1 Intramural Research Support Program SAIC, Laboratory of Experimental and Computational Biology, NCI-FCRDC, Frederick, MD 21702, USA and 3 Sackler Institute of Molecular Medicine, Department of Human Genetics and Molecular Medicine, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv 69978, Israel
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Keywords: amyloid/ß-hairpin/ß-turn/domain swapping/hinge bending/misfolding/polymerization/structural motif
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The tertiary structures of proteins undergoing such a polymerization differ from each other, yet their corresponding amyloid polymers are structurally of a similar type and have unique characteristics (Serpell et al., 1997; Sunde and Blake, 1998
). To understand the conformational changes between the native and the non-bonded polymer amyloid forms, we look for a structural motif which recurs in native proteins known to convert into non-bonded polymers. Such a motif, and the mechanism in which it plays a critical role, should be consistent with available experimental data and provide clues to how native structures undergo a conformational change to form amyloids and, in particular, to the stability and propagation of the amyloid polymer.
Different fibrils formed from different molecules are still composed of the same basic form and exhibit the same basic features (summarized in Serpell et al., 1997 and references therein): First, the fibrils are uniform, straight and unbranched; second, the fibrils yield a characteristic X-ray diffraction pattern, dominated by intense 4.7 Å meridional and weaker, ~10 Å equatorial reflections. This pattern is characteristic of a cross-ß structure. In such a structure, the ß-sheets are arranged in a way in which they run parallel to the axis of the fibril. On the other hand, the ß-strands forming the sheets are perpendicular to the axis of the fibril. Third, amyloid fibrils derived from different types of protein molecules are composed of different numbers and arrangements of protofilaments, with each protofilament consisting of such a cross-ß geometry. For example, the transthyretin fibril contains four protofilaments (Serpell et al., 1995) while the immunoglobulin light chain has five (Shirahama and Cohen, 1967
). Fourth, in vitro the amyloid fibrils are extremely stable. For example, for bovine spongiform encephalopathy it has been shown that a temperature of above 200°C is required for the destruction of the amyloid polymer (Serpell et al., 1997
). In the model for the non-bonded polymers, the ß-sheets are twisted, consistent with the larger stability of such a conformation as compared with flat sheets (Chothia, 1973
). Twisted ß-sheets form spontaneously from many oligopeptides (Chothia, 1973
; Serpell et al., 1997
). This allows a continuous pattern of ß-type hydrogen bonds along the filbril, over great lengths (Serpell et al., 1997
). These authors have further pointed out that it is possible that some amyloid polymers are composed of one pair of ß-sheets, whereas others are composed of two such pairs, as in the case of the transthyretin protofilament. Hence the conformation of the amyloid polymers is uniform and extremely stable. It is composed of one or more pairs of twisted ß-sheet structures. It provides for fast propagation and growth, once a seed amyloid is present. This suggests `sticky' ends in the protofibrils.
![]() |
The motif and amyloid polymerization |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Although to date only a relatively small number of proteins are known to form amyloids in vivo, under appropriate (high denaturant concentration or low pH) conditions, many (e.g. Guijarro et al., 1998; Litvinovich et al., 1998; Chiti et al., 1999), perhaps most proteins can convert into this stable form in vitro. This indicates that the presence of any given motif is not an absolute necessity for such a polymerization. Even in in vivo amyloidogenic proteins, the motif is not a prerequisite for amyloid formation. An unstable protein or region may partially unfold and assume such a ß-conformation. Since the bound form is remarkably stable, the equilibrium will shift in its direction, further driving the reaction.
The hypothetical model proposed here is for the elongation of the amyloid, rather than for its initiation. A schematic drawing illustrating one way in which such a mechanism may potentially be realized is given in Figure 1. We stress however, that the model is hypothetical, with currently no definitive experimental support for its existence.
|
![]() |
The domain swapping mechanism: similarities and differences |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The largest and most critical difference between domain swapping and non-bonded polymerization during amyloid formation is that whereas domain swapping may be reversible, the polymerization is an essentially irreversible process (Lomakin et al., 1996). The origin of the difference in stability may be in the conformations of the swapped parts: whereas in the `classical' domain swapping event the swapped domain is a helix, loop, a single ß-strand or an entire domain, in polymerization the swapped part is a ß-hairpin structure. In cases such as in the prion, the potential swapping domain may be a conformationally unstable structure, which may partially unfold and undergo a conversion to ß [summarized in Dobson and Karplus (1999) and references therein]. Litvinovitch et al. (1998) have recently shown how a hypothetical mechanism for fibronectin type III ß-sandwich can partially unfold and self-associate to form fibrils via a ß-strand swapping. However, even if a ß-structure exists, if it is unstable, as in the case of lysozyme, it may unfold (B.Ma and R.Nussinov, unpublished work), with subsequent formation of an altered ß-conformation. Such unfolding has recently been observed in molecular dynamic simulations.
