Preheat treatment for Mycobacterium tuberculosis Hsp16.3: correlation between a structural phase change at 60 degrees C and a dramatic increase in chaperone-like activity

Role of Molecular Interactions and Oligomerization in Chaperone Activity of Recombinant Acr from Mycobacterium tuberculosis

Background:

The chaperone activity of Mycobacterium tuberculosis Acr is an important function that helps to prevent misfolding of protein substrates inside the host, especially in conditions of hypoxia.

Objectives:

The aim of this study was to establish the correlation of structure and function of recombinant Acr proteins both before and after gel filtration chromatography. The aim was also to find the oligomeric conformation of these samples and use this information to explain differences in activit.

Material and Methods:

M. tuberculosis acr gene was cloned with an N-terminal His-tag in pET28a and expressed with IPTG induction in BL2 (DE3) competent Escherichia coli. The activity of a recombinant Acr without gel filtration was checked by preventing thermal aggregation of citrate synthase at 45°C and the chaperone activity against insulin B chain aggregation at 60°C and 37°C. On further purification using gel filtration chromatography, the protein was again tested for chaperone activity using insulin as substrate at 37°C with two types of samples without and with gel filtration designated A and B respectively. The effects of pre–heat treatment at 60 °C on chaperone activity of both A and B samples were studied by performing the chaperone assay at 37°C.

Results:

The level of expression was 40 to 50 mg /l. The protein was expressed in a soluble form at 37°C and subsequently purified by a 3 step gradient of imidazole using Ni-NTA resin. Gel filtration chromatography showed recombinant Acr to be a mixture of 9 to 15-mers, whereas Native-PAGE analysis showed a large proportion of 5 and 7 mers in the non gel-filtered sample, while non gel –filtered samples showed more proportions of higher size oligomers. The chaperone activity of non gel-filtered (A) samples was less than gel-filtered (B) samples at 37°C with 24 µM required of A for complete inhibition as compared to 6 µM of B. The chaperone activity of non gel–filtered samples at 60°C showed complete inhibition of activity at a concentration of 44 µM. Molecular interaction studies showed influence of size of oligomers on molecular coverage of insulin B chain. Pre-heat treatment improved the activity only after the gel filtration.

Conclusions:

The larger proportion of monomers in the non gel-filtered sample could explain the difference in activity as compared to the gel-filtered samples in terms of molecular interaction with insulin. Increased oligomer size favorably affected secondary structure, a finding not reported so far, and warranting further investigation. A molecular level interaction of inhibition was predicted using Avogadro number of molecules and oligomer size. The difference in activity after pre–heat treatment seemed to indicate an important role for oligomerization.

The impact of different mutations at arginine141 on the structure, subunit exchange dynamics and chaperone activity of Hsp16.3

Hsp16.3, a molecular chaperone, plays a vital role in the growth and survival of Mycobacterium tuberculosis inside the host. We previously reported that deletion of three amino acid residues (¹⁴² STN¹⁴⁴ ) from C-terminal extension (CTE) of Hsp16.3 triggers its structural perturbation and increases its chaperone activity, which reaches its apex upon the deletion of its entire CTE (¹⁴¹ RSTN¹⁴⁴ ). Thus, we hypothesized that Arg141 (R141) and Ser142 (S142) in the CTE of Hsp16.3 possibly hold the key in maintaining its native-like structure and chaperone activity. To test this hypothesis, we generated two deletion mutants in which R141 and S142 were deleted individually (Hsp16.3ΔR141 and Hsp16.3ΔS142) and three substitution mutants in which R141 was replaced by lysine (Hsp16.3R141K), alanine (Hsp16.3R141A), and glutamic acid (Hsp16.3R141E), respectively. Hsp16.3ΔS142 or Hsp16.3R141K mutant has native-like structure and chaperone activity. Deletion of R141 from the CTE (Hsp16.3ΔR141) perturbs the secondary and tertiary structure, lowers the subunit exchange dynamics and decreases the chaperone activity of Hsp16.3. But, the substitution of R141 with alanine (Hsp16.3R141A) or glutamic acid (Hsp16.3R141E) perturbs its secondary and tertiary structure. Surprisingly, such charge tampering of R141 enhances the subunit exchange dynamics and chaperone activity of Hsp16.3. Interestingly, neither the deletion of R141/S142 nor the substitution of R141 with lysine, alanine and glutamic acid affects the oligomeric mass/size of Hsp16.3. Overall, our study suggests that R141 (especially the positive charge on R141) plays a crucial role in maintaining the native-like structure as well as in regulating subunit exchange dynamics and chaperone activity of Hsp16.3.

