Together, the synthetic SPAFLA peptide corresponding to the Ser808– Ala813 sequence of human a2M, which contains the cath-epsin E-cleavage site, was selectively cleaved by cathcath-epsin
Trang 1Disruption of structural and functional integrity
Mitsue Shibata1, Hideaki Sakai1, Eiko Sakai1, Kuniaki Okamoto1, Kazuhisa Nishishita1, Yoshiyuki Yasuda2, Yuzo Kato1and Kenji Yamamoto2
1
Department of Pharmacology, Nagasaki University School of Dentistry, Japan;2Department of Pharmacology,
Graduate School of Dental Science, Kyushu University, Fukuoka, Japan
a2-Macroglobulin (a2M) is an abundant glycoprotein with
the intrinsic capacity for capturing diverse proteins for rapid
delivery into cells After internalization by the
receptor-mediated endocytosis, a2M-protein complexes were rapidly
degraded in the endolysosome system Although this is an
important pathway for clearance of both a2M and biological
targets, little is known about the nature of a2M degradation
in the endolysosome system To investigate the possible
involvement of intracellular aspartic proteinases in the
dis-ruption of structural and functional integrity of a2M in the
endolysosome system, we examined the capacity of a2M for
interacting with cathepsin E and cathepsin D under acidic
conditions and the nature of its degradation a2M was
effi-ciently associated with cathepsin E under acidic conditions to
form noncovalent complexes and rapidly degraded through
the generation of three major proteins with apparent
molecular masses of 90, 85 and 30 kDa Parallel with this reaction, a2M resulted in the rapid loss of its antiproteolytic activity Analysis of the N-terminal amino-acid sequences of these proteins revealed that a2M was selectively cleaved at the Phe811-Leu812 bond in about 100mer downstream of the bait region In contrast, little change was observed for a2M treated by cathepsin D under the same conditions Together, the synthetic SPAFLA peptide corresponding to the Ser808– Ala813 sequence of human a2M, which contains the cath-epsin E-cleavage site, was selectively cleaved by cathcath-epsin E, but not cathepsin D These results suggest the possible involvement of cathepsin E in disruption of the structural and functional integrity of a2M in the endolysosome system Keywords: a2-macroglobulin; aspartic proteinase; cathepsin D; cathepsin E; endolysosome system
1
a2-Macroglobulin (a2M) is an abundant plasma
glycopro-tein composed of four identical subunits of Mr 185 kDa
[1] a2M inhibits the activity of all classes of endopeptidases
from both endogenous and foreign sources The proteinases
cleave an accessible region of the polypeptide chain of a2M,
the bait region, thereby leading to the activation of internal
thiol esters and the subsequent conformational change that
entraps the responsible proteinase [1] Then, the
a2M-proteinase complexes are recognized by the low-density
lipoprotein receptor-related protein (LRP)/CD91 on the
surface of different cell types such as hepatocytes [2],
fibroblast-like cells [3], and monocytes/macrophages [4] and
become destined to rapid clearance and degradation in the
endolysosome system [3,5] Recent studies have also
dem-onstrated that a2M has other important intrinsic capacity
for capturing diverse molecules, including cytokines [6–9],
growth factors [10–13], hormones [14], and soluble
b-amy-loid peptide [15], for rapid delivery into cells and
degrada-tion The association of these molecules with a2M induces
neither cleavage of the a2M peptide bond [16,17] nor the a2M conformational change [18,19] The nature of this association is therefore distinct from the trapping mechan-ism for proteinases However, these molecules bound to a2M are similarly targeted to cells expressing the a2M signaling receptor and become destined to rapid degrada-tion in the endolysosome system Therefore, a2M also plays
an important part in the clearance of these molecules or regulates their biological activity Meanwhile, a2M has also been shown to mediate immune responses through the delivery of foreign antigens to macrophages [20]
It is thus considered that a2M is involved in a wide range
of physiological processes with the intrinsic capacity for capturing diverse target proteins for rapid delivery into cells and efficient degradation in the endolysosome system However, the nature of the endocytosed a2M degradation
in the endolysosome system is poorly understood A phy-siological inactivator of a2M has not yet been identified Cathepsins E and D are analogous endolysosomal aspartic proteinases in mammalian cells [21] Cathepsin E represents
a major portion of the proteolytic activity in the endosomal compartment in certain cell types such as macrophages and microglia [22–24], gastric cells [25], and antigen-presenting B cell lymphoblasts [26] Cathepsin E is also associated with the plasma membrane of various cell types such as erythrocytes [27,28], osteoclasts [29], gastric parietal cells [30], renal proximal tubule cells [30], and hepatic cells [30]
On the other hand, cathepsin D is widely distributed in almost all the mammalian cells as the most abundant
Correspondence to K Yamamoto, Department of Pharmacology,
Graduate School of Dental Science, Kyushu University,
Fukuoka 812–8582, Japan.
