Interactions and chaperone function of alphaA-crystallin with T5P gammaC-crystallin mutant

T5P gammaC-crystallin mutation is associated with Coppock-like cataract, one of the autosomal dominant congenital cataracts. It is not known why the abundant alpha-crystallin cannot prevent the mutation-related aggregation. Our previous studies indicate that the mutation changes conformation and reduces solubility and stability, but it is not known whether it is these events or the loss of interaction with other crystallins that causes the cataract. It is also not known whether the alpha-crystallin can protect T5P mutant as effectively from heat-induced aggregation as the wild-type (WT) gammaC-crystallin. To investigate the mechanism of interactions and chaperone function between alphaA- and gammaC-crystallin, human alphaA-crystallin and W9F mutant as well as WT gammaC-crystallin and T5P mutant were cloned. Interactions between alphaA- and gammaC-crystallin were studied with fluorescence resonance energy transfer (FRET), and chaperone activity was assessed by the suppression of heat-induced aggregation of substrate proteins. Conformational changes of substrate proteins were studied by spectroscopic measurements. The results indicate that the T5P mutant showed a slightly greater FRET than WT gammaC-crystallin with alphaA-crystallin, and alphaA-crystallin could effectively prevent both WT and T5P gammaC-crystallin from heat-induced aggregation. Spectroscopic measurements show that both alphaA-crystallin and gammaC-crystallin underwent only slight conformational change after chaperone binding. Together with previous results obtained with a two-hybrid system assay of interactions between alphaA- and gammaC-crystallin, the present FRET and chaperone results indicate that loss of interactions of T5P mutant with other crystallins may play a larger role than the protection afforded by chaperone-like activity in Coppock-like cataract.     

分子伴侣的功能和应用

本文综述了分子伴侣的分类、功能、作用机理、研究现状及应用前景。分子伴侣是在生物大分子的折叠、组装、转运及降解等过程中起协助作用,参与协助抗原的呈递和遗传物质的复制、转录及构象的确立,但自身并不发生任何变化的一大类广泛存在于生物体内的蛋白质分子。随着对分子伴侣的进一步研究和相关知识的不断深入,分子伴侣在生物产品开发、物种改良、抗衰老,疾病预防、诊断和治疗以及环境监测方面具有广阔的前景。     

Insights into the domains required for dimerization and assembly of human alphaB crystallin

Protein pin array technology was used to identify subunit-subunit interaction sites in the small heat shock protein (sHSP) alphaB crystallin. Subunit-subunit interaction sites were defined as consensus sequences that interacted with both human alphaA crystallin and alphaB crystallin. The human alphaB crystallin protein pin array consisted of contiguous and overlapping peptides, eight amino acids in length, immobilized on pins that were in a 96-well ELISA plate format. The interaction of alphaB crystallin peptides with physiological partner proteins, alphaA crystallin and alphaB crystallin, was detected using antibodies and recorded using spectrophotometric absorbance. Five peptide sequences including 37LFPTSTSLSPFYLRPPSF54 in the N terminus, 75FSVNLDVK82)(beta3), 131LTITSSLS138 (beta8) and 141GVLTVNGP148 (beta9) that form beta strands in the conserved alpha crystallin core domain, and 155PERTIPITREEK166 in the C-terminal extension were identified as subunit-subunit interaction sites in human alphaB crystallin using the novel protein pin array assay. The subunit-subunit interaction sites were mapped to a three-dimensional (3D) homology model of wild-type human alphaB crystallin that was based on the crystal structure of wheat sHSP16.9 and Methanococcus jannaschi sHSP16.5 (Mj sHSP16.5). The subunit-subunit interaction sites identified and mapped onto the homology model were solvent-exposed and had variable secondary structures ranging from beta strands to random coils and short alpha helices. The subunit-subunit interaction sites formed a pattern of hydrophobic patches on the 3D surface of human alphaB crystallin.     