A New Functional Model for Prediction of Chaperone Activity of the Recombinant M. tb Acr (α-Crystallin) Using Insulin as Substrate

Mycobacterium tuberculosis Acr is an important protein expressed in latent tuberculosis which is active as an oligomer in preventing misfolding of cellular proteins. In this study, Mycobacterium alpha crystallin (acr) gene was cloned and expressed in Escherichia coli (E. coli). The recombinant Acr protein was purified by Nickel-NTA resin. The oligomeric state of Acr was confirmed by gel filtration chromatography using Sephacryl S-200 and Native-PAGE. Studies of chaperone activity were performed with insulin as a substrate at different mole ratios of Acr with 2 types of samples, His tag elutes (H) and His tag elutes with gel filtration (G). It was observed that the ratio of different sizes of oligomers (9 to 24 mers) had a significant effect on chaperone activity. Using the mole ratio of Acr for both (H) and (G) samples to insulin B chain and ratio of oligomers, we determined the number of Acr molecules binding to insulin as a model substrate. We found that if 1.5% of the insulin B chains are covered completely by the (G) samples, aggregation is completely inhibited as compared to 6% with (H) samples. Pre-heat treatment studies were carried out at 37°C, 60°C, and 70°C. Far-ultraviolet Circular Dichroism (UV-CD) analysis provided fresh insights into the role of β-sheets and α-helices in chaperone activity, particularly in (H) samples suggesting a reversible conformational transition from helices to sheets. This enabled us to formulate a functional model for binding of Acr to insulin B chains which incorporated 4 types of secondary structure molecules. This might be a useful tool for analyzing in vitro preparations of recombinant Acr and build more consensuses on the structure-activity relationship especially in terms of oligomeric ratios.

Prediction of recombinant Mycobacterium tuberculosis α-crystallin oligomer chaperone activity using polynomial graphs

Background: Mycobacterial α-crystallin (Acr) is a chaperone that prevents misfolding of proteins when Mycobacterium tuberculosis is found in a latent form in the host tissue. Methods: Using insulin as a model substrate and utilizing polynomial graphs, we attempted to predict molecular-level interactions that are a function of the oligomeric state of the recombinant protein. The chaperone activity of the recombinant oligomeric Acr was measured at 60°C with Acr samples obtained before gel filtration chromatography and compared with a gel-filtered sample. Results: The polynomial graphs constructed showed improved molecular coverage of the insulin B chain by the oligomer. The 2 nd order coefficient is the one that changes with the oligomeric ratio of Acr and improves chaperone activity. Polynomial analysis suggested that it could be a useful parameter to predict chaperone activity for potential in vitro batches of M. tuberculosis Acr based on the dynamic nature of the association and disassociation of oligomers. Conclusions: The results showed that coverage of insulin B chain improved with higher ratio of 9-mer as compared to lower ratios. This was shown by both simulation plots and actual assay data. The polynomial graphs showed increase in the 2 nd order coefficient, thus suggesting the important role of oligomerisation in improved molecular coverage of insulin B chain.

Prediction of recombinant Mycobacterium tuberculosis α-crystallin oligomer chaperone activity using polynomial graphs

Background: Mycobacterial α-crystallin (Acr) is a chaperone that prevents misfolding of proteins when Mycobacterium tuberculosis is found in a latent form in the host tissue. Methods: Using insulin as a model substrate and utilizing polynomial graphs, we attempted to predict molecular-level interactions that are a function of the oligomeric state of the recombinant protein. The chaperone activity of the recombinant oligomeric Acr was measured at 60°C with Acr samples obtained before gel filtration chromatography and compared with a gel-filtered sample. Results: The polynomial graphs constructed showed improved molecular coverage of the insulin B chain by the oligomer. The 2 nd order coefficient is the one that changes with the oligomeric ratio of Acr and improves chaperone activity. Polynomial analysis suggested that it could be a useful parameter to predict chaperone activity for potential in vitro batches of M. tuberculosis Acr based on the dynamic nature of the association and disassociation of oligomers. Conclusions: The results showed that coverage of insulin B chain improved with higher ratio of 9-mer as compared to lower ratios. This was shown by both simulation plots and actual assay data. The polynomial graphs showed increase in the 2 nd order coefficient, thus suggesting the important role of oligomerisation in improved molecular coverage of insulin B chain.

