Mutation R120G in alphaB-crystallin, which is linked to a desmin-related myopathy, results in an irregular structure and defective chaperone-like function

Human alphaA- and alphaB-crystallins bind to Bax and Bcl-X(S) to sequester their translocation during staurosporine-induced apoptosis

AlphaA- and alphaB-crystallins are distinct antiapoptotic regulators. Regarding the antiapoptotic mechanisms, we have recently demonstrated that alphaB-crystallin interacts with the procaspase-3 and partially processed procaspase-3 to repress caspase-3 activation. Here, we demonstrate that human alphaA- and alphaB-crystallins prevent staurosporine-induced apoptosis through interactions with members of the Bcl-2 family. Using GST pulldown assays and coimmunoprecipitations, we demonstrated that alpha-crystallins bind to Bax and Bcl-X(S) both in vitro and in vivo. Human alphaA- and alphaB-crystallins display similar affinity to both proapoptotic regulators, and so are true with their antiapoptotic ability tested in human lens epithelial cells, human retina pigment epithelial cells (ARPE-19) and rat embryonic myocardium cells (H9c2) under treatment of staurosporine, etoposide or sorbitol. Two prominent mutants, R116C in alphaA-crystallin and R120G, in alphaB-crystallin display much weaker affinity to Bax and Bcl-X(S). Through the interaction, alpha-crystallins prevent the translocation of Bax and Bcl-X(S) from cytosol into mitochondria during staurosporine-induced apoptosis. As a result, alpha-crystallins preserve the integrity of mitochondria, restrict release of cytochrome c, repress activation of caspase-3 and block degradation of PARP. Thus, our results demonstrate a novel antiapoptotic mechanism for alpha-crystallins.

Multiple sources of passive stress relaxation in muscle fibres

The forces developed during stretch of nonactivated muscle consist of velocity-sensitive (viscous/viscoelastic) and velocity-insensitive (elastic) components. At the myofibrillar level, the elastic-force component has been described in terms of the entropic-spring properties of the giant protein titin, but entropic elasticity cannot account for viscoelastic properties, such as stress relaxation. Here we examine the contribution of titin to passive stress relaxation of isolated rat-cardiac myofibrils depleted of actin by gelsolin treatment. Monte Carlo simulations show that, up to approximately 5 s after a stretch, the time course of stress relaxation can be described assuming unfolding of 1-2 immunoglobulin domains per titin molecule. For extended periods of stress relaxation, the simulations failed to correctly describe the myofibril data, suggesting that in situ, titin-Ig domains may be more stable than predicted in earlier single-molecule atomic-force-microscopy studies. The reasons behind this finding remain unknown; simply assuming a reduced unfolding probability of domains--an effect found here by AFM force spectroscopy on titin-Ig domains in the presence of a chaperone, alpha-B-crystallin--did not help correctly simulate the time course of stress relaxation. We conclude that myofibrillar stress relaxation likely has multiple sources. Evidence is provided that in intact myofibrils, an initial, rapid phase of stress relaxation results from viscous resistance due to the presence of actin filaments.

The small heat shock protein alpha B-crystallin is a novel inhibitor of TRAIL-induced apoptosis that suppresses the activation of caspase-3

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor alpha family of cytokines that preferentially induces apoptosis in transformed cells, making it a promising cancer therapy. However, many neoplasms are resistant to TRAIL-induced apoptosis by mechanisms that are poorly understood. We demonstrate that the expression of the small heat shock protein alpha B-crystallin (but not other heat shock proteins or apoptosis-regulating proteins) correlates with TRAIL resistance in a panel of human cancer cell lines. Stable expression of wild-type alpha B-crystallin, but not a pseudophosphorylation mutant impaired in its assembly and chaperone function, protects cancer cells from TRAIL-induced caspase-3 activation and apoptosis in vitro. Furthermore, selective inhibition of alpha B-crystallin expression by RNA interference sensitizes cancer cells to TRAIL. In addition, wild-type alpha B-crystallin promotes xenograft tumor growth and inhibits TRAIL-induced apoptosis in vivo in nude mice, whereas a pseudophosphorylation alpha B-crystallin mutant impaired in its anti-apoptotic function inhibits xenograft tumor growth. Collectively, these findings indicate that alpha B-crystallin is a novel regulator of TRAIL-induced apoptosis and tumor growth. Moreover, these results demonstrate that targeted inhibition of alpha B-crystallin promotes TRAIL-induced apoptosis, thereby suggesting a novel strategy to overcome TRAIL resistance in cancer.

Structural instability caused by a mutation at a conserved arginine in the alpha-crystallin domain of Chinese hamster heat shock protein 27

Mutations in the alpha-crystallin domain of 4 of the small heat shock proteins (sHsp) (Hsp27/HspB1, alphaA-crystallin/ HspB4, alphaB-crystallin/HspB5, and HspB8) are responsible for dominant inherited diseases in humans. One such mutation at a highly conserved arginine residue was shown to cause major conformational defects and intracellular aggregation of alphaA- and alphaB-crystallins and HspB8. Here, we studied the effect of this Arg mutation on the structure and function of Hsp27. Chinese hamster Hsp27 with Arg148 replaced by Gly (Hsp27R148G) formed dimers in vitro and in vivo, which contrasted with the 12- or 24-subunit oligomers formed by the wild-type protein (Hsp27WT). Despite these alterations, Hsp27R148G had a chaperone activity almost as high as Hsp27WT. The dimers of Hsp27R148G did not further deoligomerize on phosphorylation and like the dimers formed by phosphorylated Hsp27WT were not affected by the deletion of the N-terminal WD/EPF (single letter amino acid code) motif, suggesting that mutation of Arg148, deletion of the N-terminal WD/EPF motif, and phosphorylation of Ser90 may produce similar structural perturbations. Nevertheless, the structure of Hsp27R148G appeared unstable, and the mutated protein accumulated as aggregates in many cells. Both a lower basal level of phosphorylation of Hsp27R148G and the coexpression of Hsp27WT could reduce the frequency of formation of these aggregates, suggesting possible mechanisms regulating the onset of the sHsp-mediated inherited diseases.

The pathology of cellular anti-stress mechanisms: a new frontier

Exposure to stressors is an omnipresent variable for all living organisms, which have evolved anti-stress mechanisms to deal with the consequences of stress. The chaperoning systems are among these mechanisms, and their central components are the molecular chaperones that play important roles in protein biogenesis. Recent data suggest that failure of the chaperoning systems due to defective chaperones, for example, leads to pathology. Consequently, medical researchers and practitioners must now also consider the chaperoning systems, both as potentially major players in pathogenesis and as diagnostic-prognostic indicators.

Ageing and vision: structure, stability and function of lens crystallins

The alpha-, beta- and gamma-crystallins are the major protein components of the vertebrate eye lens, alpha-crystallin as a molecular chaperone as well as a structural protein, beta- and gamma-crystallins as structural proteins. For the lens to be able to retain life-long transparency in the absence of protein turnover, the crystallins must meet not only the requirement of solubility associated with high cellular concentration but that of longevity as well. For proteins, longevity is commonly assumed to be correlated with long-term retention of native structure, which in turn can be due to inherent thermodynamic stability, efficient capture and refolding of non-native protein by chaperones, or a combination of both. Understanding how the specific interactions that confer intrinsic stability of the protein fold are combined with the stabilizing effect of protein assembly, and how the non-specific interactions and associations of the assemblies enable the generation of highly concentrated solutions, is thus of importance to understand the loss of transparency of the lens with age. Post-translational modification can have a major effect on protein stability but an emerging theme of the few studies of the effect of post-translational modification of the crystallins is one of solubility and assembly. Here we review the structure, assembly, interactions, stability and post-translational modifications of the crystallins, not only in isolation but also as part of a multi-component system. The available data are discussed in the context of the establishment, the maintenance and finally, with age, the loss of transparency of the lens. Understanding the structural basis of protein stability and interactions in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.