Analysis of the role of Hsp25 phosphorylation reveals the importance of the oligomerization state of this small heat shock protein in its protective function against TNFα- and hydrogen peroxide-induced cell death

The role of murine Hsp25 phosphorylation in the protection mediated by this protein against TNFα- or H₂O₂-mediated cytotoxicity was investigated in L929 cell lines expressing wild type (wt-) or nonphosphorylatable (mt-) Hsp25. We show that mt-Hsp25, in which the phosphorylation sites, serines 15 and 86, were replaced by alanines, is still efficient in decreasing intracellular reactive oxygen species levels and in raising glutathione cellular content, leading the protective activity of mt-Hsp25 against oxidative stress to be identical to that of wt-Hsp25. To independently investigate the role of Hsp25 phosphorylation, we blocked TNFα-induced phosphorylation of wt-Hsp25 using SB203580, a specific inhibitor of the P38 MAP kinase. This treatment did not abolish the protective activity of Hsp25 against TNFα. The pattern of Hsp25 oligomerization was also analyzed, showing mt-Hsp25 to constitutively display large native sizes, as does wt-Hsp25 after TNFα treatment in the presence of SB203580. Our results, therefore, are consistent with the possibility that the hyperaggregated form of Hsp25 is responsible for the protective activity against oxidative stress and that the phosphorylation of serines 15 and/or 86 by interfering with this structural reorganization, may lead to the inactivation of Hsp25 protective activity. J. Cell. Biochem. 69:436–452, 1998. © 1998 Wiley-Liss, Inc.     

Heat shock proteins of adult and embryonic human ocular lenses.

We investigated the presence and distribution of heat shock proteins, HSP-70 [Horwitz, J. 1992. Proc Natl Acad Sci 89:10449-10453], HSP-40, HSc-70, HSP-27, and alphabeta-crystallin in different regions of adult and fetal human lenses and in aging human lens epithelial cells. This study was undertaken because heat shock proteins may play an important role in the maintenance of the supramolecular organization of the lens proteins. Human adult and fetal lenses were dissected to separate the epithelium, superficial cortex, intermediate cortex, and nucleus. The water soluble and insoluble protein fractions were separated by SDS-PAGE, and transferred to nitrocellulose paper. Specific antibodies were used to identify the presence of heat shock proteins in distinct regions of the lens. HSP-70 [Horwitz, 1992], HSP-40, and HSc-70 immunoreactivity was mainly detected in the epithelium and superficial cortical fiber cells of the adult human lens. The small heat shock proteins, HSP-27 and alphabeta-crystallin were found in all regions of the lens. Fetal human lenses showed immunoreactivity to all heat shock proteins. An aging study revealed a decrease in heat shock protein levels, except for HSP-27. The presence of HSP-70 [Horwitz, 1992], HSP-40, and HSc-70 in the epithelium and superficial cortical fiber cells imply a regional cell specific function, whereas the decrease of heat shock protein with age could be responsible for the loss of optimal protein organization, and the eventual appearance of age-related cataract.     

Role of the N-terminal region of the crenarchaeal sHsp, StHsp14.0, in thermal-induced disassembly of the complex and molecular chaperone activity

Small heat shock protein is a ubiquitous molecular chaperone, which consists of a non-conserved N-terminal region followed by a conserved alpha-crystallin domain. To understand the role of the N-terminal region, we constructed N-terminal truncation mutants of StHsp14.0, the sHsp from Sulfolobus tokodaii strain 7. All the mutants formed a stable oligomeric complex similar to that of the wild type. Electron microscopy and size exclusion chromatography-multiangle light scattering showed that the N-terminal region should locate in the center of the oligomeric particle. The mutants exhibited reduced chaperone activity for the protection of 3-isopropylmalate dehydrogenase from thermal aggregation. This reduction correlates with lowered subunit exchange efficiency. The oligomeric structure was retained even after incubation at 90 degrees C. These results suggest that the N-terminal region of StHsp14.0 functions in the thermally induced disassembly of the complex.     

