A translation proofreader of archaeal origin imparts multi-aldehyde stress tolerance to land plants.

Aldehydes, being an integral part of carbon metabolism, energy generation, and signalling pathways, are ingrained in plant physiology. Land plants have developed intricate metabolic pathways which involve production of reactive aldehydes and its detoxification to survive harsh terrestrial environments. Here, we show that physiologically produced aldehydes, i.e., formaldehyde and methylglyoxal in addition to acetaldehyde, generate adducts with aminoacyl-tRNAs, a substrate for protein synthesis. Plants are unique in possessing two distinct chiral proofreading systems, D-aminoacyl-tRNA deacylase1 (DTD1) and DTD2, of bacterial and archaeal origins, respectively. Extensive biochemical analysis revealed that only archaeal DTD2 can remove the stable D-aminoacyl adducts on tRNA thereby shielding archaea and plants from these system-generated aldehydes. Using Arabidopsis as a model system, we have shown that the loss of DTD2 gene renders plants susceptible to these toxic aldehydes as they generate stable alkyl modification on D-aminoacyl-tRNAs, which are recycled only by DTD2. Bioinformatic analysis identifies the expansion of aldehyde metabolising repertoire in land plant ancestors which strongly correlates with the recruitment of archaeal DTD2. Finally, we demonstrate that the overexpression of DTD2 offers better protection against aldehydes than in wild type Arabidopsis highlighting its role as a multi-aldehyde detoxifier that can be explored as a transgenic crop development strategy.

Distinct localization of chiral proofreaders resolves organellar translation conflict in plants.

Plants have two endosymbiotic organelles originated from two bacterial ancestors. The transition from an independent bacterium to a successful organelle would have required extensive rewiring of biochemical networks for its integration with archaeal host. Here, using Arabidopsis as a model system, we show that plant D-aminoacyl-tRNA deacylase 1 (DTD1), of bacterial origin, is detrimental to organellar protein synthesis owing to its changed tRNA recognition code. Plants survive this conflict by spatially restricting the conflicted DTD1 to the cytosol. In addition, plants have targeted archaeal DTD2 to both the organelles as it is compatible with their translation machinery due to its strict D-chiral specificity and lack of tRNA determinants. Intriguingly, plants have confined bacterial-derived DTD1 to work in archaeal-derived cytosolic compartment whereas archaeal DTD2 is targeted to bacterial-derived organelles. Overall, the study provides a remarkable example of the criticality of optimization of biochemical networks for survival and evolution of plant mitochondria and chloroplast.

Recruitment of archaeal DTD is a key event toward the emergence of land plants.

Streptophyte algae emerged as a land plant with adaptations that eventually led to terrestrialization. Land plants encounter a range of biotic and abiotic stresses that elicit anaerobic stress responses. Here, we show that acetaldehyde, a toxic metabolite of anaerobic stress, targets and generates ethyl adducts on aminoacyl-tRNA, a central component of the translation machinery. However, elongation factor thermo unstable (EF-Tu) safeguards l-aminoacyl-tRNA, but not d-aminoacyl-tRNA, from being modified by acetaldehyde. We identified a unique activity of archaeal-derived chiral proofreading module, d-aminoacyl-tRNA deacylase 2 (DTD2), that removes N-ethyl adducts formed on d-aminoacyl-tRNAs (NEDATs). Thus, the study provides the molecular basis of ethanol and acetaldehyde hypersensitivity in DTD2 knockout plants. We uncovered an important gene transfer event from methanogenic archaea to the ancestor of land plants. While missing in other algal lineages, DTD2 is conserved from streptophyte algae to land plants, suggesting its role toward the emergence and evolution of land plants.

The extracellular chaperone haptoglobin prevents serum fatty acid-promoted amyloid fibril formation of ß2-microglobulin, resistance to lysosomal degradation, and cytotoxicity.