Fax: + 81 92 6426342, Tel.: + 81 92 6426337,
E-mail: kyama@dent.kyushu-u.ac.jp
Abbreviations: a2M, a 2 -macroglobulin; Hb, hemoglobin;
LRP, low-density lipoprotein receptor-related protein.
(Received 30 November 2002, accepted 24 January 2003)
Trang 2endolysosomal proteinase More recently, it has been
demonstrated that cathepsin E has a potential for foreign
antigen processing for presentation by class II major
histocompatibility complex[24] and possible regulation of
activities of substance P and related tachykinins [31],
whereas cathepsin D is indispensable for protection of the
onset and development of a certain type of neuronal ceroid
lipofucinosis [32] and proteolysis of proteins regulating cell
growth and tissue homeostasis [33] However, there is no
unequivocal evidence for the participation of these two
proteinases in the degradation of a2M The inherent
problem is that both cathepsins E and D were essentially
inactive at around neutral pH where a2M is very stable,
whereas they are most active at around pH 4.0 where a2M
is unstable Although previous work has suggested that
cathepsin E is inhibited by a2M at pH 6.2 [34] and 5.5 [35],
the ability of a2M to interact with cathepsins E and D below
about pH 5.0 is still uncertain In this report, we
demon-strate that a2M is selectively associated with cathepsin E
below pH 5.0 and rapidly cleaved it at a specific site distinct
from the bait region, thereby losing its structural and
functional integrity
Materials and methods
Materials
Trypsin (Type XIII) and human a2M were purchased from
Sigma-Aldrich Bovine liver cDNA was purchased from
Clontech Cathepsin E was purified from rat spleen [36] and
human erythrocytes [27] as previously described Cathepsin
D was purified from rat [37] and bovine spleen [38] as
described The fluorogenic decapeptide substrate
MOCAc-Gly-Lys-Pro-Ile-Ile-Phe-Phe-Arg-Leu-Lys(Dnp)-D-Arg-NH2
was synthesized as described previously [39] Antibodies
specific for rat cathepsin E and cathepsin D were raised in
rabbits and purified by affinity chromatography as
des-cribed previously [28] Antiserum against bovine a2M was
purchased from Yagai Research Center (Yamagata, Japan)
All other chemicals were of reagent grade and were
purchased from various commercial sources
Purification of a2M from bovine serum
a2M was purified from bovine serum as described
previ-ously [40], with a slight modification Briefly, the serum was
applied to a Ni/nitrilotriacetic acid column (QIAGEN,
1.5 cm· 3.5 cm) and eluted with 50 mM sodium acetate
buffer, pH 5.0, containing 50 mM NaCl The eluate was
applied to a Mono Q column equilibrated with 33 mM
sodium phosphate buffer, pH 6.0 The column was washed
with the same buffer and eluted with a linear gradient of
NaCl (50–500 mM) in the buffer Fractions containing a2M
were determined by assaying the inhibitory activity against
rat spleen extract at pH 3.8 using hemoglobin as a substrate
Fractions containing a2M, whose activity was determined
by inhibition of the hemoglobin (Hb)-hydrolyzing activity
of rat spleen extracts at pH 3.8, were pooled and
concen-trated and then subjected to gel filtration on Superose 6
The a2M fractions were pooled and subjected to the
second Mono Q anion exchange chromatography The
pooled a2M fractions were concentrated and dialyzed
against 20 mM Hepes buffer, pH 7.2, containing 140 mM NaCl
Assays The proteinases activity of cathepsins E and D were measured in 0.1M sodium acetate buffer, pH 3.8, using 1.5% acid-denatured Hb [27] or MOCAc-Gly-Lys-Pro-Ile-Ile-Phe-Phe-Arg-Leu-Lys(Dnp)-D-Arg-NH2 [39] as des-cribed previously
Sequencing of bovine a2M cDNA The degenerate nucleotide sequences of the primers for polymerase chain reaction (PCR) were designed based on the conserved regions of the mammalian a2M Bovine liver cDNA was used as the template for PCR The PCR products were isolated and subcloned into pBluescript II SK vector (Stratagene) and sequenced by an ABI automatic DNA sequencer model 310 (Perkin Elmer-Applied Bio-systems) Twenty-five clones were highly homologous to a2M
