine-58 is removed We find that the solution stability of both variants, especially of
b2-microglobulin from which lysine-58 is removed, is much reduced com-pared to wild-type b2-microglobulin and is strongly dependent on tem-perature and protein concentration 1H-NMR spectroscopy and amide hydrogen (1H⁄2H) exchange monitored by MS show that the overall three-dimensional structure of the variants is similar to that of wild-type
b2-microglobulin at subphysiological temperatures However, deviations do occur, especially in the arrangement of the B, D and E b-strands close to the D–E loop cleavage site at lysine-58, and the experiments suggest con-formational heterogeneity of the two variants Two-dimensional NMR spectroscopy indicates that this heterogeneity involves an equilibrium between the native-like fold and at least one conformational intermediate resembling intermediates found in other structurally altered b2 -microglo-bulin molecules This is the first single-residue resolution study of a specific
b2-microglobulin variant that has been found circulating in dialysis patients The instability and conformational heterogeneity of this variant suggest its involvement in b2-microglobulin amyloidogenicity in vivo
Abbreviations
b2m, b2-microglobulin; CE, capillary eletrophoresis; cK58-b2m, b2-microglobulin cleaved after lysine-58; dK58-b2m, b2-microglobulin with lysine-58 deleted; DRA, dialysis-related amyloidosis; DN3-b2m, b 2 -microglobulin devoid of N-terminal tripeptide; FID, free induction decay.
Trang 2The conformational behavior of b2-microglobulin (b2m)
is of interest because this molecule is involved in
dialy-sis-related amyloidosis (DRA) [1,2] This condition,
somehow induced by long-standing dialysis or renal
insufficiency, is characterized by fibrillation and
precipi-tation of b2m in osteoarticular tissues Under normal
conditions, b2m is a soluble plasma protein and also
part of the MHC class I complexes on the surface of
nucleated cells It has become clear that this compact,
seven b-stranded protein is conformationally unstable
after cleavages and truncations, and that even intact
b2m may, to a minor extent, adopt an alternative
con-formation at physiological pH [3] Amyloid fibril
forma-tion from b2m in vitro requires nonphysiological
conditions with respect to pH and ionic strength, the
presence of divalent metal ions, or some of the
trunca-tions⁄ deletions that have been reported to be present in
b2m extracted from amyloid lesions [4–6] The study of
the behavior of b2m and b2m variants is relevant not
only for DRA, but also for understanding common
pathways of fibril formation in amyloidotic conditions
such as Alzheimer’s disease, transthyretin amyloidoses,
immunoglobulin fragment amyloidosis, or some of the
many other types of amyloidoses [7]
We have previously characterized two b2m variants, the first obtained by cleavage after Lys58 (cK58-b2m), and the second by further deletion of the same residue (dK58-b2m) (Fig 1) It was shown that the concerted action of activated complement C1s and carboxypepti-dase B cleaves b2m after Lys58, leading to cK58-b2m, and removes the same residue to generate dK58-b2m [8] This limited proteolysis attacking a susceptible peptide bond residing in the loop between b-strands D and E of b2m (Fig 1) increases the conformational heterogeneity of the cleaved b2m compared with the wild-type (wt) molecule [9,10] The dK58-b2m variant may occur in vivo and has been reported to be gener-ated in sera from patients with inflammation patho-logies, cancer, and renal insufficiency [11–13] Additionally, we recently showed, using dK58-b2m-specific antibodies, that dK58-b2m circulates in the blood of many dialysis patients [14]
The conformations of cK58-b2m and dK58-b2m have not previously been probed at the single amino acid level and correlated with the solution stability of these molecules We therefore here explore the struc-tural features and stability of the Lys58-cleaved b2m variants compared with those of wt b2m by a
Lys58
wt- β2m
cK58- β2m
dK58- β2m
a
b
1
99
58 1
S S 59
1 S S
99 59
c
99
57
Lys 58
+
Fig 1 Structures of b2-microglobulin (b2m) and b2m variants (A) View of the 20 best-fitting solution structures of wild-type b2m based on NMR restraints and tethered molecular dynamics For the sake of simplicity, only the backbone is drawn, apart from the side chain of Lys58, which is highlighted Designation of b-strands A–G is indicated The local trace thickness corresponds to the spatial spreading over the best overlap of the structural family ensemble Only the first members of the solution structure families were considered Drawn with MOLMOL [34] (B) NMR-based solution structure of monomeric b2m (pdb entry: 1JNJ) in a ribbon drawing The Lys58 residue (in red) and the Cys25 and Cys80 residues (yellow) connected by a disulfide bridge are shown in the backbone trace Drawn with WEBLABVIEWERPRO 3.7 (C) Schematic drawing of the variants of b2m generated by limited proteolysis of the wild-type molecule From the single-chain wild type, a heterodimeric molecule (cK58-b2m), in which the two chains are connected by a disulfide bridge, is generated by cleavage between the Cys residues The further trimming (removal of Lys58) of cK58-b2m generates the dK58-b2m variant.