Fibril formation of ß2-microglobulin and associated inflammation occur in patients on long term dialysis. We show that the plasma protein haptoglobin prevents the fatty acid-promoted de novo fibril formation of ß2-microglobulin even at substoichiometric concentration. The fibrils are cytotoxic, and haptoglobin abolishes the cytotoxicity by preventing fibril formation. Haptoglobin does not alleviate the cytotoxicity of preformed fibrils. Fibrillar ß2-microglobulin is resistant to lysosomal degradation. However, the species of ß2-microglobulin populated in the presence of haptoglobin is susceptible to degradation. We observed that haptoglobin interacts with oligomeric prefibrillar species of ß2-microglobulin but not with monomeric or fibrillar ß2-microglobulin that may underlie the molecular mechanism. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid cross-linking to haptoglobin significantly compromises its chaperone activity, suggesting the involvement of hydrophobic surfaces. Haptoglobin is an acute phase protein whose level increases severalfold during inflammation, where local acidosis can occur. Our data show that haptoglobin prevents fibril formation of ß2-microglobulin under conditions of physiological acidosis (between pH 5.5 and 6.5) but with relatively decreased efficiency. However, compromise in its chaperone activity under these conditions is more than compensated by its increased level of expression under inflammation. Erythrolysis is known to release hemoglobin into the plasma. Haptoglobin forms a 1:1 (mol/mol) complex with hemoglobin. This complex, like haptoglobin, interacts with the prefibrillar species of ß2-microglobulin, preventing its fibril formation and the associated cytotoxicity and resistance to intracellular degradation. Thus, our study demonstrates that haptoglobin is a potential extracellular chaperone for ß2-microglobulin even in moderately acidic conditions relevant during inflammation, with promising therapeutic implications in ß2-microglobulin amyloid-related diseases.

Structural aspects and chaperone activity of human HspB3: role of the "C-terminal extension".

HspB3, an as yet uncharacterized sHsp, is present in muscle, brain, heart, and in fetal tissues. A point mutation correlates with the development of axonal motor neuropathy. We purified recombinant human HspB3. Circular dichroism studies indicate that it exhibits ß-sheet structure. Gel filtration and sedimentation velocity experiments show that HspB3 exhibits polydisperse populations with predominantly trimeric species. HspB3 exhibits molecular chaperone-like activity in preventing the heat-induced aggregation of alcohol dehydrogenase (ADH). It exhibits moderate chaperone-like activity towards heat-induced aggregation of citrate synthase. However, it does not prevent the DTT-induced aggregation of insulin, indicating that it exhibits target protein-dependent molecular chaperone-like activity. Unlike other sHsps, it has a very short C-terminal extension. Fusion of the C-terminal extension of αB-crystallin results in altered tertiary and quaternary structure, and increase in polydispersity of the chimeric protein, HspB3αB-CT. The chimeric protein shows comparable chaperone-like activity towards heat-induced aggregation of ADH and citrate synthase. However, it shows enhanced activity towards DTT-induced aggregation of insulin. Our study, for the first time, provides the structural and chaperone functional characterization of HspB3 and also sheds light on the role of the C-terminal extension of sHsps.

HspB2/myotonic dystrophy protein kinase binding protein (MKBP) as a novel molecular chaperone: structural and functional aspects.

The small heat shock protein, human HspB2, also known as Myotonic Dystrophy Kinase Binding Protein (MKBP), specifically associates with and activates Myotonic Dystrophy Protein Kinase (DMPK), a serine/threonine protein kinase that plays an important role in maintaining muscle structure and function. The structure and function of HspB2 are not well understood. We have cloned and expressed the protein in E.coli and purified it to homogeneity. Far-UV circular dichroic spectrum of the recombinant HspB2 shows a ß-sheet structure. Fluorescence spectroscopic studies show that the sole tryptophan residue at the 130(th) position is almost completely solvent-exposed. Bis-ANS binding shows that though HspB2 exhibits accessible hydrophobic surfaces, it is significantly less than that exhibited by another well characterized small HSP, αB-crystallin. Sedimentation velocity measurements show that the protein exhibits concentration-dependent oligomerization. Fluorescence resonance energy transfer study shows that HspB2 oligomers exchange subunits. Interestingly, HspB2 exhibits target protein-dependent chaperone-like activity: it exhibits significant chaperone-like activity towards dithiothreitol (DTT)-induced aggregation of insulin and heat-induced aggregation of alcohol dehydrogenase, but only partially prevents the heat-induced aggregation of citrate synthase, co-precipitating with the target protein. It also significantly prevents the ordered amyloid fibril formation of α-synuclein. Thus, our study, for the first time, provides biophysical characterization on the structural aspects of HspB2, and shows that it exhibits target protein-dependent chaperone-like activity.