Heat stress-induced localization of small heat shock proteins in mouse myoblasts: intranuclear lamin A/C speckles as target for αB-crystallin and Hsp25

We examined the effect of heat stress on localization of two sHsps, alphaB-crystallin and Hsp25, and of Hsc70, a member of a different class of heat shock proteins (Hsps), in both undifferentiated and differentiated mouse C2C12 cells. Under normal conditions, alphaB-crystallin and Hsp25 are found in the cytoplasm; only alphaB-crystallin is also found in the nucleus, distributed in a speckled pattern. Hsc70 is found to be homogeneously distributed throughout the cell. On heat stress, all these proteins translocate almost entirely into the nucleus and upon recovery relocate to the cytoplasm. Dual staining experiments using C2C12 myoblasts show that alphaB-crystallin and Hsp25, but not Hsc70, colocalize with the intranuclear lamin A/C and the splicing factor SC-35, suggesting interactions of sHsps and intranuclear lamin A/C. Interestingly, none of these proteins are found in the myotube nuclei. Upon heat stress, only Hsc70 translocates into the myotube nuclei. This differential entry of alphaB-crystallin and Hsp25 into the nuclei of myoblasts and myotubes upon heat stress may have functional role in the development and/or in the maintenance of muscle cells. Our study therefore suggests that these sHsps may be a part of the intranuclear lamin A/C network or stabilizing this specific network.

Molecular chaperone function of the Rana catesbeiana small heat shock protein, hsp30

Eukaryotic small heat shock proteins (shps) act as molecular chaperones by binding to denaturing proteins, preventing their heat-induced aggregation and maintaining their solubility until they can be refolded back to their normal state by other chaperones. In this study we report on the functional characterization of a developmentally regulated shsp, hsp30, from the American bullfrog, Rana catesbeiana. An expression vector containing the open reading frame of the hsp30 gene was expressed in Escherichia coli. Purified recombinant hsp30 was recovered as multimeric complexes and was composed of a mixture of alpha-helical and beta-sheet-like structures as determined by circular dichroism analysis. Hsp30 displayed chaperone activity since it inhibited heat-induced aggregation of citrate synthase. Furthermore hsp30 maintained heat-treated luciferase in a folding competent state. For example, heat denatured luciferase when microinjected into Xenopus oocytes did not regain enzyme activity whereas luciferase heat denatured with hsp30 regained 100% enzyme activity. Finally, hsp30 protected the DNA restriction endonuclease, PstI, from heat inactivation. PstI incubated alone at 42 degrees C lost its enzymatic function after 1 h whereas PstI supplemented with hsp30 accurately digested plasmid DNA after 4 h at the elevated temperature. These results clearly indicate a molecular chaperone role for R. catesbeiana hsp30.

Deamidation affects structural and functional properties of human alphaA-crystallin and its oligomerization with alphaB-crystallin

To determine the effects of deamidation on structural and functional properties of alphaA-crystallin, three mutants (N101D, N123D, and N101D/N123D) were generated. Deamidated alphaB-crystallin mutants (N78D, N146D, and N78D/N146D), characterized in a previous study (Gupta, R., and Srivastava, O. P. (2004) Invest. Ophthalmol. Vis. Sci. 45, 206-214) were also used. The biophysical and chaperone properties were determined in (a) homoaggregates of alphaA mutants (N101D, N123D, and N101D/N123D) and (b) reconstituted heteroaggregates of alpha-crystallin containing (i) wild type alphaA (WT-alphaA): WT-alphaB crystallins, (ii) individual alphaA-deamidated mutants:WT-alphaB crystallins, and (iii) WT-alphaA:individual alphaB-deamidated mutant crystallins. Compared with the WT-alphaA, the three alphaA-deamidated mutants showed reduced levels of chaperone activity, alterations in secondary and tertiary structures, and larger aggregates. These altered properties were relatively more pronounced in the mutant N101D compared with the mutant N123D. Further, compared with heteroaggregates of WT-alphaA and WT-alphaB, the heteroaggregates containing deamidated subunits of either alphaA- or alphaB-crystallins and their counterpart WT proteins showed higher molecular mass, altered tertiary structures, lower exposed hydrophobic surfaces, and reduced chaperone activity. However, the heteroaggregate containing WT-alphaA and deamidated alphaB subunit showed lower chaperone activity, smaller oligomers, and 3-fold lower subunit exchange rate than heteroaggregate containing deamidated alphaA- and WT-alphaB subunits. Together, the results suggested that (a) both Asn residues (Asn-101 and Asn-123) are required for the structural integrity and chaperone function of alphaA-crystallin and (b) the presence of WT-alphaB in the alpha-crystallin heteroaggregate leads to packing-induced structural changes which influences the oligomerization and modulate chaperone activity.

Interaction of the Molecular Chaperone αB-Crystallin with α-Synuclein: Effects on Amyloid Fibril Formation and Chaperone Activity

alpha-Synuclein is a pre-synaptic protein, the function of which is not completely understood, but its pathological form is involved in neurodegenerative diseases. In vitro, alpha-synuclein spontaneously forms amyloid fibrils. Here, we report that alphaB-crystallin, a molecular chaperone found in Lewy bodies that are characteristic of Parkinson's disease (PD), is a potent in vitro inhibitor of alpha-synuclein fibrillization, both of wild-type and the two mutant forms (A30P and A53T) that cause familial, early onset PD. In doing so, large irregular aggregates of alpha-synuclein and alphaB-crystallin are formed implying that alphaB-crystallin redirects alpha-synuclein from a fibril-formation pathway towards an amorphous aggregation pathway, thus reducing the amount of physiologically stable amyloid deposits in favor of easily degradable amorphous aggregates. alpha-Synuclein acts as a molecular chaperone to prevent the stress-induced, amorphous aggregation of target proteins. Compared to wild-type alpha-synuclein, both mutant forms have decreased chaperone activity in vitro against the aggregation of reduced insulin at 37 degrees C and the thermally induced aggregation of betaL-crystallin at 60 degrees C. Wild-type alpha-synuclein abrogates the chaperone activity of alphaB-crystallin to prevent the precipitation of reduced insulin. Interaction between these two chaperones and formation of a complex are also indicated by NMR spectroscopy, size-exclusion chromatography and mass spectrometry. In summary, alpha-synuclein and alphaB-crystallin interact readily with each other and affect each other's properties, in particular alpha-synuclein fibril formation and alphaB-crystallin chaperone action.

Desmin-related cardiomyopathy in transgenic mice: a cardiac amyloidosis

An R120G missense mutation in the small heat shock protein alpha-B-crystallin (CryAB(R120G)) causes desmin-related cardiomyopathy (DRM). DRM is characterized by the formation of aggregates containing CryAB and desmin, and it can be recapitulated in transgenic mice by cardiac-specific expression of the mutant protein. In this article, we show that expression of CryAB(R120G) leads to the formation of electron-dense bodies characteristic of the DRMs and identify these bodies as aggresomes, which are characteristic of the neurodegenerative diseases. Cardiomyocytes transfected with adenovirus containing CryAB(R120G) establish the necessity and sufficiency of CryAB(R120G) expression for aggresome formation. The commonality of these aggresomes with oligomeric protein aggregates found in the amyloid-related degenerative diseases was corroborated by the presence of high levels of amyloid oligomers that may represent a primary toxic species in the amyloid diseases. These oligomeric amyloid intermediates are present also in cardiomyocytes derived from many human dilated and hypertrophic cardiomyopathies.

Mutants in a small heat shock protein that affect the oligomeric state. Analysis and allele-specific suppression

Oligomerization is an essential property of small heat shock proteins (sHSPs) that appears to regulate their chaperone activity. We have examined the role of conserved hydrophobic residues that are postulated to stabilize sHSP oligomers. We identified a mutation of Synechocystis Hsp16.6 that impairs function in vivo and in vitro. The V143A mutation is in the C-terminal extension, a region predicted to form an oligomeric interaction with a hydrophobic region that includes the site of a previously characterized mutation, L66A. Both mutants were dimeric, but V143A had a stronger oligomerization defect than L66A. However, V143A protected a model substrate better than L66A. This suggests that although the two regions both play a role in oligomerization, they are not equivalent. Nevertheless, the addition of either dimeric sHSP enhanced the in vitro chaperone activity of wild type Hsp16.6, consistent with models that the sHSP dimers initiate interactions with substrates. Suppressor analysis of V143A identified mutations in the N terminus that restored activity by restabilizing the oligomer. These mutants were allele-specific and unable to suppress L66A, although they suppressed a dimeric C-terminal truncation of Hsp16.6. Conversely, suppressors of L66A were unable to suppress either V143A or the truncation, although they, like suppressors of V143A, stabilize the Hsp16.6 oligomer. We interpret these data as evidence that the mutations V143A and L66A stabilize two different dimeric structures and as further support that sHSP dimers are active species.