Trang 3combination of NMR spectroscopy, MS, and capillary
electrophoresis (CE)
Results and Discussion
Solution stability of cleaved b2m variants
monitored by1H-NMR spectroscopy
When b2m is modified by limited proteolysis cleaving
the chain between the Cys25 and Cys80 residues, a
heterodimeric molecule consisting of two chains held
together by a disulfide bridge is generated This
mole-cule (cK58-b2m) is further processed in vivo to the
dK58-b2m variant, which lacks the K58 residue
exposed in the A-chain of cK58-b2m (Fig 1) [11] The
behavior of cK58-b2m and dK58-b2m in solution was
studied by a series of one-dimensional 1H-NMR
spec-tra collected at different conditions of temperature and
protein concentration The stability of concentrated
solutions (c 0.3 mm) in the temperature range between
288 and 310 K was first investigated The
one-dimen-sional 1H spectra of cK58-b2m and dK58-b2m
collec-ted at 288 K (Fig 2A) exhibit the typical resonance
pattern of the folded protein, with a few resolved
peaks in the aliphatic and aromatic regions In
partic-ular, the upfield shifts of Val37, Ile35 and Leu23, due
to the proximity of aromatic residues such as Tyr66,
Phe30, Phe70 and Trp95 (Fig 2B), are diagnostic of
tertiary structure interactions in the hydrophobic core
and represent a signature of the native fold of the b2m
molecule (Fig 2A, lower panel) [15] When the
tem-perature is increased in steps of five degrees up to
298 K, the lower solution stability of dK58-b2m
com-pared to cK58-b2m is highlighted While the latter at
298 K maintains a folded conformation, the variant
devoid of Lys58 is less stable and undergoes slow
unfolding and aggregation over time, as shown in
Fig 3 The unfolding is evidenced by the progressive
loss of spectral spreading and the simultaneous growth
of some main envelope at the typical frequencies of
unfolded polypeptides (around 1 p.p.m.) The
format-7 8 9
cK58 288K
dK58 288K
L23 F70
A
V37 L23 L23
L23 V37 I35
I35 F70
7 8 9
wild-type 310K
V37L23
I35
L23 F70
B
Fig 2 (A) One-dimensional 1 H-NMR aliphatic (right) and aromatic
(left) region of b 2 -microglobulin (b2m) cleaved after Lys58
(cK58-b2m) and b2m with Lys58 deleted (dK58-(cK58-b2m), 0.3 m M , at 288 K and
pH 7.4, and of wild-type b2m, 0.7 m M , at 310 K at pH 6.6, observed
at 500 MHz The upfield shift of Val37, Ile35 and Leu23, which is
diagnostic of tertiary structure interactions in b2m native folding, is
highlighted (B) Representation of the b2m hydrophobic core and of
the aliphatic residues giving rise to the most upfield-shifted methyls
in the1H-NMR spectrum Val37, Ile35 and Leu23 (green) are placed
in the shielding cone of aromatic rings (red) Only the most important
residues are included in the plot The plot was drawn using
WEBLAB-VIEWERPRO 3.7 (Accelrys Inc., San Diego, CA, USA).