The oligomeric plasticity of Hsp20 of Sulfolobus acidocaldarius protects environment-induced protein aggregation and membrane destabilization

Small heat shock proteins (sHsps) are a ubiquitous family of molecular chaperones that rescue misfolded proteins from irreversible aggregation during cellular stress. Many such sHsps exist as large polydisperse species in solution, and a rapid dynamic subunit exchange between oligomeric and dissociated forms modulates their function under a variety of stress conditions. Here, we investigated the structural and functional properties of Hsp20 from thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. To provide a framework for investigating the structure-function relationship of Hsp20 and understanding its dynamic nature, we employed several biophysical and biochemical techniques. Our data suggested the existence of a ~24-mer of Hsp20 at room temperature (25 °C) and a higher oligomeric form at higher temperature (50 °C-70 °C) and lower pH (3.0-5.0). To our surprise, we identified a dimeric form of protein as the functional conformation in the presence of aggregating substrate proteins. The hydrophobic microenvironment mainly regulates the oligomeric plasticity of Hsp20, and it plays a key role in the protection of stress-induced protein aggregation. In Sulfolobus sp., Hsp20, despite being a non-secreted protein, has been reported to be present in secretory vesicles and it is still unclear whether it stabilizes substrate proteins or membrane lipids within the secreted vesicles. To address such an issue, we tested the ability of Hsp20 to interact with membrane lipids along with its ability to modulate membrane fluidity. Our data revealed that Hsp20 interacts with membrane lipids via a hydrophobic interaction and it lowers the propensity of in vitro phase transition of bacterial and archaeal lipids.

Immunodominant protein MIP_05962 from Mycobacterium indicus pranii displays chaperone activity

Tuberculosis, a contagious disease of infectious origin is currently a major cause of deaths worldwide. Mycobacterium indicus pranii (MIP), a saprophytic nonpathogen and a potent immunomodulator is currently being investigated as an intervention against tuberculosis along with many other diseases with positive outcome. The apparent paradox of multiple chaperones in mycobacterial species and enigma about the cellular functions of the client proteins of these chaperones need to be explored. Chaperones are the known immunomodulators; thus, there is need to exploit the proteome of MIP for identification and characterization of putative chaperones. One of the immunogenic proteins, MIP_05962 is a member of heat shock protein (HSP) 20 family due to the presence of α-crystallin domain, and has amino acid similarity with Mycobacterium lepraeHSP18 protein. The diverse functions of M. lepraeHSP18 in stress conditions implicate MIP_05962 as an important protein that needs to be explored. Biophysical and biochemical characterization of the said protein proved it to be a chaperone. The observations of aggregation prevention and refolding of substrate proteins in the presence of MIP_05962 along with interaction with non-native proteins, surface hydrophobicity, formation of large oligomers, in-vivo thermal rescue of Escherichia coli expressing MIP_05962, enhancing solubility of insoluble protein maltodextrin glucosidase (MalZ) under in-vivo conditions, and thermal stability and reversibility confirmed MIP_05962 as a molecular chaperone.

Conformational perturbation, hydrophobic interactions and oligomeric association are responsible for the enhanced chaperone function of Mycobacterium leprae HSP18 under pre-thermal condition

Chaperone function of HSP18 is enhanced upon pre-heating at 60 °C and above which may be due to structural alterations.

Study of the immunological profile towards Mycobacterium bovis antigens in naturally infected cattle

A number of studies have determined the contribution of Th1 and Th2 responses to the protective immunity and pathology of Mycobacterium bovis infection. However, much of that information is derived from experimentally infecting cattle with M. bovis and few data from naturally infected animals are available. The aim of this study was to characterize the immunological profile towards M. bovis antigens of naturally infected cattle by measurement of cytokine mRNA expression in PBMC, and to determine which lymphocyte subsets are involved in recall responses of PBMC from M. bovis infected cattle to M. bovis antigens. Consistent with data from cattle experimentally infected with M. bovis, naturally infected animals were found to display a Th1 cytokine profile in response to M. bovis PPDB stimulation. Production of IFN-gamma mRNA by PBMC after PPDB stimulation statistically distinguishes between infected and healthy herds, suggesting that this molecule is usable as an M. bovis-infection marker. As happens in experimentally infected cows, CD4, CD8 and gammadeltaTCR cells from a herd naturally infected with M. bovis are the predominant T cell subsets expanded in response to PPDB.