Chaperone activity of cytosolic small heat shock proteins from wheat

Small Hsps (sHsps) and the structurally related eye lens alpha-crystallins are ubiquitous stress proteins that exhibit ATP-independent molecular chaperone activity. We studied the chaperone activity of dodecameric wheat TaHsp16.9C-I, a class I cytosolic sHsp from plants and the only eukaryotic sHsp for which a high resolution structure is available, along with the related wheat protein TaHsp17.8C-II, which represents the evolutionarily distinct class II plant cytosolic sHsps. Despite the available structural information on TaHsp16.9C-I, there is minimal data on its chaperone activity, and likewise, data on activity of the class II proteins is very limited. We prepared purified, recombinant TaHsp16.9C-I and TaHsp17.8C-II and find that the class II protein comprises a smaller oligomer than the dodecameric TaHsp16.9C-I, suggesting class II proteins have a distinct mode of oligomer assembly as compared to the class I proteins. Using malate dehydrogenase as a substrate, TaHsp16.9C-I was shown to be a more effective chaperone than TaHsp17.8C-II in preventing heat-induced malate dehydrogenase aggregation. As observed by EM, morphology of sHsp/substrate complexes depended on the sHsp used and on the ratio of sHsp to substrate. Surprisingly, heat-denaturing firefly luciferase did not interact significantly with TaHsp16.9C-I, although it was fully protected by TaHsp17.8C-II. In total the data indicate sHsps show substrate specificity and suggest that N-terminal residues contribute to substrate interactions.

Desmin aggregate formation by R120G alphaB-crystallin is caused by altered filament interactions and is dependent upon network status in cells

The R120G mutation in alphaB-crystallin causes desmin-related myopathy. There have been a number of mechanisms proposed to explain the disease process, from altered protein processing to loss of chaperone function. Here, we show that the mutation alters the in vitro binding characteristics of alphaB-crystallin for desmin filaments. The apparent dissociation constant of R120G alphaB-crystallin was decreased while the binding capacity was increased significantly and as a result, desmin filaments aggregated. These data suggest that the characteristic desmin aggregates seen as part of the disease histopathology can be caused by a direct, but altered interaction of R120G alphaB-crystallin with desmin filaments. Transfection studies show that desmin networks in different cell backgrounds are not equally affected. Desmin networks are most vulnerable when they are being made de novo and not when they are already established. Our data also clearly demonstrate the beneficial role of wild-type alphaB-crystallin in the formation of desmin filament networks. Collectively, our data suggest that R120G alphaB-crystallin directly promotes desmin filament aggregation, although this gain of a function can be repressed by some cell situations. Such circumstances in muscle could explain the late onset characteristic of the myopathies caused by mutations in alphaB-crystallin.

The Selective Inhibition of Serpin Aggregation by the Molecular Chaperone, α-Crystallin, Indicates a Nucleation-dependent Specificity

Small heat shock proteins (sHsps) are a ubiquitous family of molecular chaperones that prevent the misfolding and aggregation of proteins. However, specific details about their substrate specificity and mechanism of chaperone action are lacking. alpha1-Antichymotrypsin (ACT) and alpha1-antitrypsin (alpha1-AT) are two closely related members of the serpin superfamily that aggregate through nucleation-dependent and nucleation-independent pathways, respectively. The sHsp alpha-crystallin was unable to prevent the nucleation-independent aggregation of alpha1-AT, whereas alpha-crystallin inhibited ACT aggregation in a dose-dependent manner. This selective inhibition of ACT aggregation coincided with the formation of a stable high molecular weight alpha-crystallin-ACT complex with a stoichiometry of 1 on a molar subunit basis. The kinetics of this interaction occur at the same rate as the loss of ACT monomer, suggesting that the monomeric species is bound by the chaperone. 4,4'-Dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (Bis-ANS) binding and far-UV circular dichroism data suggest that alpha-crystallin interacts specifically with a non-native conformation of ACT. The finding that alpha-crystallin does not interact with alpha1-AT under these conditions suggests that alpha-crystallin displays a specificity for proteins that aggregate through a nucleation-dependent pathway, implying that the dynamic nature of both the chaperone and its substrate protein is a crucial factor in the chaperone action of alpha-crystallin and other sHsps.

Role of the Conserved SRLFDQFFG Region of α-Crystallin, a Small Heat Shock Protein

Small heat shock proteins (sHsps) are necessary for several cellular functions and in stress tolerance. Most sHsps are oligomers; intersubunit interactions leading to changes in oligomeric structure and exposure of specific regions may modulate their functioning. Many sHsps, including alpha A- and alpha B-crystallin, contain a well conserved SRLFDQFFG sequence motif in the N-terminal region. Sequence-based prediction shows that it exhibits helical propensity with amphipathic character, suggesting that it plays a critical role in the structure and function of alpha-crystallins. In order to investigate the role of this motif in the structure and function of sHsps, we have made constructs deleting this sequence from alpha A- and alpha B-crystallin, overexpressed, purified, and studied these engineered proteins. Circular dichroism spectroscopic studies show changes in tertiary and secondary structure on deletion of the sequence. Glycerol density gradient centrifugation and dynamic light scattering studies show that the multimeric size of the mutant proteins is significantly reduced, indicating a role for this motif in higher order organization of the subunits. Both deletion mutants exhibit similar oligomeric size and increased chaperone-like activity. Urea-induced denaturation study shows that the SRLFDQFFG sequence contributes significantly to the structural stability. Fluorescence resonance energy transfer studies show that the rate of exchange of the subunits in the alpha Adel-crystallin oligomer is higher compared with that in the alpha A-crystallin oligomer, suggesting that this region contributes to the oligomer dynamics in addition to the higher order assembly and structural stability. Thus, our study shows that the SRLFDQFFG sequence is one of the critical motifs in structure-function regulation of alpha A- and alpha B-crystallin.

αB-Crystallin Modulates Protein Aggregation of Abnormal Desmin

αB-crystallin (CryAB) is the most abundant small heat shock protein in the heart. Upregulation of CryAB in desmin-related myopathy and its downregulation in end-stage congestive heart failure have both been reported. We previously demonstrated via cardiac-specific transgenesis that modest increases in normal CryAB are not detrimental to the heart, whereas expression of the R120G mutation of CryAB caused a desminopathy. It is generally believed that CryAB plays an important role in protecting the intermediate filaments, but the underlying mechanism is unclear. We hypothesized that CryAB protects the desmin filaments via preventing abnormal desmin protein from aggregating adversely. To test this hypothesis in vivo, mice expressing a desmin mutation that causes a desmin-related cardiomyopathy (D7) were bred into the R120G-CryAB transgenic (TG) background to examine the accumulation and aberrant aggregation of desmin protein. Despite lower mRNA expression of D7-des than in the D7-des TG hearts, the double-TG myocardium exhibited significantly higher desmin protein levels and dramatically more aberrant desmin aggregates than the D7-des TG hearts. The double-TG mice displayed a significantly stronger cardiac hypertrophic response, with the mice dying of congestive heart failure before 7 weeks. To explore the ability of wild-type (WT) CryAB to protect against mutant desmin, a desmin mutant was expressed in both the conventional and WT-CryAB stably transfected HEK cells. Significantly less aberrant desmin aggregation was observed in the WT-CryAB–overexpressing cells than in the HEK cells. The results suggest that CryAB modulates abnormal desmin aggregation and can serve a cardioprotective role.