Gel electrophoresis and N-terminal sequence analysis SDS/PAGE and immunoblotting were carried out follow-ing the procedure as described previously [28] For the N-terminal amino-acid sequencing, the purified bovine a2M and the cathepsin E-digested protein were separated by SDS/PAGE (8% gel) under reducing conditions and then transferred onto poly(vinylidene difluoride) membranes and stained with Coomassie blue R-250 The stained bands were excised and the adsorbed proteins were subjected to an automatic protein/peptide sequencer (Applied Biosystems Model 477A)
Interaction of a2M with cathepsin E Purified bovine a2M (140 pmol) was incubated with or without cathepsin E (70 pmol) at pH 3.8 at appropriate time intervals After neutralization, the incubation mixtures were applied to gel filtration on Superose 6 equilibrated with
10 mM sodium phosphate buffer, pH 6.0, containing
150 mM NaCl Fractions were analyzed by SDS/PAGE, immunoblotting and cathepsin E activity
Analysis of synthetic peptides treated with cathepsin E and cathepsin D
Synthetic peptides corresponding to the cleavage site region
of a2M by cathepsin E were designed and custom-synthes-ized at the Peptide Institute (Osaka) Each peptide (20 nM) was incubated with or without either cathepsin E (20 pM) or cathepsin D (20 pM) in 0.1Msodium acetate buffer, pH 3.8, and then subjected to reversed phase high-performance liquid chromatography (RP-HPLC)
column (3.9 mm· 300 mm) (Waters) The column was eluted with a gradient of acetonitorile (0–60% in 30min) in 0.1% trifluoroacetic acid at a flow rate of 1.0 mLÆmin)1 Each peak fraction was pooled and the amino-acid sequence was analyzed by an Applied Biosystems automated derivatizer-analyzer (model 477A/120)
Trang 3Effect of a2M on the hemoglobin hydrolyzing activity
of cathepsin E and cathepsin D
To assess the inhibitory capacity of a2M from various
sources for cathepsin D and cathepsin E, we first purified
bovine a2M The final preparation gave a single protein
band with an apparent molecular mass of 170 kDa when
analyzed by SDS/PAGE under reducing conditions (data
not shown) The N-terminal amino-acid sequence of this
protein was found to be AVDGKEPQYM, which was
identical to the N-terminal sequence of the intact a2M as
reported previously [41] Addition of bovine a2M, as well as
human a2M, to cathepsin E purified from human and rat
sources (0.01 ng each) at pH 3.8 resulted in the significant
decrease of Hb-hydrolyzing activity in a dose-dependent
manner though the rate and extent of inhibition by bovine
a2M were slightly but significantly higher than those by
human a2M (Fig 1) In contrast, little change was observed
for the Hb-hydrolyzing activity of cathepsin D purified from
rat and bovine spleen when a2M was added under the same
conditions This selective inhibition was further
substan-tiated by experiments using the cell extract of rat spleen, in
which cathepsin E and cathepsin D comprise 55 and 45% of
the total Hb-hydrolyzing activity [25] The cell extract
treated with discriminative antibodies specific for cathepsin
D to remove this protein showed a significant decrease in the
Hb-hydrolyzing activity by addition of either human or
bovine a2M, whereas the extract devoid of cathepsin E by
immunoprecipitation with specific antibodies to cathepsin E
showed no significant change in the Hb-hydrolyzing
activity by each a2M (not shown) The results indicate
that the selective reduction of cathepsin E activity by a2M
occurs through the specific interaction between a2M and cathepsin E
Effect of pH on the interaction of cathepsin E with a2M
Previous studies have shown that cathepsin E binds a2M at
pH 6.2 and that the enzymatic activity of the complex toward the synthetic substrate Pro-Pro-Thr-Ile-Phe-Phe-(4-NO2)-Arg-Leu is not significantly affected [34] Strong inhibition of the cathepsin E activity toward the protein substrate ribonuclease A by a2M was also observed at