Inhibition of Cu2+-mediated generation of reactive oxygen species by the small heat shock protein αB-crystallin: the relative contributions of the N- and C-terminal domains.

Oxidative stress, Cu(2+) homeostasis, and small heat shock proteins (sHsp's) have important implications in several neurodegenerative diseases. The ubiquitous sHsp αB-crystallin is an oligomeric protein that binds Cu(2+). We have investigated the relative contributions of the N- and C-terminal (C-TDαB-crystallin) domains of αB-crystallin to its Cu(2+)-binding and redox-attenuation properties and mapped the Cu(2+)-binding regions. C-TDαB-crystallin binds Cu(2+) with slightly less affinity and inhibits Cu(2+)-catalyzed, ascorbate-mediated generation of ROS to a lesser extent than αB-crystallin. [Cu(2+)]/[subunit] stoichiometries for redox attenuation by αB-crystallin and C-TDαB-crystallin are 5 and 2, respectively. Both αB-crystallin and C-TDαB-crystallin also inhibit the Fenton reaction of hydroxyl radical formation. Trypsinization of αB-crystallin bound to a Cu(2+)-NTA column and MALDI-TOF analysis of column-bound peptides yielded three peptides located in the N-terminal domain, and in-solution trypsinization of αB-crystallin followed by Cu(2+)-NTA column chromatography identified four additional Cu(2+)-binding peptides located in the C-terminal domain. Thus, Cu(2+)-binding regions are distributed in the N- and C-terminal domains. Small-angle X-ray scattering and sedimentation-velocity measurements indicate quaternary structural changes in αB-crystallin upon Cu(2+) binding. Our study indicates that an oligomer of αB-crystallin can sequester a large number (~150) of Cu(2+) ions. It acts like a "Cu(2+) sponge," exhibits redox attenuation of Cu(2+), and has potential roles in Cu(2+) homeostasis and in preventing oxidative stress.

Selective Cu2+ binding, redox silencing, and cytoprotective effects of the small heat shock proteins alphaA- and alphaB-crystallin.

Oxidative stress and Cu(2+) have been implicated in several neurodegenerative diseases and in cataract. Oxidative stress, as well as Cu(2+), is also known to induce the expression of the small heat shock proteins alpha-crystallins. However, the role of alpha-crystallins in oxidative stress and in Cu(2+)-mediated processes is not clearly understood. We demonstrate using fluorescence and isothermal titration calorimetry that alpha-crystallins (alphaA- and alphaB-crystallin and its phosphorylation mimic, 3DalphaB-crystallin) bind Cu(2+) with close to picomolar range affinity. The presence of other tested divalent cations such as Zn(2+), Mg(2+), and Ca(2+) does not affect Cu(2+) binding, indicating selectivity of the Cu(2+)-binding site(s) in alpha-crystallins. Cu(2+) binding induces structural changes and increase in the hydrodynamic radii of alpha-crystallins. Cu(2+) binding increases the stability of alpha-crystallins towards guanidinium chloride-induced unfolding. Chaperone activity of alphaA-crystallin increases significantly upon Cu(2+) binding. Alpha-crystallins rescue amyloid beta peptide, Abeta(1-40), from Cu(2+)-induced aggregation in vitro. Alpha-crystallins inhibit Cu(2+)-induced oxidation of ascorbate and, hence, prevent the generation of reactive oxygen species. Interestingly, alpha-synuclein, a Cu(2+)-binding protein, does not inhibit this oxidation process significantly. We find that the Cu(2+)-sequestering (or redox-silencing) property of alpha-crystallins confers cytoprotection. To the best of our knowledge, this is the first study to reveal high affinity (close to picomolar) for Cu(2+) binding and redox silencing of Cu(2+) by any heat shock protein. Thus, our study ascribes a novel functional role to alpha-crystallins in Cu(2+) homeostasis and helps in understanding their protective role in neurodegenerative diseases and cataract.