![]() |
Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The coordinates of the structures were retrieved from the Protein Data Bank (PDB) (Bernstein et al., 1977). A building block is considered to be a contiguous fragment with substantial interactions between its residues. The building blocks were assigned as described in Tsai et al. (1999a). For every candidate fragment of the protein, the relative buried accessible surface area (ASA) was calculated. The fragment was considered as a building block when the obtained relative buried ASA value was larger than a threshold (of 0.135 Å2). The relative buried ASA is the ASA of the first half-fragment buried by the second half of the fragment plus the ASA of the second half of the fragment which is buried by the first half-fragment divided by the total ASA of the fragment. Details for the cutting procedure are given in Tsai et al. (2000).
Salt bridges and hydrogen bonds
The presence of salt bridges was inferred when Asp and Glu side-chain carbonyl oxygen atoms were found to be within a 4.0 Å distance from the nitrogen atoms of Arg, Lys or His side-chains. When for the same pair of residues there were more than one pair of nitrogenoxygen atoms present within 4.0 Å, the salt bridge was counted only once. The presence of a hydrogen bond was inferred when two non-hydrogen atoms with opposite partial charges were found to be within a distance of 3.5 Å. Details are described in Kumar and Nussinov (1999).
Non-polar buried surface area
The non-polar buried surface area was calculated as described by Tsai and Nussinov (1997a). The buried non-polar surface area was calculated as a fraction of the buried non-polar area out of the total non-polar area. To calculate the area buried within the motif and between the motif and the rest of the protein, the fragment comprising the motif was scrutinized for the proportion of its buried surface area, both by itself and by the rest of the protein. If a residue in the motif is buried to the same extent both by a residue within the motif and by a residue in the remainder of the protein, the calculated buried surface area for that motifresidue is added to both categories. That is, it is included in the area buried within the motif and in the area buried by the rest of the protein. Thus, in our results the total non-polar buried surface area of the motif is smaller than the sum of the area buried by the motif and the area buried by the rest of the protein.
Root mean square deviations (r.m.s.d.s)
The residue by residue deviations of corresponding C pairs were inspected by superimposing the mutants on their respective wild-types.
Identification of the motif
The amyloidogenic proteins were first inspected for the presence of a ß-hairpin, which is connected to the remainder of the protein via a coil. The potential motifs were then characterized based upon the building block assignments, buried non-polar surface area and the number of hydrogen bonds and salt bridges present within the motif and between the motif and the rest of the protein.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Table I enumerates the cases and lists the sequences and the positions of the motifs in each of the cases (Table II
enumerates domain-swapped cases). Table I
gives the secondary structure assignments and the lengths of the structural elements at these sites. Figure 2
(a)(i) provide illustrations for all the cases. The figures show the assignment of the `building blocks' of the structure. Each building block is depicted in a different color. The most likely motif is in red. The regions which have been left `unassigned' into a specific building block, are in white. A fragment is left `unassigned' if by itself it is an extremely unstable building block and its score does not pass the threshold, qualifying it as a building block. Its addition to a sequentially connected building block does not increase the stability of the already identified building block (Tsai et al., 1999a
).