pH 5.5, where the a2M was cleaved by cathepsin E at the Phe-Tyr bond in the bait region [35] Similarly, complete inhibition of the cathepsin D activity toward hemoglobin by a2M was observed at pH 6.2 [34] These observations suggest that the inhibition of these enzymes by a2M at mild acidic pH values is similar to that observed with other classes of proteinases at neutral pH values To assess whether the pH is crucial for the action of a2M on these aspartic proteinases, the effect of lowering the pH below 5.5
on the association of a2M with cathepsins E and D and the structural change in a2M upon complexformation with these enzymes were analyzed After incubation of a2M at
37C for 1 h with or without cathepsin E at the indicated
pH values, the reaction mixtures were analyzed by non-denaturing PAGE (Fig 2) a2M treated with cathepsin E at
pH 5.5 migrated faster than the native a2M, indicating that a2M became bound to cathepsin E and thereby underwent conformational change into the more compact form a2M treated with cathepsin E at pH 4.5 was diffusely stained and its mobility was faster than that observed at pH 5.5 a2M treated with cathepsin E at pH 3.8 was seen near the dye front
Fig 1 Effect of bovine a2M on the proteolytic
activities of cathepsin E and cathepsin D The
Hb-hydrolyzing activity of cathepsin E
purified from human erythrocytes and rat
spleen and cathepsin D purified from bovine
spleen and rat spleen (0.01 ng each) was
measured at pH 3.8 in the absence or presence
of increasing amounts of a2M.
Trang 4To assess the mechanism of action of cathepsin E on
a2M, we next analyzed whether the mobility changes of
cathepsin E-treated a2M was due to cleavage of the bait
region or degradation of bovine a2M The cathepsin
E-treated a2M molecule was rapidly degraded to generate
two major protein bands with apparent molecular masses of
90 and 30 kDa within a 15-min incubation at pH 3.8 and
37C (Fig 3A) The N-terminal amino-acid sequences of
the 90- and 30-kDa peptides were found to be AVDGKEP
and LAIPVE, respectively Within a 30-min incubation, an
additional 85-kDa peptide was generated and its N-terminal
amino-acid sequence was identical to that of the 90-kDa
peptide All these peptides were further degraded by
prolonged incubation Similar results were obtained with
human a2M (Fig 3B), although the N-terminal amino-acid sequence of human 85-kDa peptide was not identical to that
of the 90-kDa peptide from bovine a2M The N-terminal amino-acid sequence of the 85-kDa peptide derived from human a2M corresponded to the sequence of the N-termi-nus of the original a2M, and the N-terminal amino-acid sequence of the 90-kDa was identical to the sequence starting with 685th Tyr (YESDVM) Although human a2M treated with cathepsin E generated the 30-kDa peptide, its N-terminal amino-acid sequence could not be determined
by overlapping of additional peptides in the vicinity of
30 kDa These three peptides were also generated from cathepsin E-treated human a2M at pH 4.5 and 5.5 (Fig 3C) As no detectable accumulation of the other protein bands was observed, cleavage at the other sites probably much more rapid than at the one that was slow enough to allow the cleaved fragments to build up This may explain some discrepancy in molecular sizes between the intact 170-kDa polypeptide and the generated 90- and 30-kDa fragments In contrast, no degraded protein bands were observed for the cathepsin D-treated bovine a2M under the same conditions (Fig 3A) Figure 4 shows
Fig 2 Association of cathepsin E with a2M under acidic conditions.
Cathepsin E was incubated with bovine a2M at a molar ratio of 2 : 1
at 37 C for 60 min at the indicated pH values Then the reaction
mixture was subjected to native PAGE at pH 8.9 As a control, a2M
treated with trypsin (an a2M/enzyme ratio, 1 : 1.5) at pH 7.5 and
37 C for 10 min was run on the same gel.
Fig 3 SDS/PAGE of a2M treated with cathepsin E or cathepsin D under acidic conditions a2M from bovine (A) and human sources (B, C) was incubated with cathepsin E or cathepsin D at molar ration of 1 : 1 at 37 C at the indicated pH values for various times, and then analyzed by SDS/ PAGE under reducing conditions A, bovine a2M at pH 3.8; B, human a2M at pH 3.8; C, human a2M at pH 4.5 and 5.5 A, 8% gel; B and C, 15% gel.