Inhibition of α-synuclein aggregation by small heat shock proteins

The fibrillization of α-synuclein (α-syn) is a key event in the pathogenesis of α-synucleinopathies. Mutant α-syn (A53T, A30P, or E46K), each linked to familial Parkinson's disease, has altered aggregation properties, fibril morphologies, and fibrillization kinetics. Besides α-syn, Lewy bodies also contain several associated proteins including small heat shock proteins (sHsps). Since α-syn accumulates intracellularly, molecular chaperones like sHsps may regulate α-syn folding and aggregation. Therefore, we investigated if the sHsps αB-crystallin, Hsp27, Hsp20, HspB8, and HspB2B3 bind to α-syn and affect α-syn aggregation. We demonstrate that all sHsps bind to the various α-syns, although the binding kinetics suggests a weak and transient interaction only. Despite this transient interaction, the various sHsps inhibited mature α-syn fibril formation as shown by a Thioflavin T assay and atomic force microscopy. Interestingly, HspB8 was the most potent sHsp in inhibiting mature fibril formation of both wild-type and mutant α-syn. In conclusion, sHsps may regulate α-syn aggregation and, therefore, optimization of the interaction between sHsps and α-syn may be an interesting target for therapeutic intervention in the pathogenesis of α-synucleinopathies.     

Evolution of crystallins for a role in the vertebrate eye lens

The camera eye lens of vertebrates is a classic example of the re‐engineering of existing protein components to fashion a new device. The bulk of the lens is formed from proteins belonging to two superfamilies, the α ‐crystallins and the β γ ‐crystallins. Tracing their ancestry may throw light on the origin of the optics of the lens. The α ‐crystallins belong to the ubiquitous small heat shock proteins family that plays a protective role in cellular homeostasis. They form enormous polydisperse oligomers that challenge modern biophysical methods to uncover the molecular basis of their assembly structure and chaperone‐like protein binding function. It is argued that a molecular phenotype of a dynamic assembly suits a chaperone function as well as a structural role in the eye lens where the constraint of preventing protein condensation is paramount. The main cellular partners of α ‐crystallins, the β ‐ and γ ‐crystallins, have largely been lost from the animal kingdom but the superfamily is hugely expanded in the vertebrate eye lens. Their structures show how a simple Greek key motif can evolve rapidly to form a complex array of monomers and oligomers. Apart from remaining transparent, a major role of the partnership of α ‐crystallins with β ‐ and γ ‐crystallins in the lens is to form a refractive index gradient. Here, we show some of the structural and genetic features of these two protein superfamilies that enable the rapid creation of different assembly states, to match the rapidly changing optical needs among the various vertebrates.     

Chaperone-like activity of the N-terminal region of a human small heat shock protein and chaperone-functionalized nanoparticles

Small heat shock proteins (sHsps) are molecular chaperones employed to interact with a diverse range of substrates as the first line of defense against cellular protein aggregation. The N-terminal region (NTR) is implicated in defining features of sHsps; notably in their ability to form dynamic and polydisperse oligomers, and chaperone activity. The physiological relevance of oligomerization and chemical-scale mode(s) of chaperone function remain undefined. We present novel chemical tools to investigate chaperone activity and substrate specificity of human HspB1 (B1NTR), through isolation of B1NTR and development of peptide-conjugated gold nanoparticles (AuNPs). We demonstrate that B1NTR exhibits chaperone capacity for some substrates, determined by anti-aggregation assays and size-exclusion chromatography. The importance of protein dynamics and multivalency on chaperone capacity was investigated using B1NTR-conjugated AuNPs, which exhibit concentration-dependent chaperone activity for some substrates. Our results implicate sHsp NTRs in chaperone activity, and demonstrate the therapeutic potential of sHsp-AuNPs in rescuing aberrant protein aggregation.     