|
|
|
|
Naturally occuring variants of cystatin C, gelsolin, immunoglobulin, transthyretin, lysozyme, prion and 1-antitrypsin are known. In most of the cases the wild-type itself may form polymeric amyloids, with the variants being more prone to undergo such a conformational conversion than their wild-type counterparts, as in transthyretin. On the other hand, for some of these proteins only the variants, most of the time with single mutations, polymerize and the wild-type does not, as in the cases of lysozyme and gelsolin. In
1-antitrypsin both the wild-type and the mutant form fibrils with similar efficiencies. We have inspected the residue by residue r.m.s.d.s of corresponding C
pairs of the mutational variants versus their respective native wild-type folds, where crystal structures are available. Although the results of structural superpositioning of the wild-type and the variant proteins show that overall the structures are similar, there are remarkable observations which are consistent with our model and the candidate motif swapping. The results are shown in Table IV
. Superimposing the native (wild) type and the amyloidogenic variant of lysozyme [Figure 3
(a)] shows that the red motif which is present in the wild-type swings away from the rest of the protein in the amyloidogenic variant, consistent with the potential of the red building block to flip. The r.m.s.d. measurements reflect the visual observation [Figure 3
(b)]. In the case of transthyretin we have inspected nine variants whose crystal structures are available (Table IV
). Consistently, in eight of these, the first or the second largest C
pair deviations are in the coil which connects the red building block motif to the rest of the protein [Figure 3
(c)]. In the remaining mutant the largest deviation is in the coil which connects the yellow building block to the rest of the structure. The yellow building block in transthyretin ranks second as a potential candidate motif according to our criteria. In variants of
1-antitrypsin, the largest deviations are in the long coil region which connects the red C-terminus ß-hairpin building block, the potential motif to flip by our criteria, to the rest of the protein via a small ß-strand [Figure 3
(d)]. These results clearly show that the higher deviations between the variants and the native structures are in the coil which connects the motif to the rest of the protein.
|
|
Comparisons with domain-swapped cases
We have inspected the monomers and the dimers of six domain swapped cases. The cases are enumerated in Table II. The sizes of the swapped domains, with the exception of diphtheria toxin (158 residues), are similar to the motifs in Table I
, ranging between 13 and 31 residues. The corresponding results of the calculations of the salt bridges, hydrogen bonds and buried non-polar surface areas within the swapped parts and between the swapping segments and the remainder of the structures are given below. The calculations are carried out for the native, unswapped monomers, so that a comparison can be made between the swapping domains in domain swapped cases and the motifs in our set of proteins undergoing polymerization. Three cases, interleukin-10, odorant binding protein and the Eps-SH3 dimer, are `quasi-domain swapped' cases (Schlunegger et al., 1997
). These do not exist as monomers. In our definition, we would consider them two-state folding/binding cases (Tsai et al., 1997b, 1998
). The crystal structures of the monomers of BS-RNase and
-spectrin are not available. Hence, we have included these and the `quasi-domain swapped' cases in the listing in Table II
. However, we were unable to calculate the respective monomer values of the salt bridges, hydrogen bonds and non-polar buried surface areas.
Several points should be highlighted here. First, the swapped region is consistently at the N-terminus or at the C-terminus of the protein. Second, there is a relatively long, flexible coil connecting the motif to the remainder of the structure, allowing it to flip. Third, in none of these cases is the swapped part a ß-hairpin motif. Instead, it is an -helix, a strand or an entire domain. In this regard, cystatin C is of particular interest. There is experimental evidence that cystatin C forms an SDS-resistant dimer (Wei et al., 1998
). This may suggest a domain-swapped dimer. In the dimer, the interdigitation due to the swapping may render the dimer SDS resistant. Fourth, the number of salt bridges and of hydrogen bonds connecting the swapped domain to the remainder of the structure is small. A larger number is frequently observed within the swapped domain (Table III
). Fifth, the buried non-polar surface area within the swapped domain is usually (with the exception of CksHs and interleukin-5) larger than that observed between the motif and the remainder of the structure. In particular, sixth, the absolute value of the buried non-polar surface area between the motif and the remainder of the structure is variable and can be large (Table III
). This, however, does not prevent the domain-swapping event from taking place.
We have also inspected the residue by residue r.m.s.d.s of corresponding C pairs from the native and respective mutants where structures are available. The results are listed in Table V
and depicted in Figure 4
. In the barnase mutants the first or second largest deviations are in the coil which connects the swapping domain to the rest of the protein [Figure 4
(a)]. Similarly, in staphylococcal nuclease the second largest deviations are found in the coil which connects the swapping domain to the rest of the protein [Figure 4
(b)]. As shown in Table V
, higher deviations are also found in a long coil connecting the
-helix to the ß-strand. Long flexible coils usually show larger deviations. Still, this latter coil is in the middle of the structure. Thus, a flipping event of the region connected to this long coil is considerably more difficult. In the BP-RNase A mutants, the first or second largest deviations again lie in the coil which connects the swapping domain to the rest of the protein [Figure 4
(c)]. These results are consistent with those of the candidate motifs in proteins known to undergo non-bonded polymerization (Table IV
). There, too, the deviations of the mutational variants are higher in the coils connecting the motifs to the remainder of the structure.
|
|
![]() |
Discussion and conclusions |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Conformational low-energy transitions involving hinge-based motions are frequently observed in proteins. In native proteins they are critically important for binding, catalysis and motility. Hinge-bending motions can involve movements of fragments or of larger domains. In general, the binding interface between these and the rest of the protein does not involve tight packing (Gerstein et al., 1994). A more extreme case of hinge-bending motions is illustrated in domain swapping. A revealing example is bovine seminal ribonuclease, where domain swapping may take place after hours or days (Piccoli et al., 1992
; D'Alessio, 1995
). The rate of the swapping event reflects the population time of the conformation of the monomer in the swapped form.