Identification of novel Mycobacterium bovis antigens by dissection of crude protein fractions

Culture filtrate and cell extracts from Mycobacterium bovis cultures contain molecules which could promote protective immunity to tuberculosis in animals. Different protein fractions of M. bovis cultures were obtained by elution electrophoresis and were tested in experimentally infected cattle. The fractions that elicited gamma interferon (IFN-gamma) responses were resolved by two-dimensional gel electrophoresis, and individual proteins were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. The open reading frames were cloned, expressed as their recombinant forms, and retested with naturally and experimentally infected animals. Eleven protein fractions were highly reactive, from which the Rv1636, HspX, Rv0138, Rv2524, EsxI, and Rv3740 recombinant proteins were obtained. EsxI and HspX were the antigens most recognized by the IFN-gamma release assay. In summary, a proteomic approach allowed the identification of novel antigens useful for the diagnosis of bovine tuberculosis.

Posttranslational modulation on the biological activities of molecular chaperones

Molecular chaperones are a family of proteins that were first noticed to exist about 45 years ago from their increased transcription under heat shock conditions. As a result, the regulation of their encoding genes has been subject to extensive studies. Recent studies revealed that the biological activities of molecular chaperones can also be effectively modulated at the protein level. The ways of modulation so far elucidated include allosteric effect, covalent modification, protein-protein interaction, and conformational alteration induced by such macro-environmental conditions as temperature and pH. These latter aspects were reviewed here. Emphasized here is the importance of such immediate structural alterations that lead to an immediate activity increase, providing the immediate protection needed for the cells to survive the stress conditions.

The dramatically increased chaperone activity of small heat-shock protein IbpB is retained for an extended period of time after the stress condition is removed

sHSP (small heat-shock protein) IbpB (inclusion-body-binding protein B) from Escherichia coli is known as an ATP-independent holding chaperone which prevents the insolubilization of aggregation-prone proteins by forming stable complexes with them. It was found that the chaperone function of IbpB is greatly modulated by the ambient temperature, i.e. when the temperature increases from normal to heat-shock, the chaperone activity of IbpB is dramatically elevated to a level that allows it to effectively bind the aggregation-prone client proteins. Although it is generally believed that the release and refolding of the client protein from the sHSPs depends on the aid of the ATP-dependent chaperones such as Hsp (heat-shock protein) 70 and Hsp100 when the ambient temperature recovers from heat-shock to normal, the behaviour of the sHSPs during this recovery stage has not yet been investigated. In the present study, we examined the behaviour and properties of IbpB upon temperature decrease from heat-shock to normal. We found that IbpB, which becomes functional only under heat-shock conditions, retains the chaperone activity for an extended period of time after the heat-shock stress condition is removed. A detail comparison demonstrates that such preconditioned IbpB is distinguished from the non-preconditioned IbpB by a remarkable conformational transformation, including a significant increase in the flexibility of the N- and C-terminal regions, as well as enhanced dynamic subunit dissociation/reassociation. Intriguingly, the preconditioned IbpB displayed a dramatic decrease in its surface hydrophobicity, suggesting that the exposure of hydrophobic sites might not be the sole determinant for IbpB to exhibit chaperone activity. We propose that the maintenance of the chaperone activity for such ‘holdases’ as sHSPs would be important for cells to recover from heat-shock stress.