Evidence for specific subunit distribution and interactions in the quaternary structure of α‐crystallin

The quaternary structure of α‐crystallin is dynamic, a property which has thwarted crystallographic efforts towards structural characterization. In this study, we have used collision‐induced dissociation mass spectrometry to examine the architecture of the polydisperse assemblies of α‐crystallin. For total α‐crystallin isolated directly from fetal calf lens using size‐based chromatography, the αB‐crystallin subunit was found to be preferentially dissociated from the oligomers, despite being significantly less abundant overall than the αA‐crystallin subunits. Furthermore, upon mixing molar equivalents of purified αA‐ and αB‐crystallin, the levels of their dissociation were found to decrease and increase, respectively, with time. Interestingly though, dissociation of subunits from the αA‐ and αB‐crystallin homo‐oligomers was comparable, indicating that strength of the αA:αA, and αB:αB subunit interactions are similar. Taken together, these data suggest that the differences in the number of subunit contacts in the mixed assemblies give rise to the disproportionate dissociation of αB‐crystallin subunits. Limited proteolysis mass spectrometry was also used to examine changes in protease accessibility during subunit exchange. The C‐terminus of αA‐crystallin was more susceptible to proteolytic attack in homo‐oligomers than that of αB‐crystallin. As subunit exchange proceeded, proteolysis of the αA‐crystallin C‐terminus increased, indicating that in the hetero‐oligomeric form this tertiary motif is more exposed to solvent. These data were used to propose a refined arrangement for the interactions of the α‐crystallin domains and C‐terminal extensions of subunits within the α‐crystallin assembly. In particular, we propose that the palindromic IPI motif of αB‐crystallin gives rise to two orientations of the C‐terminus. Proteins 2010. © 2010 Wiley‐Liss, Inc.     

The folding landscape of an alpha-lytic protease variant reveals the role of a conserved beta-hairpin in the development of kinetic stability

Most secreted bacterial proteases, including alpha-lytic protease (alphaLP), are synthesized with covalently attached pro regions necessary for their folding. The alphaLP folding landscape revealed that its pro region, a potent folding catalyst, is required to circumvent an extremely large folding free energy of activation that appears to be a consequence of its unique unfolding transition. Remarkably, the alphaLP native state is thermodynamically unstable; a large unfolding free energy barrier is solely responsible for the persistence of its native state. Although alphaLP folding is well characterized, the structural origins of its remarkable folding mechanism remain unclear. A conserved beta-hairpin in the C-terminal domain was identified as a structural element whose formation and positioning may contribute to the large folding free energy barrier. In this article, we characterize the folding of an alphaLP variant with a more favorable beta-hairpin turn conformation (alphaLP(beta-turn)). Indeed, alphaLP(beta-turn) pro region-catalyzed folding is faster than that for alphaLP. However, instead of accelerating spontaneous folding, alphaLP(beta-turn) actually unfolds more slowly than alphaLP. Our data support a model where the beta-hairpin is formed early, but its packing with a loop in the N-terminal domain happens late in the folding reaction. This tight packing at the domain interface enhances the kinetic stability of alphaLP(beta-turn), to nearly the same degree as the change between alphaLP and a faster folding homolog. However, alphaLP(beta-turn) has impaired proteolytic activity that negates the beneficial folding properties of this variant. This study demonstrates the evolutionary limitations imposed by the simultaneous optimization of folding and functional properties.     

Structure and hydration of the amylopectin trisaccharide building blocks--Synthesis, NMR, and molecular dynamics

To gain insight into the molecular details and hydration of amylopectin, the five constituting trisaccharides have been chemically synthesized as their methyl alpha-glycosides. All five trisaccharides were subjected to 950 MHz NMR spectroscopy for complete assignment and nanosecond molecular dynamics trajectories were calculated to study the structure and dynamics of the trisaccharides in aqueous solution. Systematic analysis of the simulation data revealed several examples of bridging water molecules playing an important role in the stabilization of specific amylopectin conformations, which was also supported by the experimental NMR data such as interresidue NOE's and heteronuclear scalar couplings between nuclei from neighboring residues. Although alpha-maltotriose, alpha-iso-maltotriose, alpha-panose and alpha-isopanose are relatively well characterized structures, the study also includes one less characterized trisaccharide with the structure alphaGlcp(1-->4)alphaGlcp(1-->6)alphaGlcp. This trisaccharide, tentatively labelled alpha-forkose, is located at the branch point of amylopectin, forking the amylopectin into two strands that align into double-helical segments. The results show that the conformation of alpha-forkose takes a natural bend form which fits well into the structure of the double-helical segment of amylopectin. As the only trisaccharide in this study the structure of alpha-forkose is not significantly influenced by the hydration. In contrast, alpha-isopanose takes a restricted, but rather extended form due to an exceptionally strong localized water density. The two homo-linkage oligomers, alpha-maltotriose and alpha-iso-maltotriose, showed to be the most extended and the most flexible trimers, respectively, providing regular structure for crystalline domains and maximum linker flexibility for amorphous domains.     