Expression and function of small heat shock protein genes during Xenopus development

The hsp30 small heat shock protein family is a stress-inducible group of molecular chaperones in the frog, Xenopus laevis. Hsp30 genes are intronless and present in clusters. Expression of these genes are developmentally regulated likely at the level of chromatin structure. Also heat-induced hsp30 transcripts and protein are enriched in selected embryonic tissues. In vitro studies revealed that multimeric hsp30 binds to heat denatured target protein, inhibits their aggregation and maintains them in a folding-competent state until reactivated by other cellular chaperones. Finally optimal chaperone activity and secondary structure of hsp30 can be inhibited by phosphorylation or mutagenesis of the C-terminal end.

Hyperproliferation and p53 status of lens epithelial cells derived from alphaB-crystallin knockout mice

alphaB-Crystallin, a major protein of lens fiber cells, is a stress-induced chaperone expressed at low levels in the lens epithelium and numerous other tissues, and its expression is enhanced in certain pathological conditions. However, the function of alphaB in these tissues is not known. Lenses of alphaB-/- mice develop degeneration of specific skeletal muscles but do not develop cataracts. Recent work in our laboratory indicates that primary cultures of alphaB-/- lens epithelial cells demonstrate genomic instability and undergo hyperproliferation at a frequency 4 orders of magnitude greater than that predicted by spontaneous immortalization of rodent cells. We now demonstrate that the hyperproliferative alphaB-/- lens epithelial cells undergo phenotypic changes that include the appearance of the p53 protein as shown by immunoblot analysis. Sequence analysis showed a lack of mutations in the p53 coding region of hyperproliferative alphaB-/- cells. However, the reentry of hyperproliferative alphaB-/- cells into S phase and mitosis after DNA damage by gamma-irradiation were consistent with impaired p53 checkpoint function in these cells. The results demonstrate that expression of functionally impaired p53 is one of the factors that promote immortalization of lens epithelial cells derived from alphaB-/- mice. Fluorescence in situ hybridization using probes prepared from centromere-specific mouse P1 clones of chromosomes 1 and 9 demonstrated that the hyperproliferative alphaB-/- cells were 30% diploid and 70% tetraploid, whereas wild type cells were 83% diploid. Further evidence of genomic instability was obtained when the hyperproliferative alphaB-/- cells were labeled with anti-beta-tubulin antibodies. Examination of the hyperproliferative alphaB-/- mitotic profiles revealed the presence of cells that failed to round up for mitosis, or arrested in cytokinesis, and binucleated cells in which nuclear division had occurred without cell division. These results suggest that the stress protein and molecular chaperone alphaB-crystallin protects cells from acquiring impaired p53 protein and genomic instability.

Crystallins, genes and cataract

Far from being a physical entity, assembled of inanimate structural proteins, the ocular lens epitomizes the biological ingenuity that sustains an essential and near-perfect physical system of immaculate optics. Crystallins (alpha, beta, and gamma) provide transparency by dint of their high concentration, but it is debatable whether proteins that provide transparency are any different, biologically or structurally, from those that are present in non-transparent structures or tissues. It is becoming increasingly clear that crystallins may have a plethora of metabolic and regulatory functions, both within the lens as well as outside of it. Alpha-crystallins are members of a small heat shock family of proteins and beta/gamma-crystallins belong to the family of epidermis-specific differentiation proteins. Crystallin gene expression has been studied from the perspective of the lens specificity of their promoters. Mutations in alpha-, beta-, and gamma-crystallins are linked with the phenotype of the loss of transparency. Understanding catalytic, non-structural properties of crystallins may be critical for understanding the malfunction in molecular cascades that lead to cataractogenesis and its eventual therapeutic amelioration.

Structural perturbation and enhancement of the chaperone-like activity of alpha-crystallin by arginine hydrochloride

Structural perturbation of alpha-crystallin is shown to enhance its molecular chaperone-like activity in preventing aggregation of target proteins. We demonstrate that arginine, a biologically compatible molecule that is known to bind to the peptide backbone and negatively charged side-chains, increases the chaperone-like activity of calf eye lens alpha-crystallin as well as recombinant human alphaA- and alphaB-crystallins. Arginine-induced increase in the chaperone activity is more pronounced for alphaB-crystallin than for alphaA-crystallin. Other guanidinium compounds such as aminoguanidine hydrochloride and guanidine hydrochloride also show a similar effect, but to different extents. A point mutation, R120G, in alphaB-crystallin that is associated with desmin-related myopathy, results in a significant loss of chaperone-like activity. Arginine restores the activity of mutant protein to a considerable extent. We have investigated the effect of arginine on the structural changes of alpha-crystallin by circular dichroism, fluorescence, and glycerol gradient sedimentation. Far-UV CD spectra show no significant changes in secondary structure, whereas near-UV CD spectra show subtle changes in the presence of arginine. Glycerol gradient sedimentation shows a significant decrease in the size of alpha-crystallin oligomer in the presence of arginine. Increased exposure of hydrophobic surfaces of alpha-crystallin, as monitored by pyrene-solubilization and ANS-fluorescence, is observed in the presence of arginine. These results show that arginine brings about subtle changes in the tertiary structure and significant changes in the quaternary structure of alpha-crystallin and enhances its chaperone-like activity significantly. This study should prove useful in designing strategies to improve chaperone function for therapeutic applications.

Structural and functional defects caused by point mutations in the alpha-crystallin domain of a bacterial alpha-heat shock protein

The diverse family of alpha-crystallin-type small heat shock proteins (alpha-Hsps or sHsps) is characterised by a central, moderately conserved alpha-crystallin domain. Oligomerisation followed by dissociation of subparticles is thought to be a prerequisite for chaperone function. We demonstrate that HspH, a bacterial alpha-Hsp from the soybean-symbiont Bradyrhizobium japonicum, assembles into dynamic complexes freely exchanging subunits with homologous and heterologous complexes. The importance of the alpha-crystallin domain for oligomerisation and chaperone activity was tested by site-directed mutagenesis of 12 different residues. In contrast to mammalian alpha-Hsps, the majority of these mutations elicited severe structural and functional defects in HspH. The individual exchange of five amino acid residues throughout the alpha-crystallin domain was found to compromise oligomerisation to various degrees. Assembly defects resulting in complexes of reduced size correlated with greatly decreased or abolished chaperone activity, reinforcing that complete oligomerisation is required for functionality. Mutation of a highly conserved glycine (G114) at the C-terminal end of the alpha-crystallin domain specifically impaired chaperone activity without interfering with oligomerisation properties, indicating that this residue is critical for substrate interaction. The structural and functional importance of this and other residues is discussed in the context of a modeled three-dimensional structure of HspH.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

Mechanism of chaperone function in small heat-shock proteins. Phosphorylation-induced activation of two-mode binding in alphaB-crystallin

The consequences of alphaB-crystallin phosphorylation on its chaperone activity were investigated using a detailed analysis of the recognition and binding of destabilized T4 lysozyme (T4L) mutants by alphaB-crystallin phosphorylation mimics containing combinations of serine to aspartate substitutions. The T4L site-directed mutants were selected to constitute an energetic ladder of progressively destabilized proteins having similar structures in the folded state. alphaB-crystallin and its variants differentially recognize the T4L mutants, binding the more destabilized ones to a larger extent. Furthermore, the aspartate substitutions result in an increase in the extent of binding to the same T4L mutant and in the appearance of biphasic binding isotherms. The latter indicates the presence of two modes of binding characterized by different affinities and different numbers of binding sites. The transition to two-mode binding can also be induced by temperature or pH activation of the second mode. The similarity between the phosphorylation, pH, and temperature effects suggests a common structural origin. The location of the phosphorylation sites in the N-terminal domain and the hypothesized burial of this domain in the core of the oligomeric structure are consistent with a critical role for the destabilization of the quaternary structure in the process of recognition and binding by small heat-shock proteins.