The ability of Mycobacterium avium subsp. paratuberculosis to enter bovine epithelial cells is influenced by preexposure to a hyperosmolar environment and intracellular passage in bovine mammary epithelial cells

Mycobacterium avium subsp. paratuberculosis is the cause of Johne's disease in cattle and other ruminants. M. avium subsp. paratuberculosis infection of the bovine host is not well understood; however, it is assumed that crossing the bovine intestinal mucosa is important in order for M. avium subsp. paratuberculosis to establish infection. To examine the ability of M. avium subsp. paratuberculosis to infect bovine epithelial cells in vitro, Madin-Darby bovine kidney (MDBK) epithelial cells were exposed to M. avium subsp. paratuberculosis. It was observed that bacteria can establish infection and replicate within MDBK cells. M. avium subsp. paratuberculosis also has been reported to infect mammary tissue and milk, and we showed that M. avium subsp. paratuberculosis infects bovine mammary epithelial cells (MAC-T cell line). Using polarized MAC-T cell monolayers, it was also determined that M. avium subsp. paratuberculosis crosses apical and basolateral surfaces with approximately the same degree of efficiency. Because M. avium subsp. paratuberculosis can be delivered to the naïve host by milk, it was investigated whether incubation of M. avium subsp. paratuberculosis with milk has an effect on invasion of MDBK cells. M. avium subsp. paratuberculosis exposed to milk entered epithelial cells with greater efficiency than M. avium subsp. paratuberculosis exposed to broth medium or water (P < 0.01). Growth of M. avium subsp. paratuberculosis within MAC-T cells also resulted in augmented ability to subsequently infect bovine MDBK cells (P < 0.001). Microarray analysis of intracellular M. avium subsp. paratuberculosis RNA indicates the increased transcription of genes which might be associated with an invasive phenotype.

Chaperone-Like Activity of Mycobacterium tuberculosis Hsp16.3 Does Not Require Its Intact (Native) Structures

Small heat shock proteins (sHsps) were found to exhibit efficient chaperone-like activities under stress conditions although their native structures are severely disturbed. Here, using an alternative approach (site-directed mutagenesis), we obtained two structurally and functionally distinct Mycobacterium tuberculosis Hsp16.3 single-site mutant proteins. The G59W mutant protein (with Gly59 substituted by Trp) is capable of exhibiting efficient chaperone-like activity even under non-stress conditions although its secondary, tertiary, and quaternary structures are very different from that of the wild type protein. By contrast, the G59A mutant protein (with Gly59 substituted by Ala) resembles with the wild type protein in structure and function. These observations suggest that the Gly59 of the Hsp16.3 protein is critical for its folding and assembly. In particular, we propose that the exhibition of chaperone-like activity for Hsp16.3 does not require its intact (native) structures but requires the disturbance of its native structures (i.e., the native structure-disturbed Hsp16.3 retains its chaperone-like activity or even becomes more active). In addition, the behavior of such an active mutant protein (G59W) also strongly supports our previous suggestion that Hsp16.3 exhibits chaperone-like activity via oligomeric dissociation.

Anti-Vpr activity of a yeast chaperone protein

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) exerts multiple effects on viral and host cellular activities during viral infection, including nuclear transport of the proviral integration complex, induction of cell cycle G(2) arrest, and cell death. In this report, we show that a fission yeast chaperone protein Hsp16 inhibits HIV-1 by suppressing these Vpr activities. This protein was identified through three independent genome-wide screens for multicopy suppressors of each of the three Vpr activities. Consistent with the properties of a heat shock protein, heat shock-induced elevation or overproduction of Hsp16 suppressed Vpr activities through direct protein-protein interaction. Even though Hsp16 shows a stronger suppressive effect on Vpr in fission yeast than in mammalian cells, similar effects were also observed in human cells when fission yeast hsp16 was expressed either in vpr-expressing cells or during HIV-1 infection, indicating a possible highly conserved Vpr suppressing activity. Furthermore, stable expression of hsp16 prior to HIV-1 infection inhibits viral replication in a Vpr-dependent manner. Together, these data suggest that Hsp16 inhibits HIV-1 by suppressing Vpr-specific activities. This finding could potentially provide a new approach to studying the contribution of Vpr to viral pathogenesis and to reducing Vpr-mediated detrimental effects in HIV-infected patients.