Trang 4ion of large aggregates is suggested by the broadening
linewidth and the related decrease of the overall
integ-ral value under equivalent NMR acquisition
condi-tions Over the )2 ⁄ 12 p.p.m region, the spectra of
dK58-b2m shown in Fig 3 exhibit signal losses of
16% and 33%, respectively, corresponding to 10 and
41 h at 298 K In the absence of overt precipitation,
this suggests the formation of aggregates with
substan-tially larger linewidths The loss of stability and the
formation of large, soluble aggregates in dK58-b2m
solutions at 310 K over time were suggested previously
by CE analyses, and evidenced by size-exclusion
chromatography with light-scattering detection In
these experiments a well-defined aggregate formation
with an aggregate size of about 50 nm or
5· 106gÆmol)1 was noted [10] No estimate of the aggregate dimensions by measurement of translational diffusion coefficients using diffusion-ordered 2D-NMR spectroscopy experiments [16,17] was possible in the present study, because the relatively low sample con-centration (0.3 mm) prevented reliable exponential fit-ting of the experimental data
Upon further increase of the temperature to 310 K, cK58-b2m eventually slowly undergoes the same unfolding–aggregation process as observed for dK58-b2m (data not shown) In accordance with earlier observations using other methods [10], this thermal transition is irreversible (data not shown)
Fig 3 One-dimensional 1 H-NMR traces of
b2-microglobulin (b2m) cleaved after Lys58 (cK58-b2m) and b2m with Lys58 deleted (dK58-b2m) at 298 K and pH 7.4 At 298 K, cK58-b2m exhibits the typical folded protein spectrum, whereas dK58-b2m undergoes an unfolding–aggregation process that is monit-ored at 0, 10 and 41 h from the temperature setting The intensity of the upfield-shifted resonances of Leu23, Ile35 and Val37 gradu-ally diminishes, while the envelope around
1 p.p.m increases Simultaneous changes are observed in the aromatic region, invol-ving a loss of signal dispersion The overall integral value is reduced after 41 h.
Trang 5A different behavior is found when obtaining a
ser-ies of one-dimensional1H spectra at 310 K using more
dilute solutions of cleaved b2m variants (c 0.05 mm)
In contrast to the results at 0.3 mm, the unfolding–
aggregation process at a concentration of 0.05 mm is
very slow This is indicated by only a minor decrease
of the diagnostic upfield-shifted peaks of Leu40,
Val37, Ile35 and Leu23, even after 4 days (Fig 4)
Nevertheless, a slight and continuous modification of
the tertiary structure is evident from the slow overall
drift of the resonance system with a pattern suggesting
loss of conformational homogeneity After some 60 h,
for both cK58-b2m and dK58-b2m, the presence of
shoulders within the monitored isolated peaks indicates
the presence of two or more conformers in equilibrium
(peak shoulders are indicated by asterisks in Fig 4)
Further evidence for conformational heterogeneity
comes from several other envelope changes that appear
when the spectra are superimposed (data not shown)
Protein aggregation monitored by capillary
electrophoresis
In contrast to wt b2m, which is freely soluble in
physiological buffers up to at least 10 mgÆmL)1
(0.85 mm), the cleaved variants, in particular dK58-b2m, are prone to aggregation at high protein concentrations, especially at increased temperatures Visible precipitation occurs over time at concentra-tions higher than 2 mg mL)1 (0.17 mm) for the dK58 variant; the cK58 variant is more stable The aggregation behavior at different concentrations and temperatures was characterized by CE (Fig 5) In these experiments, the changes in the amount of sol-uble material were followed over time As shown in Fig 5A, a 1 mgÆmL)1 (0.09 mm) dK58-b2m solution incubated at increasing temperature initially exhibits
a shift in the conformational equilibrium between the fast (f) and slow (s) species to more of the (s) species, which is believed to be a partly unfolded intermediate (as can be seen below) Subsequently, at higher temperatures, an irreversible loss of soluble material occurs In Fig 5B, an analysis of soluble material over time at a fixed sample temperature
of 308 K at two different protein concentrations, 0.9 mgÆmL)1 (0.08 mm) and 2.5 mgÆmL)1 (0.22 mm), clearly show the loss of solubility in the higher-concentration solutions of both variants, whereas at lower concentrations both species have constant peak areas from 0 to 24 h This dependence of the
Fig 4 Details of one-dimensional 1 H-NMR traces of diluted b2-microglobulin (b2m) with Lys58 deleted (dK58-b2m) and b2m cleaved after Lys58 (cK58-b2m) solutions (0.05 m M ) at 310 K and pH 7.4 At low concentration, the unfolding process is very slow, as indicated by an only very minor decrease of the intensity of the diagnostic peaks of Leu23, Ile35, Val37 and Leu40, even after some days The presence of one or more conformational isomers, indicated by the resonance splitting of some isolated peaks (highlighted by asterisks), is particularly manifest in the spectra recorded after more than 100 h of incubation at 310 K, but may also be noticed after 60 h and to some degree in the very first recorded spectra (t ¼ 0 h) The increasing splitting between the peaks assigned to Ile35 H d1
and Leu23 Hd2, which is especially noticeable in the left panel, is consistent with a slow, continuous modification of tertiary structure, which takes place at 310 K in the dilute protein solution.