Binding determinants of the small heat shock protein, αB‐crystallin: recognition of the ‘IxI’ motif

Small heat shock proteins (sHSPs) play a central role in protein homeostasis under conditions of stress by binding partly unfolded, aggregate‐prone proteins and keeping them soluble. Like many sHSPs, the widely expressed human sHSP, αB‐crystallin (‘αB’), forms large polydisperse multimeric assemblies. Molecular interactions involved in both sHSP function and oligomer formation remain to be delineated. A growing database of structural information reveals that a central conserved α‐crystallin domain (ACD) forms dimeric building blocks, while flanking N‐ and C‐termini direct the formation of larger sHSP oligomers. The most commonly observed inter‐subunit interaction involves a highly conserved C‐terminal ‘IxI/V’ motif and a groove in the ACD that is also implicated in client binding. To investigate the inherent properties of this interaction, peptides mimicking the IxI/V motif of αB and other human sHSPs were tested for binding to dimeric αB‐ACD. IxI‐mimicking peptides bind the isolated ACD at 22°C in a manner similar to interactions observed in the oligomer at low temperature, confirming these interactions are likely to exist in functional αB oligomers.     

Structural determinants of multimerization and dissociation in 2-Cys peroxiredoxin chaperone function

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Influence of hPin1 WW N-terminal domain boundaries on function, protein stability, and folding

An N-terminally truncated and cooperatively folded version (residues 6-39) of the human Pin1 WW domain (hPin1 WW hereafter) has served as an excellent model system for understanding triple-stranded beta-sheet folding energetics. Here we report that the negatively charged N-terminal sequence (Met1-Ala-Asp-Glu-Glu5) previously deleted, and which is not conserved in highly homologous WW domain family members from yeast or certain fungi, significantly increases the stability of hPin1 WW (approximately 4 kJ mol(-1) at 65 degrees C), in the context of the 1-39 sequence based on equilibrium measurements. N-terminal truncations and mutations in conjunction with a double mutant cycle analysis and a recently published high-resolution X-ray structure of the hPin1 cis/trans-isomerase suggest that the increase in stability is due to an energetically favorable ionic interaction between the negatively charged side chains in the N terminus of full-length hPin1 WW and the positively charged epsilon-ammonium group of residue Lys13 in beta-strand 1. Our data therefore suggest that the ionic interaction between Lys13 and the charged N terminus is the optimal solution for enhanced stability without compromising function, as ascertained by ligand binding studies. Kinetic laser temperature-jump relaxation studies reveal that this stabilizing interaction has not formed to a significant extent in the folding transition state at near physiological temperature, suggesting a differential contribution of the negatively charged N-terminal sequence to protein stability and folding rate. As neither the N-terminal sequence nor Lys13 are highly conserved among WW domains, our data further suggest that caution must be exercised when selecting domain boundaries for WW domains for structural, functional, or thermodynamic studies.     