Inspection of the non-polar buried surface area both in amyloidogenic proteins and in documented domain-swapping cases, shows that it is variable and can be fairly large. Moreover, the analysis presented here illustrates that the number of salt bridges and hydrogen bonds in both amyloidogenic and domain-swapping cases is limited. Consistent with the results obtained here, in an extensive analysis of hinge-bending transitions of known cases, we have observed that while the non-polar buried surface area between the hinging domains can be large, the number of salt bridges and hydrogen bonds is small (N.Sinha, S.Kumar and R.Nussinov, unpublished work).
The X-ray fiber diffraction pattern of the filaments shows that they consist of continuous, twisted ß-sheets, arranged along the protofibril axis. While it is well known that ß-structures are particularly stable, a continuation of ß-sheets across the intermolecular interface is unlikely to produce a filament with such an exceptionally high stability. Inspection of protein crystal or NMR structures reveals that twisted ß-sheets are frequently continued between independently folding hydrophobic units, across their interface. Such a continuation is observed between domains and between subunits of an oligomer. Figure 5 illustrates two examples, depicting such a twisted ß-sheet propagation, between hydrophobic folding units. Yet, despite the extended propagating ß-sheets, in all of these cases, motions are observed between these structural components, with melting temperatures within the generally observed ranges, i.e. well below 100°C. On the other hand, an interdigitation of ß-hairpins between domains or subunits, locking them together, has not been observed to date in globular, functional, native proteins. Proteins are well known to be only marginally stable. This is essential for their function. Interdigitated, interlocked ß-sheets, across domains or subunits, are likely to be conformationally too stable, hindering the motion which is necessary for activity. Hence, there is a selection in nature against such conformations.
|
Two steps are involved in the polymerization. The first is the conformational change of the native monomer; the second is the binding of the `open', flipped, monomer to the growing interdigitating polymer. If the barriers for the interconversion step are low, the rate-limiting step might be expected to be the binding, via a diffusioncollision process (Karplus and Weaver, 1976). If the barriers are high, the conformational interconversion may be the rate-limiting step. In the case of the polymerization, the rate of the reaction is very slow. This again is reminiscent of domain swapping (e.g. bovine seminal ribonuclease; Piccoli et al., 1992; D'Alessio, 1995). However, once a seed is introduced, if the concentration is high, the reaction proceeds at a much faster rate (Lomakin et al., 1996
; Dobson and Karplus, 1999
). Hence here the rate-limiting step involves the formation of a seed polymer. This is similar to the case of supercool water. The water would stay in the liquid state until an ice seed is introduced into it (Tsai et al., 1999b
).
Although here we have focused largely on cases in which the ß-hairpins are already present in the native forms, this condition does not always hold. A potential example is the case of the prion. Furthermore, it has been shown that depending on the conditions, most proteins can form amyloids, illustrating dynamic landscapes. Even if the ß-structure exists, if it is unstable, it may unfold and reform in an alternative way. Additionally, even if a stable ß-structure exists it is not necessarily the case that it swaps to form amyloids. Alternative, unstable building blocks may partially unfold, with subsequent participation in swapping. In particular, it is important to note that while here we have largely kept the monomeric structure intact, this is unlikely to hold universally. There is substantial evidence that partially folded conformations are critical intermediates on the pathway to fibril formation.
The validity of our model is further supported by analysis of mutations. The r.m.s.d.s between the native and the mutants are always small (Tables IV and V), as may be expected between such close mutational variants. Nevertheless, when analyzed with respect to specific positions, namely, with respect to matching C
pairs between the two (native and mutant) proteins, the positions showing the largest deviations (ranking between first and third) are consistently in the coil regions connecting the motifs to the rest of the structures [Table IV
, Figure 3
(a)(d)]. Consistently, the deviations obtained for domain swapping cases are also the largest in the coils connecting the swapping domain to the remainder of the protein [Table V
, Figure 4
(a)(c)]. This is despite the fact that the actual location of the mutations varies, suggesting that the regions which are most prone to respond to the sequence alterations are at the proposed hinge region. This may also suggest that these regions are inherently susceptible to the changes in physiological conditions, such as pH changes, which lead to amyloid fiber formation in some wild-type proteins, as in transthyretin. This is also consistent with a large-scale mutational analysis (Sinha and Nussinov, 2001
).