Effect of phosphorylation on alpha B-crystallin: differences in stability, subunit exchange and chaperone activity of homo and mixed oligomers of alpha B-crystallin and its phosphorylation-mimicking mutant.

Phosphorylation appears to be one of the modulators of chaperone functions of small heat shock proteins. However, the role of phosphorylation is not completely understood. We have investigated the structural and functional consequences of a phosphorylation-mimicking mutation in alpha B-crystallin, a small heat shock protein with chaperone activity. We have used a phosphorylation-mimicking mutant, 3D alpha B-crystallin, in which all the three phosphorylatable serine residues are replaced with aspartic acid. 3D alpha B-Crystallin showed enhanced chaperone-like activity towards DTT-induced aggregation of insulin, heat-induced aggregation of citrate synthase and SDS-induced amyloid fibril formation of alpha-synuclein. Fluorescence and circular dichroism spectroscopic studies showed that 3D alpha B-crystallin exhibits lower stability towards urea-induced denaturation compared to alpha B-crystallin. Subunit exchange studies using fluorescence resonance energy transfer showed that 3D alpha B-crystallin exhibits an observable increase in subunit exchange compared to alpha B-crystallin. Since only part of alpha B-crystallin is phosphorylated in vivo, our subunit exchange studies indicate that formation of mixed oligomers between the unphosphorylated and phosphorylated subunits are likely to play a role in vivo. Our study shows that mixed-oligomer formation modulates the chaperone-like activity. We propose that the degree of phosphorylation of the alpha B-crystallin oligomers and temperature are key modulators to achieve a wide range of chaperone capabilities of the small heat shock protein, alpha -crystallin.

Mixed oligomer formation between human alphaA-crystallin and its cataract-causing G98R mutant: structural, stability and functional differences.

Mutation of the glycine 98 residue to arginine in alphaA-crystallin has been shown to cause presenile cataract in an Indian family. Our earlier study showed that the mutant protein exhibits folding defects that lead to aggregation and inclusion body formation in Escherichia coli. Despite the presence of a normal copy, the pathology is seen in the heterozygous individuals. Formation of mixed oligomers between wild-type and the mutant subunits might be crucial for manifestation of such dominant negative character. We have investigated the role of G98R mutation in alphaA-crystallin in its structural stability and subunit exchange. G98R alphaA-crystallin unfolds at lower concentrations of urea compared to wild-type alphaA-crystallin. The mutant protein is more susceptible to proteolysis than the wild-type protein and transiently populates fragments that are prone to aggregation. Subunit exchange studies using fluorescence resonance energy transfer show that the mutant protein forms mixed oligomers with the wild-type protein. The mutant protein is more susceptible to thermal aggregation, whereas mixed oligomer formation leads to a decreased propensity to aggregate. Co-expression of wild-type alphaA-crystallin with G98R alphaA-crystallin in E. coli rescues the mutant alphaA-crystallin from formation of inclusion bodies. These observations may underlie the molecular basis for the presenile onset, not congenital cataract, in spite of severe folding defect and aggregation of the mutant. Our study shows that the mixed oligomers of wild-type and G98R alphaA-crystallin exhibit properties dominated by those of the mutant protein in structural aspects, oligomeric size, urea-induced unfolding and, more importantly, the chaperone activity, which may provide the molecular basis for presenile cataract formation in affected individuals.

Interaction of mammalian Hsp22 with lipid membranes.