Phosphorylation of Ser45 and Ser59 of αB-crystallin and p38/extracellular regulated kinase activity determine αB-crystallin-mediated protection of rat brain astrocytes from C2-ceramide- and staurosporine-induced cell death

We previously demonstrated that αB-crystallin and protease-activated receptor (PAR) are involved in protection of astrocytes against C2-ceramide- and staurosporine-induced cell death [Li et al. (2009) J. Neurochem.110, 1433-1444]. Here, we further investigated the mechanism of cytoprotection by αB-crystallin. Our current data revealed that after down-regulation of αB-crystallin by siRNA, cell death caused by C2-ceramide and staurosporine is increased. Furthermore, we investigated the mechanism of cytoprotection of astrocytes by intracellular αB-crystallin. Application of specific inhibitors of p38 and extracellular regulated kinase (ERK) abrogates the protection of astrocytes by over-expression of αB-crystallin. Thus, p38 and ERK contribute to protective processes by αB-crystallin. To reveal the molecular mechanism of αB-crystallin-mediated cytoprotection, we mimicked phosphorylation or unphosphorylation of αB-crystallin. In these experiments, we found that the phosphorylation of αB-crystallin at Ser45 and Ser59 is required for protection. Ser19 phosphorylation of αB-crystallin does not contribute to protection. Moreover, we detected that PAR-2 activation increases the phosphorylation level of αB-crystallin at Ser59, but does not affect the expression level of αB-crystallin. Thus, endogenous αB-crystallin has protective capacity employing a mechanism, which involves regulation of the phosphorylation status of αB-crystallin and p38 and ERK activity. Moreover, we report that PAR-2 activation evokes the phosphorylation of αB-crystallin to increase astrocytes survival.     

Crystal structures of E. coli CcmG and its mutants reveal key roles of the N-terminal beta-sheet and the fingerprint region

CcmG, also designated DsbE, functions as a periplasmic protein thiol:disulfide oxidoreductase and is required for cytochrome c maturation. Here we report the crystal structures of Escherichia coli CcmG and its two mutants, P144A and the N-terminal fifty seven-residue deletion mutant, and two additional deletion mutants were studied by circular dichroism. Structural comparison of E. coli CcmG with its deletion mutants reveals that the N-terminal beta-sheet is essential for maintaining the folding topology and consequently maintaining the active-site structure of CcmG. Pro144 and Glu145 are key residues of the fingerprint region of CcmG. Pro144 is in cis-configuration, and it makes van der Waals interactions with the active-site disulfide Cys80-Cys83 and forms a C--H...O hydrogen bond with Thr82, helping stabilize the active-site structure. Glu145 forms a salt-bridge and hydrogen-bond network with other residues of the fingerprint region and with Arg158, further stabilizing the active-site structure. The cis-configuration of Pro144 makes the backbone nitrogen and oxygen of Ala143 exposed to solvent, favorable for interacting with binding partners. The key role of cis-Pro144 is verified by the P144A mutant, which contains trans-Ala144 and displays redox property changes. Structural comparison of E. coli CcmG with the recently reported structure of CcmG in complex with the N-terminal domain of DsbD reveals that Tyr141 undergoes conformational changes upon binding DsbD. A cis-proline located at the N-terminus of the first beta-strand of the betabetaalpha motif of the thioredoxin-like domain is a conserved structural feature of the thioredoxin superfamily.     

Integrity of N‐ and C‐termini is important for E. coli Hsp31 chaperone activity

Hsp31 is a stress‐inducible molecular chaperone involved in the management of protein misfolding at high temperatures and in the development of acid resistance in starved E. coli. Each subunit of the Hsp31 homodimer consists of two structural domains connected by a flexible linker that sits atop a continuous tract of nonpolar residues adjacent to a hydrophobic bowl defined by the dimerization interface. Previously, we proposed that while the bowl serves as a binding site for partially folded species at physiological temperatures, chaperone function under heat shock conditions requires that folding intermediates further anneal to high‐affinity binding sites that become uncovered upon thermally induced motion of the linker. In support of a mechanism requiring that client proteins first bind to the bowl, we show here that fusion of a 20‐residue‐long hexahistidine tag to the N‐termini of Hsp31 abolishes chaperone activity at all temperatures by inducing reversible structural changes that interfere with substrate binding. We further demonstrate that extending the C‐termini of Hsp31 with short His tags selectively suppresses chaperone function at high temperatures by interfering with linker movement. The structural and functional sensitivity of Hsp31 to lengthening is consistent with the high degree of conservation of class I Hsp31 orthologs and will serve as a cautionary tale on the implications of affinity tagging.