Mimicking phosphorylation of alphaB-crystallin on serine-59 is necessary and sufficient to provide maximal protection of cardiac myocytes from apoptosis

AlphaB-crystallin (alphaBC), a small heat shock protein expressed in high levels in the heart, is phosphorylated on Ser-19, 45, and 59 after stress. However, it is not known whether alphaBC phosphorylation directly affects cell survival. In the present study, constructs were prepared that encode forms of alphaBC harboring Ser to Ala (blocks phosphorylation) or Ser to Glu (mimics phosphorylation) mutations at positions 19, 45, and 59. The effects of each form on apoptosis of cultured cardiac myocytes after hyperosmotic or hypoxic stress were assessed. Compared with controls, cells that expressed alphaBC with Ser to Ala substitutions at all three positions, alphaBC(AAA), exhibited more stress-induced apoptosis. Cells expressing either alphaBC(AAE) or (EEE) exhibited 3-fold less apoptosis than cells expressing alphaBC(AAA), indicating that phosphorylation of Ser-59 confers protection. alphaBC is known to bind to procaspase-3 and to decrease caspase-3 activation. Compared with cells expressing alphaBC(AAA), the activation of caspase-3 was decreased by 3-fold in cells expressing alphaBC(AAE). These results demonstrate that mimicking the phosphorylation of alphaBC on Ser-59 is necessary and sufficient to confer caspase-3 inhibition and protection of cardiac myocytes against hyperosmotic or hypoxic stress. These findings provide direct evidence that alphaBC(S59P) contributes to the cardioprotection observed after physiologically relevant stresses, such as transient hypoxia. Identifying the targets of alphaBC(S59P) will reveal important details about the mechanism underlying the cytoprotective effects of this small heat shock protein.

Acute effects of desmin mutations on cytoskeletal and cellular integrity in cardiac myocytes

Mutations in desmin have been associated with a subset of human myopathies. Symptoms typically appear in the second to third decades of life, but in the most severe cases can manifest themselves earlier. How desmin mutations lead to aberrant muscle function, however, remains poorly defined. We created a series of four mutations in rat desmin and tested their in vitro filament assembly properties. RDM-G, a chimera between desmin and green fluorescent protein, formed protofilament-like structures in vitro. RDM-1 and RDM-2 blocked in vitro assembly at the unit-length filament stage, while RDM-3 had more subtle effects on assembly. When expressed in cultured rat neonatal cardiac myocytes via adenovirus infection, these mutant proteins disrupted the endogenous desmin filament to an extent that correlated with their defects in in vitro assembly properties. Disruption of the desmin network by RDM-1 was also associated with disruption of plectin, myosin, and alpha-actinin organization in a significant percentage of infected cells. In contrast, expression of RDM-2, which is similar to previously characterized human mutant desmins, took longer to disrupt desmin and plectin organization and had no significant effect on myosin or alpha-actinin organization over the 5-day time course of our studies. RDM-3 had the mildest effect on in vitro assembly and no discernable effect on either desmin, plectin, myosin, or alpha-actinin organization in vivo. These results indicate that mutations in desmin have both direct and indirect effects on the cytoarchitecture of cardiac myocytes.

Alpha-crystallin

Alpha A and alpha B-crystallins are a major protein component of the mammalian eye lens. Being a member of the small heat-shock protein family they possess chaperone-like function. The alpha-crystallins and especially alpha B is also found outside the lens having an extensive tissue distribution. Alpha B-crystallin is found to be over-expressed in many neurological diseases, and mutations in alpha A or B-crystallin can cause cataract and myopathy. This review deals with some of the unique properties of the alpha-crystallins emphasizing especially what we don't know about its function and structure.

[Small heat shock proteins participate in the regulation of cellular aggregates of misfolded protein]

Molecular chaperones participate in folding of many proteins and several families are known to exist in mammalian cells including the small heat shock protein (sHSP) family. sHSPs have a molecular mass of 15-30 kDa and are known to be induced and phosphorylated in response to various stimuli. There are several reports describing the correlation between sHSPs and degenerative diseases. We have been reported that Hsp27 and alpha B-crystallin were recruited to aggresome when cells were treated with proteasome inhibitors. Expression of Hsp27 suppresses the cell death induced by expression of expanded polyglutamine via down regulation of the oxidative stress pathway. Recently, a missense mutation in alpha B-crystallin, R120G, has been found in a French family suffering from desmin-related myopathy. Moreover, transgenic mice expressing R120G alpha B-crystallin exhibit symptoms similar to desmin-related myopathy. We recently examined the interaction of R120G alpha B-crystallin and Hsp27 in mammalian cells and found that expression of R120G alpha B-crystallin caused formation of inclusion bodies and co-expression of Hsp27 inhibited this formation of inclusion bodies. Clarification of physiological roles of sHSPs in degenerative diseases may lead to the development of new therapy.

Role of the C-terminal extensions of alpha-crystallins. Swapping the C-terminal extension of alpha-crystallin to alphaB-crystallin results in enhanced chaperone activity

Several small heat shock proteins contain a well conserved alpha-crystallin domain, flanked by an N-terminal domain and a C-terminal extension, both of which vary in length and sequence. The structural and functional role of the C-terminal extension of small heat shock proteins, particularly of alphaA- and alphaB-crystallins, is not well understood. We have swapped the C-terminal extensions between alphaA- and alphaB-crystallins and generated two novel chimeric proteins, alphaABc and alphaBAc. We have investigated the domain-swapped chimeras for structural and functional alterations. We have used thermal and non-thermal models of protein aggregation and found that the chimeric alphaB with the C-terminal extension of alphaA-crystallin, alphaBAc, exhibits dramatically enhanced chaperone-like activity. Interestingly, however, the chimeric alphaA with the C-terminal extension of alphaB-crystallin, alphaABc, has almost lost its activity. Pyrene solubilization and bis-1-anilino-8-naphthalenesulfonate binding studies show that alphaBAc exhibits more solvent-exposed hydrophobic pockets than alphaA, alphaB, or alphaABc. Significant tertiary structural changes are revealed by tryptophan fluorescence and near-UV CD studies upon swapping the C-terminal extensions. The far-UV CD spectrum of alphaBAc differs from that of alphaB-crystallin whereas that of alphaABc overlaps with that of alphaA-crystallin. Gel filtration chromatography shows alteration in the size of the proteins upon swapping the C-terminal extensions. Our study demonstrates that the unstructured C-terminal extensions play a crucial role in the structure and chaperone activity, in addition to generally believed electrostatic "solubilizer" function.

Early defect in the expression of mouse sperm DNAJ 1, a member of the DNAJ/heat shock protein 40 chaperone protein family, in the spinal cord of the wobbler mouse, a murine model of motoneuronal degeneration

Prevention of protein misfolding is ensured by chaperone proteins, including the heat shock proteins (HSP) of the DNAJ/HSP40 family. Detection of abnormal protein aggregates in various neurodegenerative diseases has led to the proposal that altered chaperone activity contributes to neurodegeneration. Msj-1, a DNAJ/HSP40 protein located around the spermatozoa acrosome, was recently found to be down-regulated in the testis of wobbler mutant mice. Wobbler is an unidentified recessive mutation which triggers progressive motoneuron degeneration with abnormal intracellular protein accumulations, and defective spermatozoa maturation. Here, we examined Msj-1 expression in the spinal cord of the mutants and their controls. Msj-1 transcripts were amplified by reverse transcription-polymerase chain reaction from mutant and wild-type spinal cord RNA. Sequencing of Msj-1 coding region revealed no change in the mutant. In contrast, decreased Msj-1 mRNA levels were observed in five to six-week-old wobbler mice spinal cord, when motoneuron degeneration is at its apex, as compared to controls. A similar decrease was observed in two-week-old wobbler spinal cord, when the number of motoneurons is still unaltered, indicating that the decreased mRNA content is intrinsic to the mutant and not simply related to the loss of cells expressing Msj-1. Assays of Msj-1 protein levels yielded similar results. Immunofluorescent labeling revealed numerous Msj-1-ir motoneurons in five-week-old control spinal cord while no signal was observed in age-matched wobbler. Our results show, therefore, that Msj-1 expression is down-regulated in both organs affected by the wobbler mutation, the CNS and the testis, and that this defect precedes the first histological signs of motoneuron degeneration. These results provide the first example of an association between transcriptional repression of a chaperone protein and a neurodegenerative process.