Hsp22/HspB8 is a member of the small heat-shock protein family, whose function is not yet completely understood. Our immunolocalization studies in a human neuroblastoma cell line, SK-N-SH, using confocal microscopy show that a significant fraction of Hsp22 is localized to the plasma membrane. We therefore investigated its interactions with lipid vesicles in vitro. Intrinsic tryptophan fluorescence is quenched in the presence of lipid vesicles derived from either bovine brain lipid extract or purified lipids. Time-resolved fluorescence studies show a decrease in the lifetimes of the tryptophan residues. Both of these results indicate burial of some tryptophan residues of Hsp22 upon interaction with lipid vesicles. Membrane interactions also lead to increase in fluorescence polarization of Hsp22. Gel-filtration chromatography shows that Hsp22 binds stably with lipid vesicles; the extent of binding depends on the nature of the lipid. Hsp22 binds more strongly to vesicles made of lipids containing a phosphatidic acid, phosphatidylinositol or phosphatidylserine headgroup (known to be present in the inner leaflet of plasma membrane) compared with lipid vesicles made of a phosphatidylcholine head-group alone. Far-UV CD spectra reveal conformational changes upon binding to the lipid vesicles or in membrane-mimetic solvent, trifluoroethanol. Thus our fluorescence, CD and gel-filtration studies show that Hsp22 interacts with membrane and this interaction leads to stable binding and conformational changes. The present study therefore clearly demonstrates that Hsp22 exhibits potential membrane interaction that may play an important role in its cellular functions.

The cataract-causing mutation G98R in human alphaA-crystallin leads to folding defects and loss of chaperone activity.

PURPOSE: The objective of this study is to understand the molecular basis of cataract that develops due to the mutation of the glycine-98 residue to arginine in alphaA-crystallin. METHODS: The glycine-98 residue was mutated to arginine by site-directed mutagenesis. The expression, structural and chaperone properties and thermal stability of the mutant, G98RalphaA-crystallin have been studied. The secondary and tertiary structure of the wild type and the mutant protein was studied using circular dichroism and fluorescence spectroscopy. The quaternary structure was studied by gel filtration chromatography and dynamic light scattering. Chaperone activity studies were carried out using DTT-induced aggregation of insulin. RESULTS: Unlike the wild type protein, the heterologous expression of G98R alphaA-crystallin in E. coli results in the formation of inclusion bodies. Upon dissolving the inclusion bodies in 3 M urea and subjecting to refolding, it yielded a clear solution. The refolded mutant protein exhibits altered secondary, tertiary and quaternary structure, which lacks chaperone function, and is susceptible to heat-induced aggregation. CONCLUSIONS: The G98R mutation in alphaA-crystallin results in altered folding and becomes aggregation-prone leading to formation of large oligomers lacking chaperone function. Tendency to aggregate and loss of chaperone activity could be contributing to turbidity and loss of transparency of the lens.

Fibrillogenic and non-fibrillogenic ensembles of SDS-bound human alpha-synuclein.

Fibril formation of alpha-synuclein is associated with several neurodegenerative diseases, including Parkinson's disease in humans. The anionic detergent sodium dodecyl sulfate (SDS) can accelerate the fibril formation in vitro. However, the molecular basis of this acceleration is not clear. Our study shows that native alpha-synuclein exhibits relatively less fibril growth despite providing fibril seeds for nucleation. The presence of SDS promotes the seeded fibril growth in a concentration-dependent manner, with an optimal concentration of 0.5-0.75 mM. We used isothermal calorimetry, hydrophobic dye binding and circular dichroism spectroscopy to characterize the protein-detergent interactions as a function of the concentration of SDS. Interaction of SDS with alpha-synuclein when studied by isothermal titration calorimetry and hydrophobic dye-binding reveals a similar characteristic optimal behavior between 0.5 mM and 0.75 mM SDS. The study shows two types of ensembles of alpha-synuclein and SDS: the fibrillogenic ensembles formed with optimal concentration of SDS around 0.5-0.75 mM are characterized by enhanced accessible hydrophobic surfaces and extended to partially helical conformation, while the less or non-fibrillogenic ensembles formed above 2 mM SDS are characterized by less accessible hydrophobic surfaces and maximal helical content. Little or no fibrillogenicity of the ensembles observed above 2 mM SDS could be partly because of the observed intrinsic instability of the fibrils under the condition.

AlphaB-crystallin, a small heat-shock protein, prevents the amyloid fibril growth of an amyloid beta-peptide and beta2-microglobulin.