Temperature-dependent subunit exchange and chaperone-like activities of Hsp16.3, a small heat shock protein from Mycobacterium tuberculosis

Small heat shock proteins (sHsps) usually exist as oligomers that undergo dynamic oligomeric dissociation/re-association, with the dissociated oligomers as active forms to bind substrate proteins under heat shock conditions. In this study, however, we found that Hsp16.3, one sHsp from Mycobacterium tuberculosis, is able to sensitively modulate its chaperone-like activity in a range of physiological temperatures (from 25 to 37.5 degrees C) while its native oligomeric size is still maintained. Further analysis demonstrated that Hsp16.3 exposes higher hydrophobic surfaces upon temperatures increasing and that a large soluble complex between Hsp16.3 and substrate is formed only in the condition of heating temperature up to 35 and 37.5 degrees C. Structural analysis by fluorescence anisotropy showed that Hsp16.3 nonameric structure becomes more dynamic and variable at elevated temperatures. Moreover, subunit exchange between Hsp16.3 oligomers was found to occur faster upon temperatures increasing as revealed by fluorescence energy resonance transfer. These observations indicate that Hsp16.3 is able to modulate its chaperone activity by adjusting the dynamics of oligomeric dissociation/re-association process while maintaining its static oligomeric size unchangeable. A kinetic model is therefore proposed to explain the mechanism of sHsps-binding substrate proteins through oligomeric dissociation. The present study also implied that Hsp16.3 is at least capable of binding non-native proteins in vivo while expressing in the host organism that survives at 37 degrees C.

Small heat shock protein Hsp16.3 modulates its chaperone activity by adjusting the rate of oligomeric dissociation

Small heat shock proteins usually exist as oligomers and appear to undergo dynamic dissociation/reassociation, with oligomeric dissociation being a prerequisite for their chaperone activities. However, contradictory cases were also reported that chaperone activities could be enhanced with no change or even increase in oligomeric sizes. Using Hsp16.3 as a model system, our studies show the following: (1) Although a preheat (over 60 degrees C) treatment or the presence of low concentrations of urea (around 0.8M) hardly caused any change in the oligomeric size of Hsp16.3 proteins when examined by size exclusion chromatography, its chaperone activities were increased significantly. (2) Further analysis using the unique pore-gradient polyacrylamide gel electrophoresis revealed a dramatic increase in the tendency of oligomeric dissociation for both the preheated and urea-containing Hsp16.3. (3) Meanwhile, for both cases, an apparent increase in the rate constants of oligomeric dissociation was also observed, as determined by utilizing conjugated fluorescence probes whose quantum yield increases accompanying oligomeric dissociation. (4) Moreover, the fluorescence anisotropy analysis also demonstrated that the oligomeric structures for the preheated or urea-containing Hsp16.3 proteins seem to be more dynamic and variable. In light of these observations, we propose that the small heat shock proteins like Hsp16.3 can modulate their chaperone activities by adjusting the rate of oligomeric dissociation in responding to environmental changes. Results obtained here also suggest that small heat shock proteins might be able to "remember" their stress experiences via certain structural alterations which will allow them to act as better chaperones when the stress conditions reappear.

Disulfide bonds convert small heat shock protein Hsp16.3 from a chaperone to a non-chaperone: implications for the evolution of cysteine in molecular chaperones

Molecular chaperones mainly function in assisting newly synthesized polypeptide folding and protect non-native proteins from aggregation, with known structural features such as the ability of spontaneous folding/refolding and high conformational flexibility. In this report, we verified the assumption that the lack of disulfide bonds in molecular chaperones is a prerequisite for such unique structural features. Using small heat shock protein (one sub-class of chaperones) Hsp16.3 as a model system, our results show the following: (1) Cysteine-free Hsp16.3 wild type protein can efficiently exhibit chaperone activity and spontaneously refold/reassemble with high conformational flexibility. (2) Whereas Hsp16.3 G89C mutant with inter-subunit disulfide bonds formed seems to lose the nature of chaperone proteins, i.e., under stress conditions, it neither acts as molecular chaperone nor spontaneously refolds/reassembles. Structural analysis indicated that the mutant exists as an unstable molten globule-like state, which incorrectly exposes hydrophobic surfaces and irreversibly tends to form aggregates that can be suppressed by the other molecular chaperone (alpha-crystallin). By contrast, reduction of disulfide bond in the Hsp16.3 G89C mutant can significantly recover its character as a molecular chaperone. In light of these results, we propose that disulfide bonds could severely disturb the structure/function of molecular chaperones like Hsp16.3. Our results might not only provide insights into understanding the structural basis of chaperone upon binding substrates, but also explain the observation that the occurrence of cysteine in molecular chaperones is much lower than that in other protein families, subsequently being helpful to understand the evolution of protein family.