Mechanism of chaperone function in small heat shock proteins. Two-mode binding of the excited states of T4 lysozyme mutants by alphaA-crystallin

To elucidate the mechanism of alphaA-crystallin chaperone function, a detailed thermodynamic analysis of its binding to destabilized, site-directed mutants of T4 lysozyme was carried out. The selected mutants form a ladder of stabilities spanning the 5-10 kcal/mol range of free energy of unfolding. The crystal structures of the majority of the mutants have been previously determined and found to be similar to that of the wild type with no evidence of static local unfolding. Complex formation between alphaA-crystallin and T4 lysozyme was observed directly via the changes in the electron paramagnetic resonance lineshape of a nitroxide introduced at a non-destabilizing, solvent exposed site in T4 lysozyme. AlphaA-crystallin differentially interacts with the mutants, binding the more destabilized ones to a larger extent despite the similar structure of their native states. Our results suggest that the states recognized by alphaA-crystallin are non-native excited states distinct from the unfolded state. Stable complexes are formed when the free energy of binding to alphaA-crystallin is on the order of the free energy associated with the transition from the excited state to the native state. Biphasic binding isotherms reveal two modes of interactions with distinct affinities and stoichiometries. Highly destabilized mutants preferentially bind to the high capacity mode, suggesting conformational preference in the use of each mode. Furthermore, binding can be enhanced by increased temperature and pH, which may be reflecting conformational changes in alphaA-crystallin oligomeric structure.

Ischemic protection and myofibrillar cardiomyopathy: dose-dependent effects of in vivo deltaPKC inhibition

To delineate the in vivo cardiac functions requiring normal delta protein kinase C (PKC) activity, we pursued loss-of-function through transgenic expression of a deltaPKC-specific translocation inhibitor protein fragment, deltaV1, in mouse hearts. Initial results using the mouse alpha-myosin heavy chain (alphaMHC) promoter resulted in a lethal heart failure phenotype. Viable deltaV1 mice were therefore obtained using novel attenuated mutant alphaMHC promoters lacking one or the other thyroid response element (TRE-1 and -2). In transgenic mouse hearts, deltaV1 decorated cytoskeletal elements and inhibited ischemia-induced deltaPKC translocation. At high levels, deltaV1 expression was uniformly lethal, with depressed cardiac contractile function, increased expression of fetal cardiac genes, and formation of intracardiomyocyte protein aggregates. Ultrastructural and immunoconfocal analyses of these aggregates revealed focal cytoskeletal disruptions and localized concentrations of desmin and alphaB-crystallin. In individual cardiomyocytes, cytoskeletal abnormalities correlated with impaired contractile function. Whereas desmin and alphaB-crystallin protein were increased approximately 4-fold in deltaV1 hearts, combined overexpression of these proteins at these levels was not sufficient to cause any detectable cardiac pathology. At low levels, deltaV1 expression conferred striking resistance to postischemic dysfunction, with no measurable effects on basal cardiac structure, function, or gene expression. Intermediate expression of deltaV1 conferred modest basal contractile depression with less ischemic protection, associated with abnormal cardiac gene expression, and a histological picture of infrequent cardiomyocyte cytoskeletal deformities. These results validate an approach of deltaPKC inhibition to protect against myocardial ischemia, but indicate that there is a threshold level of deltaPKC activation that is necessary to maintain normal cardiomyocyte cytoskeletal integrity.

The small heat shock protein alpha B-crystallin negatively regulates apoptosis during myogenic differentiation by inhibiting caspase-3 activation

Myoblasts respond to growth factor deprivation either by differentiating into multinucleated myotubes or by undergoing apoptosis; hence, the acquisition of apoptosis resistance by myogenic precursors is essential for their development. Here we demonstrate that the expression of the small heat shock protein alpha B-crystallin is selectively induced in C2C12 myoblasts that are resistant to differentiation-induced apoptosis, and we show that this induction occurs at an early stage in their differentiation in vitro. In contrast, the expression of several known anti-apoptotic proteins (FLIP, XIAP, Bcl-x(L)) was not altered during myogenesis. We also demonstrate that ectopic expression of alpha B-crystallin, but not the closely related small heat shock protein Hsp27, renders C2C12 myoblasts resistant to differentiation-induced apoptosis. Furthermore, we show that the myopathy-causing R120G alpha B-crystallin mutant is partly impaired in its cytoprotective function, whereas a pseudophosphorylation alpha B-crystallin mutant that mimics stress-induced phosphorylation is completely devoid of anti-apoptotic activity. Finally, we demonstrate that alpha B-crystallin negatively regulates apoptosis during myogenesis by inhibiting the proteolytic activation of caspase-3, whereas the R120G and pseudophosphorylation mutants are defective in this function. Taken together, our findings indicate that alpha B-crystallin is a novel negative regulator of myogenic apoptosis that directly links the differentiation program to apoptosis resistance.

Subunit exchange, conformational stability, and chaperone-like function of the small heat shock protein 16.5 from Methanococcus jannaschii

Hsp16.5, isolated from the hyperthermophilic Archaea Methanococcus jannaschii, is a member of the small heat-shock protein family. Small Hsps have 12- to 42-kDa subunit sizes and have sequences that are conserved among all organisms. The recently determined crystal structure of Hsp16.5 indicates that it consists discretely of 24 identical subunits. Using fluorescence resonance energy transfer, we show that at temperatures above 60 degrees C, the subunits of MjHsp16.5 freely and reversibly exchange with a rate constant of exchange at 68 degrees C of 0.067 min(-1). The subunit exchange reactions were strongly temperature-dependent, similar to the exchange reactions of the alpha-crystallins. The exchange reaction was specific to MjHsp16.5 subunits, as other sHsps such as alpha-crystallin were not structurally compatible and could not integrate into the MjHsp16.5 oligomer. In addition, we demonstrate that at temperatures as high as 70 degrees C, MjHsp16.5 retains its multimeric structure and subunit organization. Using insulin and alpha-lactalbumin as model target proteins, we also show that MjHsp16.5 at 37 degrees C is a markedly inefficient chaperone compared with other sHsps with these substrates. The results of this study support the hypothesis that MjHsp16.5 has a dynamic quaternary structure at temperatures that are physiologically relevant to M. jannaschii.

Mutation or deletion of the C-terminal tail affects the function and structure of Xenopus laevis small heat shock protein, hsp30

Small heat shock proteins (shsps) act as molecular chaperones by preventing heat-induced aggregation and unfolding of cellular proteins by a mechanism that is still unclear. Previously we found that the C-terminal end of Xenopus shsp, hsp30C (30C), was essential for optimal chaperone activity. Examination of the C-terminal tail of 30C revealed that it had a net negative charge. Involvement of this negative charge in chaperone activity was assessed by the creation of two mutants, D209G (Asp converted to the more neutrally charged and less polar Gly at position 209) and D209/213G (Asp to Gly at position 209 and 213). Compared to 30C and D209G, D209/213G was impaired in inhibiting heat-induced citrate synthase aggregation. In rabbit reticulocyte lysate and Xenopus oocyte microinjection refolding assays the mutants were not as efficient as 30C in maintaining heat-treated luciferase in a folding competent state. Circular dichroism analysis revealed that D209G was similar in secondary structure to 30C whereas D209/213G displayed a loss of alpha-helical-like and beta-sheet structure. Also, C-terminal truncation of 30C or 30D (an hsp30 isoform) resulted in a loss of secondary structure and function. This study clearly shows that mutation of aspartic acid residues in the C-terminal end of hsp30 or its truncation disrupts secondary structure and impairs its chaperone activity.