Trang 6solution stability of cleaved b2m on its concentration
is in agreement with the NMR results presented
above
MS analysis of global conformation by amide hydrogen (1H⁄2H) exchange
We have previously shown that native wt b2m and dK58-b2m undergo transient cooperative unfolding, evidenced by a correlated isotopic exchange of amide hydrogens [10] This type of exchange mechanism (EX1) leads to the appearance of distinct bimodal isotopic envelopes in the mass spectra The lower mass peak of this envelope represents the population of mol-ecules that has not yet undergone cooperative unfold-ing; the higher mass peak represents the population of molecules that has been in the unfolded state and thus undergone correlated exchange To investigate the structural stability of the folded states of wt b2m, cK58-b2m and dK58-b2m, the exchange kinetics of the folded populations were determined at 298 K (Fig 6)
At this temperature, a gradual mass increase with exchange time is observed for the lower-mass popula-tion This is due to the noncorrelated exchange mech-anism, which in structural terms can be explained by small-amplitude fluctuations within the protected core The noncorrelated isotopic exchange kinetics shown in Fig 7 was determined by the mass difference of the lower-mass populations relative to the fully deuterated control Fig 7 shows that at the shortest deuteration period (t¼ 0.5 min), all three proteins contain the same number (i.e 32; this number is also displayed in Fig 6)
of 1H atoms not yet exchanged for deuterium This indicates that an identical number of protecting hydro-gen bonds exists in the folded states of wt b2m, cK58-b2m and dK58-cK58-b2m Furthermore, the cleaved variants, cK58-b2m and dK58-b2m, exhibit very similar noncor-related exchange kinetics (Fig 7) This indicates that the stability of the hydrogen bond network that confers protection against isotopic exchange is almost identical for these proteins However, with prolonged incubation this network appears to be slightly more stable in wt b2m than in the cleaved species (Fig 7)
Thus, in accordance with the NMR results, the glo-bal conformation of wt b2m appears to be conserved
in the cleaved forms of b2m Note that in these experi-ments only the slowly exchanging hydrogens are monitored Thus, contributions from amide hydrogens
in loop regions and from new termini generated in the cleaved variants are not expected to affect the exchange count
Two-dimensional NMR characterization The detailed interpretation of the 1H-NMR spectra of b2m variants is based on the parent spectra of wt b2m obtained at different temperature and pH values
0
20
40
60
80
100
0.22 mM dK58-β2m 0.08 mM dK58-β2m 0.08 mM cK58- β2m 0.21 mM cK58- β2m
Incubation time (min) at 35 °C
B
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Time (min)
A 200 nm
A
Fig 5 Capillary electrophoresis separation of b2-microglobulin (b2m)
with Lys58 deleted (dK58-b2m) incubated at different
tempera-tures (A) Separation profiles to show that sample temperature
(indicated in the figure) influences the ratio between f and s
con-formers of dK58-b2m in CE All CE experiments were performed
at a constant capillary temperature of 278 K to preserve the
dis-tribution of conformers in the injected samples Shown are
over-layed electropherograms with time windows showing the s and f
conformer peaks Samples were 1 mgÆmL)1 dK58-b2m
electro-phoresed at 278 K using 90 lA constant current after injection for
2 s (B) Aggregation propensity of b2m cleaved after Lys58
(cK58-b2m) and dK58-b2m at different protein concentrations
Sol-uble material was monitored by CE as a function of incubation
time Samples of 0.9 mgÆmL)1 (triangles) or 2.5 mgÆmL)1 (circles)
b2m variants (cK58, open symbols; dK58, filled symbols) were
kept at 308 K, and aliquots (2 s injections of high-concentration
samples and 4 s injections of low-concentration samples) were
analyzed by CE performed at constant current of 80 lA, with the
capillary cooling fluid maintained at 278 K Samples also contained
0.2 mgÆmL)1 of a marker peptide Shown are the summed peak
areas P (total area of f + s peaks) divided by the marker peak
area M at different time points as a percentage of the initial
value of P ⁄ M at the onset of the experiments where the sample
temperature was brought from 278 K to 308 K.