AlphaB-crystallin, a small heat-shock protein, exhibits molecular chaperone activity. We have studied the effect of alphaB-crystallin on the fibril growth of the Abeta (amyloid beta)-peptides Abeta-(1-40) and Abeta-(1-42). alphaB-crystallin, but not BSA or hen egg-white lysozyme, prevented the fibril growth of Abeta-(1-40), as revealed by thioflavin T binding, total internal reflection fluorescence microscopy and CD spectroscopy. Comparison of the activity of some mutants and chimaeric alpha-crystallins in preventing Abeta-(1-40) fibril growth with their previously reported chaperone ability in preventing dithiothreitol-induced aggregation of insulin suggests that there might be both common and distinct sites of interaction on alpha-crystallin involved in the prevention of amorphous aggregation of insulin and fibril growth of Abeta-(1-40). alphaB-crystallin also prevents the spontaneous fibril formation (without externally added seeds) of Abeta-(1-42), as well as the fibril growth of Abeta-(1-40) when seeded with the Abeta-(1-42) fibril seed. Sedimentation velocity measurements show that alphaB-crystallin does not form a stable complex with Abeta-(1-40). The mechanism by which it prevents the fibril growth differs from the known mechanism by which it prevents the amorphous aggregation of proteins. alphaB-crystallin binds to the amyloid fibrils of Abeta-(1-40), indicating that the preferential interaction of the chaperone with the fibril nucleus, which inhibits nucleation-dependent polymerization of amyloid fibrils, is the mechanism that is predominantly involved. We found that alphaB-crystallin prevents the fibril growth of beta2-microglobulin under acidic conditions. It also retards the depolymerization of beta2-microglobulin fibrils, indicating that it can interact with the fibrils. Our study sheds light on the role of small heat-shock proteins in protein conformational diseases, particularly in Alzheimer's disease.

Metal ion-dependent effects of clioquinol on the fibril growth of an amyloid {beta} peptide.

Although metal ions such as Cu(2+), Zn(2+), and Fe(3+) are implicated to play a key role in Alzheimer disease, their role is rather complex, and comprehensive understanding is not yet obtained. We show that Cu(2+) and Zn(2+) but not Fe(3+) renders the amyloid beta peptide, Abeta(1-40), nonfibrillogenic in nature. However, preformed fibrils of Abeta(1-40) were stable when treated with these metal ions. Consequently, fibril growth of Abeta(1-40) could be switched on/off by switching the molecule between its apo- and holo-forms. Clioquinol, a potential drug for Alzheimer disease, induced resumption of the Cu(2+)-suppressed but not the Zn(2+)-suppressed fibril growth of Abeta(1-40). The observed synergistic effect of clioquinol and Zn(2+) suggests that Zn(2+)-clioquinol complex effectively retards fibril growth. Thus, clioquinol has dual effects; although it disaggregates the metal ion-induced aggregates of Abeta(1-40) through metal chelation, it further retards the fibril growth along with Zn(2+). These results indicate the mechanism of metal ions in suppressing Abeta amyloid formation, as well as providing information toward the use of metal ion chelators, particularly clioquinol, as potential drugs for Alzheimer disease.

Critical balance of electrostatic and hydrophobic interactions is required for beta 2-microglobulin amyloid fibril growth and stability.

Investigation of factors that modulate amyloid formation of proteins is important to understand and mitigate amyloid-related diseases. To understand the role of electrostatic interactions and the effect of ionic cosolutes, especially anions, on amyloid formation, we have investigated the effect of salts such as NaCl, NaI, NaClO(4), and Na(2)SO(4) on the amyloid fibril growth of beta(2)-microglobulin, the protein involved in dialysis-related amyloidosis. Under acidic conditions, these salts exhibit characteristic optimal concentrations where the fibril growth is favored. The presence of salts leads to an increase in hydrophobicity of the protein as reported by 8-anilinonaphthalene-1-sulfonic acid, indicating that the anion interaction leads to the necessary electrostatic and hydrophobic balance critical for amyloid formation. However, high concentrations of salts tilt the balance to high hydrophobicity, leading to partitioning of the protein to amorphous aggregates. Such amorphous aggregates are not competent for fibril growth. The order of anions based on the lowest concentration at which fibril formation is favored is SO(4)(2)(-) > ClO(4)(-) > I(-) > Cl(-), consistent with the order of their electroselectivity series, suggesting that preferential anion binding, rather than general ionic strength effect, plays an important role in the amyloid fibril growth. Anion binding is also found to stabilize the amyloid fibrils under acidic condition. Interestingly, sulfate promotes amyloid growth of beta(2)-microglobulin at pH between 5 and 6, closer to its isoelectric point. Considering the earlier studies on the role of glycosaminoglycans and proteoglycans (i.e., sulfated polyanions) on amyloid formation, our study suggests that preferential interaction of sulfate ions with amyloidogenic proteins may have biological significance.