Chaperone and antichaperone activities of trigger factor

Reduced denatured lysozyme tends to aggregate at neutral pH and competition between productive folding and aggregation substantially reduces the efficiency of refolding. Trigger factor, a folding catalyst and chaperone can, depending on the concentration of trigger factor and the solution conditions, cause either a substantial increase (chaperone activity) or a substantial decrease (antichaperone activity) in the recovery of native lysozyme as compared with spontaneous refolding. When trigger factor is working as a chaperone, the reactivation rates of lysozyme are decelerated and aggregation decreases with increasing trigger factor concentrations. Under conditions where antichaperone activity of trigger factor dominates, the reactivation rates of lysozyme are accelerated and aggregation is increased. Trigger factor and lysozyme were both released from the aggregates on re-solubilization with urea indicating that trigger factor participates directly in aggregate formation and is incorporated into the aggregates. The apparently dual effect of trigger factor toward refolding of lysozyme is a consequence of the peptide binding ability and may be important in regulation of protein biosynthesis.

Identification of a region in alcohol dehydrogenase that binds to alpha-crystallin during chaperone action

alpha-Crystallin, the major eye lens protein and a member of the small heat-shock protein family, has been shown to protect the aggregation of several proteins and enzymes under denaturing conditions. The region(s) in the denaturing proteins that interact with alpha-crystallin during chaperone action has not been identified. Determination of these sites would explain the wide chaperoning action (promiscuity) of alpha-crystallin. In the present study, using two different methods, we have identified a sequence in yeast alcohol dehydrogenase (ADH) that binds to alpha-crystallin during chaperone-like action. The first method involved the incubation of alpha-crystallin with ADH peptides at 48 degrees C for 1 h followed by separation and analysis of bound peptides. In the second method, alpha-crystallin was first derivatized with a photoactive trifunctional cross-linker, sulfosuccinimidyl-2[6-(biotinamido)-2-(p-azidobenzamido)-hexanoamido]ethyl-1,3di-thiopropionate (sulfo-SBED), and then complexed with ADH at 48 degrees C for 1 h in the dark. The complex was photolyzed and digested with protease, and the biotinylated peptide fragments were isolated using an avidin column and then analyzed. The amino acid sequencing and mass spectral analysis revealed the sequence YSGVCHTDLHAWHGDWPLPVK (yeast ADH(40-60)) as the alpha-crystallin binding site in ADH. The interaction was further confirmed by demonstrating complex formation between alpha-crystallin and a synthetic peptide representing the binding site of ADH.

Effects of modifications of alpha-crystallin on its chaperone and other properties

The role of alpha-crystallin, a small heat-shock protein and chaperone, may explain how the lens stays transparent for so long. alpha-Crystallin prevents the aggregation of other lens crystallins and proteins that have become unfolded by 'trapping' the protein in a high-molecular-mass complex. However, during aging, the chaperone function of alpha-crystallin becomes compromised, allowing the formation of light-scattering aggregates that can proceed to form cataracts. Within the central part of the lens there is no turnover of damaged protein, and therefore post-translational modifications of alpha-crystallin accumulate that can reduce chaperone function; this is compounded in cataract lenses. Extensive in vitro glycation, carbamylation and oxidation all decrease chaperone ability. In the present study, we report the effect of the modifiers malondialdehyde, acetaldehyde and methylglyoxal, all of which are pertinent to cataract. Also modification by aspirin, which is known to delay cataract and other diseases, has been investigated. Recently, two point mutations of arginine residues were shown to cause congenital cataract. 1,2-Cyclohexanedione modifies arginine residues, and the extent of modification needed for a change in chaperone function was investigated. Only methylglyoxal and extensive modification by 1,2-cyclohexanedione caused a decrease in chaperone function. This highlights the robust nature of alpha-crystallin.

Glucocorticoid treatment induces expression of small heat shock proteins in human satellite cell populations: consequences for a desmin-related myopathy involving the R120G alpha B-crystallin mutation

A missense mutation (R120G) of the molecular chaperone alpha B-crystallin has recently been linked to a familial form of desmin-related myopathy characterized by intrasarcoplasmic aggregates of desmin. It was previously demonstrated that the mutant R120G had a defective chaperone-like function. However, the cellular and physiopathological consequences of R120G mutant expression in human muscle cells are as yet unclear. Thus, we developed a cellular model for the study of this R120G alpha B-crystallin-related desmin-related myopathy. We demonstrate that dexamethasone enhances alpha B-crystallin and HSP27 expression in normal and desmin-related myopathy-derived muscle cells. In the undifferentiated desmin-related myopathy satellite cell population no intracytoplasmic aggregates were observed. However, in differentiated satellite cells derived from a related myopathy patient, we observed an enhanced plasma membrane localization of alpha B-crystallin following glucocorticoid. We also observed that the protective effect against stress of alpha B-crystallin is altered upon glucocorticoid-induced small heat shock protein expression for the desmin-related myopathy-derived cells.

Analysis of crystallin-crystallin interactions by surface plasmon resonance

The mechanism of aggregation and insolubilization of lens proteins was examined based on the kinetics of crystallin-crystallin interaction determined by the surface plasmon resonance method on a BlAcore system. Lens proteins are composed mainly of three types crystallins, alpha-, beta-, and gamma-crystallin. The present study indicated that alpha-crystallin shows marked self-interaction. Furthermore, this interaction was shown to be due to alphaA-crystallin, which is a subunit of alpha-crystallin. It was also clarified that this mutual interaction of aA-crystallin decreases abruptly after the age of 20 years. On the other hand, it was assumed that alphaB-crystallin, the other subunit of alpha-crystallin, may play an important role in interactions with beta- and gamma-crystallin, while a-crystallin shows chaperone-like activity. Based on the present results, alphaA- and betaB-crystallin may play different roles when alpha-crystallin displays chaperone-like activity, and also that the decreased chaperone-like activity of a-crystallin may finally result in cataract formation following aggregation and insolubilization of lens proteins.

The R116C mutation in alpha A-crystallin diminishes its protective ability against stress-induced lens epithelial cell apoptosis

alphaA-crystallin is a small heat-shock protein expressed preferentially in the lens and is detected during the early stages of lens development. Recent work indicates that the expression of alphaA-crystallin enhances lens epithelial cell growth and resistance to stress conditions. Mutation of the arginine 116 residue to cysteine (R116C) in alphaA-crystallin has been associated with congenital cataracts in humans. However, the physiological consequences of this mutation have not been analyzed in lens epithelial cells. In the present study, we expressed wild type or R116C alphaA-crystallin in the human lens epithelial cell line HLE B-3. Immunofluorescence and confocal microscopy indicated that both wild type and R116C alphaA-crystallin were distributed mainly in the cytoplasm of lens epithelial cells. Size-exclusion chromatography indicated that the size of the alphaA-crystallin aggregate in lens epithelial cells increased from 500 to 600 kDa for the wild type protein to >2 MDa in the R116C mutant. When cells were exposed to physiological levels of UVA radiation, wild type alphaA-crystallin protected cells from apoptotic death as shown by annexin labeling and flow cytometric analysis, whereas the R116C mutant had a 4- to 10-fold lower protective ability. UVA-irradiated cells expressing the wild type protein had very low TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) staining, whereas cells expressing R116C mutant had a high level of TUNEL staining. F-actin was protected in UVA-treated cells expressing the wild type alphaA-crystallin but was either clumped around the apoptotic cells or was absent in apoptotic cells in cultures expressing the R116C mutant. Structural changes caused by the R116C mutation could be responsible for the reduced ability of the mutant to protect cells from stress. Our study shows that comparing the stress-induced apoptotic cell death is an effective way to compare the protective abilities of wild type and mutant alphaA-crystallin. We propose that the diminished protective ability of the R116C mutant in lens epithelial cells may contribute to the pathogenesis of cataract.

Sequence and spatial expression of zebrafish (Danio rerio) alphaA-crystallin

PURPOSE

To determine the nucleotide sequence, amino acid sequence and tissue specificity of zebrafish alphaA-crystallin.

METHODS

RACE, both 3' and 5', was used to clone the zebrafish alphaA-crystallin gene. The peptide sequence of the encoded protein was deduced and compared to cavefish, shark, amphibian, bird and human orthologues using the CLUSTAL W algorithm. alphaA-crystallin transcript was evaluated in brain, heart, lens, liver, skeletal muscle/skin, and spleen by semi-quantitative RT-PCR.