Trang 7[15,18] and requires the checking and redetermination
of most of the resonance assignments of the molecule
under investigation according to the standard
meth-odology, i.e going through scalar and dipolar
connec-tivity patterns for each amino acid residue [19] This
work could be almost entirely completed for
cK58-b2m, but only partially for dK58-b2m The difficulty
with both variants, particularly dK58-b2m, is due to
their thermal lability (unfolding and aggregation) This
prevented the use of optimal temperatures (e.g 310 K)
to improve data quality with concentrated samples
(e.g 0.5 mm) Increasing the temperature up to 320 K,
whenever possible, generally improves the NMR data
quality for 10–15 kDa proteins by reducing linewidths
and thus favoring spectral analysis As a compromise
in the present study, the two-dimensional TOCSY
and NOESY spectra of cK58-b2m were obtained at
298 K, whereas the best results with dK58-b2m were
generated at 310 K by working with a very dilute
sample (0.05 mm)
The assignment lists (supplemental Tables 1 and 2) indicate an overall conservation of the resonance fre-quencies with respect to the corresponding wt values and thus confirm the retention of the main features of the native structure in both variants The backbone Ha chemical shifts of cK58-b2m and dK58-b2m (wherever assignments were available) were compared to the corresponding values of the wt protein, as shown in Fig 8 As expected, the largest deviations of Ha chem-ical shifts of cK58-b2m are found in proximity to the cleavage site, more specifically in fragments 56–58 and 59–64, i.e at the opened loop D–E, and at the start of strand E [15] Interestingly, similar deviations are also found in fragment 26–35, i.e at the end of strand B and at loop B–C, which faces the D–E region Accord-ing to the well-established correlation between Ha chemical shifts and secondary structure in polypeptides [20], the shifts of cK58-b2m Ha resonances compared
to wt b2m reflect changes in the backbone arrangement within the D–E as well as in the B–C loop Thus, two
m/z
Da
Min
0
0.5
dK58, 9+
30 80 160 100% D
ox
ox
54 Da
dK58 ox
ox
32 Da
32 55
Fig 6 Global amide hydrogen (1H ⁄ 2
H) exchange analysis of the folded
conforma-tions of b2-microglobulin (b2m) cleaved after
Lys58 (cK58-b2m) and b2m with Lys58
dele-ted (dK58-b2m) at 298 K in deuteradele-ted
NaCl⁄ P i The proteins were incubated
pairwise in deuterated NaCl ⁄ P i buffer After
various periods of deuteration, isotopic
exchange was quenched by acidification.
Subsequently, the samples were desalted at
quench conditions and analyzed by ESI-MS.
Shown are the ESI mass spectra of a
mix-ture of cK58-b2m and dK58-b2m obtained
after various deuteration periods (given in
minutes in the figure) at 298 K Left panel:
deconvoluted ESI mass spectra Right
panel: ESI mass spectra of the m ⁄ z region
with the [M + 9H]9+ions The spectra
obtained at t ¼ 0 min (i.e lowest traces)
were obtained from 1 H2O Ox,
Met99-oxidized species.