Fig 4 Effects of substrate/enzyme ratios on generation of the cathepsin E-cleavage peptides of a2M Bovine a2M was incubated with cathepsin
E at various a2M/enzyme ratios at pH 3.8 and 37 C for 30 min, and then the reaction products were analyzed by SDS/PAGE (6%) under reducing conditions.
Trang 5SDS/PAGE profiles of bovine a2M treated with cathepsin
E at different molar ratios at 37C and 30 min at pH 3.8
The a2M-cathepsin E complexgave the 90- and 30-kDa
peptides at a a2M/cathepsin E molar ratio of below 10
More than 90% of the original a2M disappeared at the
molar ratio of approx 2 : 1, where the generation of the
90- and 30-kDa peptides reached the maximal value
N-terminal sequencing of bovine a2M and sequencing its cDNA
Because the inhibition capacity of bovine a2M for cathepsin
E was stronger than that of human a2M, and because there
is no report on the sequence of the subunit of bovine a2M so far, we determined the partial amino-acid sequence of
Fig 5 Comparison of the partial amino-acid sequence of bovine a2M with those of human, rat, and mouse a2M species The partial amino-acid sequence of bovine a2M, including the bait region to the thiol ester bond site, was shown aligned with those of human, rat and mouse a2M species This region corresponds to the residues 538–954 of human a2M Shading indicates identity relative to the bovine a2M sequence, and numbering is relative to the human a2M The bait region is underlined and the thiol ester bond site is double-underlined The arrow and the boxindicate the cathepsin E cleavage site and the sequence used for peptide synthesis, respectively Human, rat and mouse a2M sequences are from refs [45], [46] and [47], respectively.
Trang 6bovine a2M predicted from its cDNA sequence
(corres-ponding to the residues 538–954 of human a2M), which
contained the bait region Comparison of the amino-acid
sequence of bovine a2M determined with those of human,
rat, and mouse origins revealed that the sequence was
strongly related to those of these species (Fig 5) Although
the overall sequence of this region of bovine a2M was
significantly similar to those of human, mouse, and rat
sources (59%, 62%, and 55% identities, respectively), the
bait region (corresponding to the residues 666–706 of
human a2M) was dissimilar to those of other species and
of different length The N-terminal amino-acid sequences of
the 90- (AVDGKEP) and 30-kDa peptides (LAIPVE)
generated from cathepsin E-treated bovine a2M
correspon-ded to the sequences of N-terminus and starting with 812th
Leu of the intact a2M, respectively It is worth emphasizing
that the amino-acid sequences of cathepsin E-cleavage
sites (shown by the box) were highly conserved among
mammalian species Taken together, these results indicate
that cathepsin E specifically cleaved at the Phe811-Leu812
bond present in about 100mer downstream of the bait
region of both bovine and human a2M
Analysis of the specific cleavage of a2M by cathepsin E
with various synthetic peptides
To further confirm the specific cleavage of a2M by cathepsin
E, synthetic peptides of a2M including the cathepsin E
cleavage site region were designed and synthesized These
synthetic peptides were incubated with cathepsin E or
cathepsin D at pH 3.8 and 37C for various time intervals
and then the reaction products were analyzed by HPLC C18
column The hexapeptide SPAFLA corresponding to the
cleavage site of human a2M was efficiently cleaved by
cathepsin E at the Phe-Leu bond, whereas it was not cleaved
by cathepsin D (Table 1) The hepta peptide SSAFLAF
corresponding to the cleavage site of bovine a2M was also
cleaved by cathepsin E However, differing from the peptide
SPAFLA, this peptide was efficiently cleaved by cathepsin
D On the other hand, the pentapeptides SPAFL and
SSAFL were not cleaved either cathepsin E or cathepsin D,
indicating that the presence of Ala in the P¢2 site is crucial for
the selective cleavage of a2M by cathepsin E and that the
addition of Phe to the P¢3 site causes a loss of this selective
action of cathepsin E In agreement with these results, the