Polymerization takes place since the free energy of the bound, polymerized form is lower than that of the unbound, native form of the protein. Here we address the problem of how the barrier from the unbound to the polymerized form is lowered. If, however, polymerized protofilaments are not formed, despite partially denaturing conditions, as in the case of the WW domain (Koepf et al., 1999), we may infer that the free energy of the bound, amyloid form is higher than that of the native form. Koepf et al. (1999) suggest that the reason may reside in its strong hydrophobic core. Alternatively, it may suggest that in the polymerized form there are unfavorable interactions, such as exposure of the large aromatic residues to water or steric hindrance either in the formation of the protofilaments or in their assembly to the fibrillar structure.
What are the conditions for a ß-hairpin to be able to swap? First, the ß-hairpin motif should preferably be at the edge of the structure, rather than buried within it. Second, it should not have salt bridges or too many hydrogen bonds, connecting it to the remainder of the structure. While a smaller extent of buried surface area would lower the barrier for the swapping, swapping would eventually take place with a more extensive buried surface area, as long as the polymerized form is more stable than the native conformation. This is evident from domain swapped cases. Third, it should preferably be at the amino or carboxy termini, resulting in a single hinge. In this regard, it is revealing to inspect concanavalin A (PDB: 1jbc). Although the structure of concanavalin A is very similar to that of serum amyloid P component (SAP), it does not bind to non-bonded polymeric amyloid fibrils, like SAP. Assigning concanavalin A into its constituent building blocks and comparing it to serum amyloid P component (Figure 2) we notice two interesting points. First, the NH2- and COOH-termini of concanavalin A are in the middle of a ß-sheet. However, in contrast, in the case of serum amyloid P component, they are at the edge of sheet. Second, the building block assignment results suggest that flipping a ß-hairpin at the other end of the ß-sheet appears to be easier for SAP than for concanavalin A. That is, concanavalin A is more stable than serum amyloid P component at the edge of their two seven-stranded ß-sandwich.
![]() |
Notes |
---|
4 To whom correspondence should be addressed, at the USA address. E-mail: ruthn{at}ncifcrf.gov
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bennett,M.J., Choe,S. and Eisenberg,D. (1994) Proc. Natl Acad. Sci. USA, 91, 31273131.[Abstract]
Bennett,M.J., Schlunegger,M.P. and Eisenberg, D. (1995) Protein Sci., 4, 24552468.
Bernstein,F., Koetzle,T., Williams,G., Meyer,E.J., Brice,M., Rodgers,J., Kennard,O., Shimanuchi,T. and Tasumi,M. (1977) J. Mol. Biol., 112, 535542.[ISI][Medline]
Chiti,F., Webster,P., Taddei,N., Clark,A., Stefani,M., Ramponi,G. and Dobson,C.M. (1999) Proc. Natl Acad. Sci. USA, 96, 35903594.
Chothia,C. (1973) J. Mol. Biol., 75, 295302.[ISI][Medline]
D'Alessio,G. (1995) Nature Struct. Biol., 2, 1113.[ISI][Medline]
Dobson,C.M. and Karplus,M. (1999) Curr. Opin. Struct. Biol., 9, 92101.[Medline]
Emsley,J. et al. (1994) Nature, 367, 338345.[ISI][Medline]
Fink,A.L. (1998) Fold. Des., 3, R9R23.[ISI][Medline]
Gerstein,M., Lesk,A. and Chothia,C. (1994) Biochemistry, 33, 67396749.[ISI][Medline]
Gewurz,H., Zhang,X.H. and Lint,T.F. (1995) Curr. Opin. Immunol., 7, 5464.[ISI][Medline]
Guijarro,J.I., Sunde,M., Jones,J.A., Campbell,I.D. and Dobson,C.M. (1998) Proc. Natl Acad. Sci. USA, 95, 42244228.