Trang 8opposite changes of secondary structure are found at
the end of strand D and the beginning of strand E, i.e
a further loss in D and a stabilization in E of the local
b-structure geometry Compared with wt b2m, the
cK58-b2m molecule is thus most conformationally
different in the cleavage site region (D–E loop), with
additional involvement of the adjacent residues of
strands D and E, and the facing residues of loop B–C
Unfortunately, this analysis could not be extended to
dK58-b2m, because of the ambiguous assignment of
residues from these regions of the molecule
Conformational heterogeneity of cK58-b2m and
dK58-b2m
The whole NMR dataset for cK58-b2m and
dK58-b2m revealed the occurrence of at least two different
conformers for each molecule These conformers were
undergoing slow exchange on the chemical shift
time-scale Examination of the two-dimensional maps
obtained with concentrated cK58-b2m at 298 K
showed a generalized resonance doubling at the
loca-tions and to the extent reported in Fig 9 The features
of the pattern of the second conformer resemble the
features of a minor monomeric intermediate occurring
along the b2m-refolding pathway that was named I2
and initially identified by Chiti et al in wt b2m [3]
The I2 conformer was subsequently also detected in
real-time NMR experiments [21] Further analysis of
other amyloidogenic b2m variants, and in particular of
the species devoid of the N-terminal tripeptide,
DN3-b2m, has shown that the I2 conformer is in equi-librium with the fully folded species [21,22] This indi-cates that it can be precisely identified through NMR characterization In the case of DN3-b2m, the observa-tion of resonance doubling for the side chain signals of residues Val9, Ser11, Leu23, Val37 and Ala79, which are all close to one or more aromatic residues in the cluster of Tyr26, Tyr66, Phe70, Tyr78 and Trp95 [15], strongly suggested that I2 corresponds to a slightly destabilized fold that has the overall conformation of
wt b2m, but exhibits a looser packing of its hydropho-bic core This interpretation was recently challenged by Kameda et al [23], who reported evidence in favor of
a slow trans–cis isomerization of Pro32 during refold-ing of b2m Whatever the origin of the conformational equilibrium that gives rise to the slow refolding step of b2m, the proposed correspondence of the second form observed in the cK58-b2m spectra with the I2 con-former identified in DN3-b2m is based on the similar-ity of the resonance doubling patterns of the two variants This analogy is visualized in Fig 10, where details of NOESY spectra are shown In spite of the different conditions of temperature and pH, the close similarity of the patterns is readily appreciated The excellent resolution of the resonances in Fig 10 could not be exploited for quantitation of the relative con-centrations of the two forms because, in general, NOESY cross-peak amplitudes are determined by the actual motional characteristics of the connected nuc-lear pairs, and thus may differ between distinct con-formers [24] Many other resonance doublings were observed (the most relevant are reported in Fig 9), all consistent with the expected pattern of the I2 interme-diate that was unambiguously recognized in previous studies of other b2m variants [21,22] The best estimate
of the equilibrium populations of the fully folded and
I2 forms for cK58-b2m at 298 K was obtained by using, for each conformer, the pair of TOCSY connec-tivities assigned to Val37 Hc1–Hc2 Taking into account the partial overlap of the specific cross-peaks, the resulting relative amount of I2 at 298 K was
19 ± 9% of the total protein The occurrence of an I2 intermediate in equilibrium with the main species was also deduced from the dK58-b2m NMR spectra, although the lower resolution made it necessary to rely more on peak shape distortion than on actual peak separation (Figs 4 and 11B)
Two conformers of the b2m variants cleaved at Lys58 were also detected by CE as previously reported [9] and are shown in Figs 5A and 11A The precise nat-ure of the slow conformer peak (labelled ‘s’ in Fig 5A and 11A) could not be unequivocally determined in these experiments The two populations observed in CE
0
5
10
15
20
25
30
35
Exchange time[min]
dK58-2m cK58-2m wt-2m
Fig 7 Noncorrelated exchange kinetics of the folded conformations
of b2-microglobulin (b2m) cleaved after Lys58 (cK58-b2m) (triangles),
b2m with Lys58 deleted (dK58-b2m) (crosses), and wild-type (wt)