cathepsin E-induced a2M degradation was significantly inhibited by these three peptides, most strongly by SPAFLA (Fig 6)
Structural changes in a2M upon complex formation with cathepsin E as determined by gel filtration analysis a2M was first incubated with cathepsin E (a molar ratio of
14 : 1) at pH values between 5.5 and 3.8 at various time intervals The reaction mixtures were adjusted to pH 6.0 and then subjected to gel filtration on Superose 6 At every
pH value, more than 80% of cathepsin E rapidly disappeared from the original position corresponding to a molecular mass as high as 80 kDa and appeared at the position where a2M was eluted However, the a2M-cathepsin E complexwas rapidly dissociated at pH 3.8 within a 2-min incubation (Fig 7B) Parallel to this change,
a significant amount of the original a2M disappeared and a
Table 1 The ability of cathepsin E and cathepsin D to cleave synthetic peptides The synthetic peptides (20 n M ) containing the cathepsin E-cleavage site of a2M were incubated with or without either cathepsin E or cathepsin D (20 p M each) at pH 3.8 and 37 C for the indicated time The samples were then subjected to a HPLC C18 column chromatography and the resultant peak fractions were subjected to the amino-acid sequence analyzer The values are expressed as a percentage of the initial intact peptide The asterisk indicates the values for SPAF and SSAF generated by cleavage of SPAFLA and SSAFLAF, respectively ND, not determined.
Peptides
Fig 6 Effects of synthetic peptides on degradation of a2M by cathepsin
E Bovine a2M was incubated with cathepsin E at pH 3.8 and 37 C for 30 min in the absence or presence of various synthetic peptides (500 l M ), and then the reaction products were analyzed by SDS/ PAGE under reducing conditions Peptides used are Ser-Ser-Ala-Phe-Leu (1), Ser-Pro-Ala-Phe-Ser-Ser-Ala-Phe-Leu (2), and Ser-Pro-Ala-Phe-Ser-Ser-Ala-Phe-Leu-Ala (3).
Trang 7few protein peaks were produced After the 1-h incubation
at pH 3.8, the original a2M completely disappeared and
generated two major protein peaks at the positions
corresponding to molecular masses as high as 90 and
30 kDa In agreement with this change cathepsin E
reappeared at the original position with no loss of activity
On the other hand, the dissociation of cathepsin E from the
a2M complexwas relatively slow at pH 4.5 and 5.5
(Fig 7A) Although the formation of a2M–cathepsin E
complexwas rapid at pH 5.5 and 4.5, the rate of
dissociation of this complexwas pH-dependent On the
other hand, the antiproteolytic activity of a2M for trypsin
was lost by incubation with cathepsin E in a dose-dependent manner (Fig 8)
Discussion a2M has a wide range of physiological activities via its interaction with various target proteins, such as the control
of the activity of proteinases, the regulation of the activities
of numerous cytokines and growth factors, and the enhancement of antigen presentation Once taken up inside the cell the a2M–protein complexes are rapidly degraded in the endolysosome system However, the fate of a2M in the
Fig 7 Gel filtration on Superose 6 of a2M treated with cathepsin E at various pH values Bovine a2M was incubated with cathepsin E at 37 C at the indicated pH values The reaction mixtures were adjusted to pH 6.0 and then run on a Superose 6 column equilibrated with 10 m M sodium phosphate buffer, pH 6.0, containing 150 m M NaCl Fractions were analyzed by the cathepsin E activity at pH 3.8 with the synthetic substrate MOCAc-Gly-Lys-Pro-Ile-Ile-Phe-Phe-Arg-Leu-Lys(Dnp)- D -Arg-NH 2 , SDS/PAGE and immunoblotting with antibodies to cathepsin E (a) The elution profiles of a2M and cathepsin E when each protein was run on the column independently The a2M was treated with cathepsin E at pH 4.5 for 1 h (b), at pH 5.5 for 1 h (c), at pH 3.8 for 45 s (d), at pH 3.8 for 2 min (e), and at pH 3.8 for 1 h (f) The solid and dotted lines illustrate the cathepsin E activity and the absorbance at 280 nm, respectively.
Trang 8complexes and the nature of its degradation are not clear.