Inouye,H., Domingues,F.S., Damas,A.M., Saraiva,M.J., Lundgren,E., Sandgren,O. and Kirschner,D.A. (1998) Amyloid: Int J. Exp. Clin. Invest., 5, 163174.[ISI]
Karplus,M. and Weaver,D.L. (1976) Nature, 260, 404406.[ISI][Medline]
Kelly,J.W. (1996) Curr. Opin. Struct. Biol., 6, 1117.[ISI][Medline]
Kelly,J.W. and Lansbury,P.T. (1994) Amyloid: Int. J. Exp. Clin. Invest., 1, 186205.[ISI]
Kirschner,D.A., Elliott-Bryant,R., Szumowski,K.E., Gonnerman,W.A., Kindy,M.S., Sipe,J.D. and Cathcart,E.S. (1998) J. Struct. Biol., 124, 8898.[ISI][Medline]
Koepf,E.K., Petrassi,H.M., Sudol,M. and Kelly,J.W. (1999) Protein Sci., 8, 841853.[Abstract]
Kumar,S. and Nussinov,R. (1999) J. Mol. Biol., 293, 12411255.[ISI][Medline]
Lansbury,P.T. et al. (1995) Nature Struct. Biol., 2, 990998.[ISI][Medline]
Litvinovich,S.V., Brew,S.A., Aota,S., Akiyama,S.K., Haudenschild,C. and Ingham,K.C. (1998) J. Mol. Biol., 280, 245258.[ISI][Medline]
Lomakin,A., Chung,D.S., Benedek,G.B., Kirschner,D.A. and Teplow,D.B. (1996) Proc. Natl Acad. Sci. USA, 93, 11251129.
Nussinov, R. and Wolfson, H.J. (1991) Proc. Natl Acad. Sci. USA, 88, 1049510499.[Abstract]
Peterson,S.A., Klabunde,T., Lashuel,H.A., Purkey,H., Sacchettini,J.C. and Kelly,J.W. (1998) Proc. Natl Acad. Sci. USA, 95, 1295612960.
Piccoli,R., Tamburrini,M., Piccialli,G., Di Donato,A., Parente,A. and D'Alessio,G. (1992) Proc. Natl Acad. Sci. USA, 89, 18701874.[Abstract]
Ratnaswamy,G., Koepf,E., Bekele,H., Yin,H. and Kelly,J.W. (1999) Chem. Biol., 6, 293304.[ISI][Medline]
Saraiva,M.J.M. (1995) Hum. Mutat., 5, 191196.[ISI][Medline]
Schlunegger,M.P., Bennett,M.J. and Eisenberg,D. (1997) Adv. Protein Chem., 50, 61122.[ISI][Medline]
Serpell,L.C., Sunde,M., Fraser,P.E., Luther,P.K., Morris,E. and Sandgren,O. (1995) J. Mol. Biol., 254, 113118.[ISI][Medline]
Serpell,L.C., Sunde,M. and Blake,C.C.F. (1997) Cell Mol. Life Sci., 53, 871887.[ISI][Medline]
Sinha,N. and Nussinov,R. (2001) Proc. Natl Acad. Sci. USA, 98, 31393144.
Sipe,J.D. (1992) Annu. Rev. Biochem., 61, 947975.[ISI][Medline]
Shirahama,T. and Cohen,A.S. (1967) J. Cell Biol., 33, 679706.
Sunde,M. and Blake,C.C.F. (1998) Rev. Biophys., 31, 139.
Tsai,C.J. and Nussinov,R. (1997a) Protein Sci., 6, 2442.
Tsai,C.J. and Nussinov,R. (1997b) Protein Sci., 6, 14261437.
Tsai,C.J., Xu,D. and Nussinov,R. (1998) Fold. Des., 3, R71R80.[ISI][Medline]
Tsai,C.J., Maizel,J.V. and Nussinov,R. (1999a) Protein Sci., 8, 15911604.[Abstract]
Tsai,C.J., Kumar,S., Ma,B. and Nussinov,R. (1999b) Protein Sci., 8, 11901190.
Tsai,C.J., Maizel,J.V. and Nussinov,R. (2000). Proc. Natl Acad. Sci. USA, in press.
Xu,D., Tsai,C.J. and Nussinov,R. (1998) Protein Sci., 7, 533544.
Wei,L., Berman,Y., Castano,E.M., Cadene,M., Beavis,R.C., Devi,L. and Levy,E. (1998) J. Biol. Chem., 273, 1180611814.
Received July 15, 2000; revised September 20, 2000; accepted November 8, 2000.