b2m (circles) Shown are mass shifts (expressed as loss of protected
protiated residues to adjust for differences in chain lengths) at 298 K
as a function of time incubated in deuterated NaCl ⁄ P i
Trang 9separations of samples kept at 298 K gave, for the
slow-migrating conformer, concentrations of 38% ±
2% for cK58-b2m and 30% ± 4% for dK58-b2m
(triplicate experiments ± SD), relative to the total
peak area, independently of the total b2m
concentra-tions used (examples are shown in Fig 11A) CE
sepa-rations are accomplished at low temperature in
10–12 min Thus, solution states are sampled under dynamic conditions where the conformers are being separated from each other, whereas NMR spectra record steady-state solution distributions Such ences in experimental conditions may explain the differ-ences in the relative concentration estimates for the two conformers However, both the NMR and CE
approa-dK58- β2m compared to β2m-wt
-0,08
-0,06
-0,04
-0,02
0
0,02
0,04
0,06
0,08
0,1
0,12
residue
cK58-β2m compared to β2m-wt
T4 P5 K6
Q8 V9 Y10
H13 P14
N17 G18 G18 K19
V37 D38L39
G43 E44
E69 F70
-0,3
-0,25
-0,2
-0,15
-0,1
-0,05
0
0,05
0,1
0,15
0,2
residue
Fig 8 Two-dimensional NMR study of b2-microglobulin (b2m) cleaved after Lys58 (cK58-b2m) and b2m with Lys58 deleted (dK58-b2m)
at pH 7.4 The assigned backbone H a chemical shifts of 0.3 m M cK58-b2m at 298 K, and of 0.05 m M dK58-b2m at 310 K, are compared with the corresponding values of the wild-type (wt) species obtained at 310 K and pH 6.6 The DdH a values (p.p.m.) are reported as (Dvariant ) Dwt) Residue labels are omitted in regions where the resonance assignment was ambiguous.
Trang 10ches strongly support the notion of conformational
het-erogeneity of the cK58-b2m and dK58-b2m variants
Conclusions
Although cK58-b2m is unlikely to have a long lifetime
in vivo, where the exposed Lys58 is rapidly cleaved off
by endogeneous carboxypeptidase B activity [25], this
variant was included in our study because it is more
sta-ble in solution than dK58-b2m and thus more accessista-ble
to analysis It has very similar characteristics in all the
MS and CE analyses However, we found for both b2m
variants that the protein concentrations required for
high-resolution NMR spectroscopy were detrimental to
their stability in solution The two cleaved b2m species
have a pronounced propensity to undergo
temperature-dependent unfolding and aggregation In addition, the data show the occurrence of conformational heterogen-eity in cK58-b2m and dK58-b2m solutions, which is consistent with their thermal lability Despite these diffi-culties, detailed characterization of the conformational states of the cK58-b2m and dK58-b2m variants has now been accomplished, and has made it possible, by reference to the NMR pattern of the DN3 variant of b2m, to identify a minor conformational species that also exists in the conformational equilibrium of the cleaved b2m variants This conformer is a monomeric intermediate (I2) occurring on the b2m-refolding path-way These findings are consistent with the existence, in addition to the folded conformation, of a less abundant form with amyloidogenic features, which has also been suggested by CE experiments [9,10,22]
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
H assignment
Fig 9 Resonance doublings in b2 -micro-globulin (b2m) cleaved after Lys58 (cK58-b2-m) indicating conformational heterogeneity The proton chemical shifts of the alternative conformer (I 2 ) of cK58-b2m are compared with the corresponding values of the nat-ively folded form of cK58-b2m in the graph The two conformers are recognized in TOCSY and NOESY maps, obtained at
298 K and pH 7.4 The DdH values (p.p.m.) are reported as (dI2) dN), where N stands for the natively folded form Only the most relevant deviations are shown Resonance doubling observed elsewhere was less pronounced in terms of Dd.
p.p.m.
10.4 10.6
10.8
7.0
7.2
7.4
7.6
7.8
8.0
p.p.m.
10.4 10.6 10.8
7.0 7.2 7.4 7.6 7.8 8.0
W95 N W95 I2
W95 N
W95 N
W95 N
W95 I2
Fig 10 Details of two-dimensional NMR NOESY maps of b2-microglobulin (b2m) cleaved after Lys58 (cK58-b2m) (left) and b2m devoid of the N-terminal tripeptide (DN3-b2m) (right), recorded at 500 and
800 MHz, respectively The intraresidue connectivities H e1 –H d1 (top) and H e1 –H f2 (bottom) of Trp95 are indicated for the nat-ively folded form (N) and for the I2form, in equilibrium under the chosen experimental conditions.