RESULTS

The 173 amino acid sequence of zebrafish alphaA-crystallin was determined to be 73% and 86% similar to its human and cavefish orthologues, respectively. We detected high expression of zebrafish alphaA-crystallin in the lens and very low expression in liver and spleen.

CONCLUSIONS

Few amino acids identified as being functionally important to chaperone function differ between zebrafish and mammalian alphaA-crystallin. The expression of alphaA-crystallin is mainly confined to the lens in both taxa. These data suggest that zebrafish alphaA-crystallin plays a physiologically limited role outside of the zebrafish lens, similar to its mammalian orthologues.

Alpha-crystallin-type heat shock proteins: socializing minichaperones in the context of a multichaperone network

Alpha-crystallins were originally recognized as proteins contributing to the transparency of the mammalian eye lens. Subsequently, they have been found in many, but not all, members of the Archaea, Bacteria, and Eucarya. Most members of the diverse alpha-crystallin family have four common structural and functional features: (i) a small monomeric molecular mass between 12 and 43 kDa; (ii) the formation of large oligomeric complexes; (iii) the presence of a moderately conserved central region, the so-called alpha-crystallin domain; and (iv) molecular chaperone activity. Since alpha-crystallins are induced by a temperature upshift in many organisms, they are often referred to as small heat shock proteins (sHsps) or, more accurately, alpha-Hsps. Alpha-crystallins are integrated into a highly flexible and synergistic multichaperone network evolved to secure protein quality control in the cell. Their chaperone activity is limited to the binding of unfolding intermediates in order to protect them from irreversible aggregation. Productive release and refolding of captured proteins into the native state requires close cooperation with other cellular chaperones. In addition, alpha-Hsps seem to play an important role in membrane stabilization. The review compiles information on the abundance, sequence conservation, regulation, structure, and function of alpha-Hsps with an emphasis on the microbial members of this chaperone family.

Ectopic expression of desmin in the epidermis of transgenic mice permits development of a normal epidermis

Cell architecture is largely based on the interaction of cytoskeletal proteins, which include intermediate filaments (IF), microfilaments, microtubules, as well as their type-specific membrane-attachment structures and associated proteins. In order to further our understanding of IF proteins and to address the fundamental issue whether different IF perform unique functions in different tissues, we expressed a desmin transgene in the basal epidermis of mice. Ectopic expression of desmin led to the formation of an additional, keratin-independent IF cytoskeleton and did not interfere with the keratin-desmosome interaction. We show that ectopic expression of a type III IF protein in basal keratinocytes did not interfere with the normal epidermal architecture and the program of terminal differentiation. This demonstrated that keratinocytes suffered no obvious detrimental effects from extra desmin filaments in their cytoplasm. In addition, we asked whether stable expression of desmin could rescue K5 null mice, which served as a model for severe EBS. Transgenic mice ectopically expressing desmin in the basal layer were mated with K5 heterozygous deficient animals to generate desmin rescue mice and analysed. In summary, our study support the notion that the different IF like desmin or keratins composing a IF network in vivo are central to cytoskeletal architecture and design in cells.

Conformational change and destabilization of cataract gammaC-crystallin T5P mutant

Human lens gammaC-crystallin and T5P mutant were cloned, and their biophysical properties and thermodynamic stability were studied. CRYGC (T5P) is one of the many gamma-crystallin mutant genes for autosomal dominant congenital cataracts. This mutation is associated with Coppock-like cataract, and has the phenotype of a dust-like opacity of the fetal lens nucleus. During cloning and overexpression, the majority of T5P mutant was found in the inclusion body. This property is unique among the many cataract gamma-crystallin mutant genes. It is thus worthwhile to study what factors contribute to this unique property of gammaC-crystallin. One possibility is changes in conformation and stability, which can be studied using spectroscopic measurements. In this study, conformational change was studied by circular dichroism and fluorescence measurements, and conformational stability was determined by thermal unfolding probed by Trp fluorescence and time-dependent light scattering. The T5P mutation obviously changes conformation and decreases conformational stability.

Detection of protein-protein interactions among lens crystallins in a mammalian two-hybrid system assay

alpha-Crystallin consists of two subunits, alphaA and alphaB, and each can form an oligomer by itself or with the other. The aggregation arises from interdomain interactions. However, it is not known whether such interactions also exist among alpha-, beta-, and gamma-crystallins. This heterogeneous crystallin interaction is far weaker than the homogeneous crystallin interaction and is difficult to detect by conventional spectroscopic measurements. We used a mammalian two-hybrid system in this study. The major crystallin components, alphaA-, alphaB-, betaB2-, and gammaC-crystallin genes, were subcloned into the DNA binding domain and transcription activation domain vectors of the two-hybrid system, and they were cotransfected along with a chloramphenicol acetyltransferase (CAT) reporter vector into HeLa cells. Chloramphenicol acetyltransferase activity indicated that there were interactions between alphaA- (or alphaB-) and betaB2- or gammaC-crystallins but with an intensity of one-third that of alphaA-alphaB interactions. Hsp27, a member of the family of the small heat-shock proteins, showed a similar interaction property with alphaB-crystallin. Using the N- and C-terminal domain-truncated mutants, we demonstrated that both domains were important in the alphaA-crystallin self-interaction, but that only the C-terminal domain was important in the alphaB-crystallin self-interaction. These results show that the two-hybrid system can detect interactions among various crystallins and may be used in mapping interaction domains.

Effect of trifluoroethanol on the structural and functional properties of alpha-crystallin

Alpha crystallin is an eye lens protein with a molecular weight of approximately 800 kDa. It belongs to the class of small heat shock proteins. Besides its structural role, it is known to prevent the aggregation of beta- and gamma-crystallins and several other proteins under denaturing conditions and is thus believed to play an important role in maintaining lens transparency. In this communication, we have investigated the effect of 2,2,2-trifluoroethanol (TFE) on the structural and functional features of the native alpha-crystallin and its two constituent subunits. A conformational change occurs from the characteristic beta-sheet to the alpha-helix structure in both native alpha-crystallin and its subunits with the increase in TFE levels. Among the two subunits, alphaA-crystallin is relatively stable and upon preincubation prevents the characteristic aggregation of alphaB-crystallin at 20% and 30% (v/v) TFE. The hydrophobicity and chaperone-like activity of the crystallin subunits decrease on TFE treatment. The ability of alphaA-crystallin to bind and prevent the aggregation of alphaB-crystallin, despite a conformational change, could be important in protecting the lens from external stress. The loss in chaperone activity of alphaA-crystallin exposed to TFE and the inability of peptide chaperone--the functional site of alphaA-crystallin--to stabilize alphaB-crystallin at 20-30% TFE suggest that the site(s) involved in subunit interaction and chaperone-like function are quite distinct.

Functional analysis of a small heat shock/alpha-crystallin protein from Artemia franciscana. Oligomerization and thermotolerance

Oviparously developing embryos of the brine shrimp, Artemia franciscana, synthesize abundant quantities of a small heat shock/alpha-crystallin protein, termed p26. Wild-type p26 functions as a molecular chaperone in vitro and is thought to help encysted Artemia embryos survive severe physiological stress encountered during diapause and anoxia. Full-length and truncated p26 cDNA derivatives were generated by PCR amplification of p26-3-6-3, then cloned in either pET21(+) or pRSETC and expressed in Escherichia coli BL21(DE3). All constructs gave a polypeptide detectable on Western blots with either p26 specific antibody, or with antibody to the His(6) epitope tag encoded by pRSETC. Full-length p26 in cell-free extracts of E. coli was about equal in mass to that found in Artemia embryos, but p26 lacking N- and C-terminal residues remained either as monomers or small multimers. All p26 constructs conferred thermotolerance on transformed E. coli, although not all formed oligomers, and cells expressing N-terminal truncated derivatives of p26 were more heat resistant than bacteria expressing p26 with C-terminal deletions. The C-terminal extension of p26 is seemingly more important for thermotolerance than is the N-terminus, and p26 protects E. coli against heat shock when oligomer size and protein concentration are low. The findings have important implications for understanding the functional mechanisms of small heat shock/alpha-crystallin proteins.