Oligomeric Hsp33 with enhanced chaperone activity: gel filtration, cross-linking, and small angle x-ray scattering (SAXS) analysis.

Hsp33, an Escherichia coli cytosolic chaperone, is inactive under normal conditions but becomes active upon oxidative stress. It was previously shown to dimerize upon activation in a concentration- and temperature-dependent manner. This dimer was thought to bind to aggregation-prone target proteins, preventing their aggregation. In the present study, we report small angle x-ray scattering (SAXS), steady state and time-resolved fluorescence, gel filtration, and glutaraldehyde cross-linking analysis of full-length Hsp33. Our circular dichroism and fluorescence results show that there are significant structural changes in oxidized Hsp33 at different temperatures. SAXS, gel filtration, and glutaraldehyde cross-linking results indicate, in addition to the dimers, the presence of oligomeric species. Oxidation in the presence of physiological salt concentration leads to significant increases in the oligomer population. Our results further show that under conditions that mimic the crowded milieu of the cytosol, oxidized Hsp33 exists predominantly as an oligomeric species. Interestingly, chaperone activity studies show that the oligomeric species is much more efficient compared with the dimers in preventing aggregation of target proteins. Taken together, these results indicate that in the cell, Hsp33 undergoes conformational and quaternary structural changes leading to the formation of oligomeric species in response to oxidative stress. Oligomeric Hsp33 thus might be physiologically relevant under oxidative stress.

DNA-mediated assembly of weakly interacting DNA-binding protein subunits: in vitro recruitment of phage 434 repressor and yeast GCN4 DNA-binding domains.

The specificity of DNA-mediated protein assembly was studied in two in vitro systems, based on (i) the DNA-binding domain of bacteriophage 434 repressor cI (amino acid residues 1-69), or (ii) the DNA-binding domain of the yeast transcription factor GCN4, (amino acids 1-34) and their respective oligonucleotide cognates. In vivo, both of these peptides are part of larger protein molecules that also contain dimerization domains, and the resulting dimers recognize cognate palindromic DNA sequences that contain two half-sites of 4 bp each. The dimerization domains were not included in the peptides tested, so in solution-in the presence or absence of non-cognate DNA oligonucleotides-these molecules did not show appreciable dimerization, as determined by pyrene excimer fluorescence spectroscopy and oxidative cross-linking monitored by mass spectrometry. Oligonucleotides with only one 4 bp cognate half-site were able to initiate measurable dimerization, and two half-sites were able to select specific dimers even from a heterogeneous pool of molecules of closely related specificity (such as DNA-binding domains of the 434 repressor and their engineered mutants that mimic the binding helix of the related P22 phage repressor). The fluorescent technique allowed us to separately monitor the unspecific, ionic interaction of the peptides with DNA which produced a roughly similar signal in the case of both cognate and non-cognate oligonucleotides. But in the former case, a concomitant excimer fluorescence signal showed the formation of correctly positioned dimers. The results suggest that DNA acts as a highly specific template for the recruitment of weakly interacting protein molecules that can thus build up highly specific complexes.

The IXI/V motif in the C-terminal extension of alpha-crystallins: alternative interactions and oligomeric assemblies.