The present findings on the biochemical nature of the
interaction of cathepsin E with a2M are unique and
unexpected This is the first report of the structural and
functional disruption of a2M integrity by cathepsin E It
is surprising that cathepsin E can be associated with a2M
at very low pH values and rapidly cleaves it at the specific
site distinct from the bait region Under mild acidic
conditions, a2M appears to interact with cathepsin E [35],
as well as other proteinases, to be cleaved a peptide bond
in the bait region and thereby undergoes a conformational
change similar to that occurring at neutral pH values [42],
where cathepsin E is essentially inactive [27,36] However,
at pH values below 5.0, a2M is unstable and is likely to
lose the proteinase-binding activity [43], where cathepsin E
is more active The endolysosomal compartment is the
major site of endogenous protein degradation
Degrada-tion of a2M, like other endocytosed proteins, is known to
occur rapidly inside the endolysosomes, where the pH is
maintained below 5.0 The intravacuolar pH is important
because most of the endolysosomal hydrolases including
cathepsins E and D require a pH of 3.5–5.0 for maximal
activity Similarly, microbicidal systems such as
peroxi-dase-hydrogen peroxide need such an acid pH for optimal
activity [44]
At pH values between 3.8 and 5.5, cathepsin E selectively
bound a2M and cleaved it at the Phe811-Leu812 bond at
a distance from the bait region Therefore, the cathepsin
E–a2M interaction below pH 5.0 appears to be unique and
is different from that occurring at mild acidic and neutral
pH values Namely, cathepsin E is efficiently associated with
a2M at pH values below 5.0 without loss of the proteolytic
activity and rapidly cleaves it at the Phe811-Leu812 bond
distinct from the bait region and then leaves the associated
site Considering that a very low pH treatment results in
dissociation of a2M into the dimers, which do not
reassociate normally but tend to aggregate [43], it is more
likely that cathepsin E interact with the distinct region from the bait region of the a2M Under these conditions, however, the analogous aspartic proteinase cathepsin D neither interacts with a2M nor cleaves it As previous work has demonstrated that the action of cathepsin D, as well as cathepsin E, toward protein substrates was blocked by a2M at pH 6.2 [34], a slight decrease in pH below 5.5 is very likely to cause an additional conformational change
of a2M and thereby abolish the ability of cathepsin D to bind a2M
This study also described for the first time the cloning and sequencing of partial cDNA for bovine a2M, as its primary structure is likely to provide significant informa-tion regarding the finding that bovine a2M is more sensitive to cathepsin E digestion than that from other species (data not shown) Analysis of the amino-acid sequence of bovine a2M deduced from isolated cDNA clones revealed that its gross structure was homologous to those of human, rat, and mouse a2M, although the bait region was dissimilar to that of either a2M from other species In particular, the sequence around the cathepsin E cleavage site (Phe811–Leu812 bond) was highly conserved
in all of the other species To further confirm the selective cleavage of the Phe811–Leu812 bond in a2M by cathepsin
E, we synthesized some peptides corresponding to part of the cleavage site and analyzed the susceptibility of these peptides to cleavage by cathepsin E The hexapeptide SPAFLA corresponding to part of the cathepsin E cleavage site (the Ser808–Ala813 sequence of human a2M) was selectively cleaved at the Phe-Leu bond by cathepsin E, but not cathepsin D The peptides SPAFL and SSAFL corresponding to the sequence Ser808–Leu812
of human and bovine a2M, respectively, were not cleaved
by either cathepsin E or cathepsin D In agreement with the results, the peptide SPAFLA significantly inhibited the degradation of a2M by cathepsin E These results indicate that the presence of Ala in the P¢2 is essential for selective cleavage the synthetic peptides by cathepsin E The presence of Pro in the P3 site in a2M, however, is unlikely
to be crucial for its selective cleavage, as the 809th residue
in bovine a2M is Ser in place of Pro found in other species
Acknowledgements
We thank Dr Haruki Uemura (Department of Protozoology, Institute of Tropical Medicine, Nagasaki University) for helpful discussion and technical advice and Drs Tsutomu Iwamoto (1st Department of Oral and Maxillofacial Surgery, Nagasaki University School of Dentistry), Masayo Okaji, Kazuhiro Kanaoka, Fumio Hashimoto and Yasuhiro Kobayashi (Department Orthodontics, Nagasaki University School of Dentistry) for fruitful discussions.
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Fig 8 Effect of cathepsin E-treated a2M on the antiproteolytic activity
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