PURPOSE: Alpha-crystallin, a hetero-oligomer of alphaA- and alphaB-crystallin, is involved in maintaining eye lens transparency, primarily by its structural packing and chaperone activity. alphaA- and alphaB-crystallin share significant sequence homology, which is almost exclusively restricted to the central, conserved "alphaA-crystallin domain". The flanking N-terminal domain and C-terminal extension are highly variable both in sequence and length. Mutations and age-related post-translational modifications of these proteins are associated with congenital and age-onset cataracts. Interestingly, most mutations or truncations in the C-terminal extensions of the alpha-crystallins and other alpha-sHsps like Hsp27 lead to pathology. It is therefore important to understand the structure/function relationship of this region. Sequence alignment of the C-terminal extensions of alphaA- and alphaB-crystallin with other homologues shows a conserved IXI/V motif. The purpose of this study was to investigate the role of this conserved motif, IPV in alphaA-crystallin and IPI in alphaB-crystallin (corresponding to residues 159-161 in both crystallins), in the structure and chaperone activity. METHODS: The isoleucine/valine residues in the IPV motif of alphaA-crystallin and the IPI motif of alphaB-crystallin were mutated to glycine and studied the secondary and tertiary structure of the mutant proteins using circular dichroism and fluorescence spectroscopy, and the quaternary structure using glycerol density gradient centrifugation and dynamic light scattering. Chaperone activity was studied at 37 degrees C and 25 degrees C using DTT induced aggregation of insulin as a model system. We have performed fluorescence resonance energy transfer (FRET) experiments to investigate the interactions of this motif in homo- and hetero-oligomers. Since alphaB-crystallin is devoid of Cys residues, we have introduced a Cys residue flanking the IPI motif (T162CalphaB-crystallin) to facilitate fluorescence labeling studies. RESULTS: Unlike in other homologues from plants or prokaryotes, mutation of the isoleucine/valine residues in alpha-crystallins does not result in oligomer dissociation or loss of chaperone activity. On the contrary, the mutant proteins retain their capacity to oligomerize and show enhanced chaperone activity at 37 degrees C. The mutants also exhibit significantly higher chaperone-like activity at 25 degrees C. FRET experiments show that the region spanning the IPI/V motif comes in proximity either to the beta-strands of the "alpha-crystallin" domain or the corresponding IPI/V region of another subunit. CONCLUSIONS: Our mutational studies show that the IPI/V motif has a propensity to participate in inter-subunit interactions, either with regions in the alpha-crystallin domain or with the corresponding IPI/V region on another monomer. These interactions are important in the structure and function of alpha-crystallins. This motif also appears to be important in the temperature dependent chaperone-like activity of the alpha-crystallins. The propensity of the IPI/V motif to form multiple inter-subunit interactions may contribute to the diversity in structure and function seen in the alpha-crystallin/sHsp family.

Mammalian Hsp22 is a heat-inducible small heat-shock protein with chaperone-like activity.

A newly identified 22 kDa protein that interacts with Hsp27 (heat-shock protein 27) was shown to possess the characteristic alpha-crystallin domain, hence named Hsp22, and categorized as a member of the sHsp (small Hsp) family. Independent studies from different laboratories reported the protein with different names such as Hsp22, H11 kinase, E2IG1 and HspB8. We have identified, on the basis of the nucleotide sequence analysis, putative heat-shock factor 1 binding sites upstream of the Hsp22 translation start site. We demonstrate that indeed Hsp22 is heat-inducible. We show, in vitro, chaperone-like activity of Hsp22 in preventing dithiothreitol-induced aggregation of insulin and thermal aggregation of citrate synthase. We have cloned rat Hsp22, overexpressed and purified the protein to homogeneity and studied its structural and functional aspects. We find that Hsp22 fragments on storage. MS analysis of fragments suggests that the fragmentation might be due to the presence of labile peptide bonds. We have established conditions to improve its stability. Far-UV CD indicates a randomly coiled structure for Hsp22. Quaternary structure analyses by glycerol density-gradient centrifugation and gel filtration chromatography show that Hsp22 exists as a monomer in vitro, unlike other members of the sHsp family. Hsp22 exhibits significantly exposed hydrophobic surfaces as reported by bis-8-anilinonaphthalene-l-sulphonic acid fluorescence. We find that the chaperone-like activity is temperature dependent. Thus Hsp22 appears to be a true member of the sHsp family, which exists as a monomer in vitro and exhibits chaperone-like activity.