Activation mechanism of HSP16.5 from Methanococcus jannaschii

The small heat shock proteins are the ubiquitous proteins found in a wide range of organisms and function as molecular chaperones by binding to the folding intermediates of their substrates. Although the crystal structure of HSP16.5, a small heat shock protein from Methanococcus jannaschii, revealed that it is a hollow sphere composed of 24 identical subunits, its activation mechanism remains unclear. We found out that HSP16.5 is active only at high temperatures and forms a stable complex with substrate in a stoichiometric manner. We also observed that the conformational change of HSP16.5 is correlated with the increasing hydrophobic site and its activation as a molecular chaperone. However, it is revealed that the conformational change is not accompanied with the change of the secondary structure of a subunit, but correlated with the increasing diameter of HSP16.5. Therefore, it is proposed that the activation mechanism of HSP16.5 involves temperature induced conformational change with size increment of the complex resulting in the exposure of hydrophobic substrate-binding site.

Reversible methionine sulfoxidation of Mycobacterium tuberculosis small heat shock protein Hsp16.3 and its possible role in scavenging oxidants

Mycobacterium tuberculosis (TB) small heat shock protein Hsp16.3 was found to be a major membrane protein that is most predominantly expressed under oxidative stress and is localized to the thickened cell envelope. Gene knock-out studies indicate that the Hsp16.3 protein is required for TB to grow in its host macrophage cells. The physiological function of Hsp16.3 has not yet revealed. Our analyses via mass spectrometry, conformation-dependent trypsin digestion, nondenaturing pore gradient electrophoresis, ANS-binding fluorescence measurements, and circular dichroism demonstrate that the three and only the three methionine residues (cysteine and tryptophan residues, which can also be readily oxidized by such oxidant as H(2)O(2), are absent in Hsp16.3) can be readily sulfoxidized with H(2)O(2) treatment in vitro, and the methionine sulfoxide can be effectively reduced back to the methionine form. Interconversion between the methionine and methioninesulfoxide has been confirmed by selective oxidation and reduction. The sulfoxidation leads to a small degree of conformational change, which in turn results in a significant decrease of the chaperone-like activity. Data presented in this report strongly implicate that reversible sulfoxidation/desulfoxidation of methionine residues may occur in Hsp16.3, which serves as a way to scavenger reactive oxygen or nitrogen species abundantly present in macrophage cells, thus protecting the plasma membrane and other components of M. tuberculosis allowing their survival in such bacteriocidal hosts.

Mycobacterium tuberculosis Hsp16.3 nonamers are assembled and re-assembled via trimer and hexamer intermediates

Hsp16.3, a small heat shock protein from Mycobacterium tuberculosis proposed to form specific trimer-of-trimers structures, acts as a molecular chaperone in vitro. The assembly and re-assembly mechanisms of this oligomeric protein were studied and compared using in vitro transcription/translation and denaturization/renaturization systems. Analysis using a combination of non-denaturing pore gradient polyacrylamide gel electrophoresis, chemical cross-linking, and size-exclusion chromatography demonstrate that the predominant form of Hsp16.3 produced in the in vitro transcription/translation system is the trimer, which can be further assembled into a nonameric structure via a hexamer intermediate in the presence of purified exogenous Hsp16.3 proteins. Meanwhile, an "inert" Hsp16.3 dimer, which does not seem to participate in nonamer assembly but may be involved in forming other forms of Hsp16.3, was also detected in the in vitro expression system. On the other hand, our current data clearly show that the re-assembly of Hsp16.3 nonamers also occurs via a very similar mechanism, with the formation of trimers and hexamers. The presence of high levels of macromolecular crowding protein agent in the in vitro expression system promoted the formation of the nonamers to a very limited degree, indicating that the assembly of proteins like Hsp16.3 may depend mainly on its own concentration instead of those of the macromolecules in the environment.

Monodisperse Hsp16.3 nonamer exhibits dynamic dissociation and reassociation, with the nonamer dissociation prerequisite for chaperone-like activity

Small heat-shock proteins (sHsps) of various origins exist commonly as oligomers and exhibit chaperone-like activities in vitro. Hsp16.3, the sHsp from Mycobacterium tuberculosis, was previously shown to exist as a monodisperse nonamer in solution when analyzed by size-exclusion chromatography and electron cryomicroscropy. This study represents part of our effort to understand the chaperone mechanism of Hsp16.3, focusing on the role of the oligomeric status of the protein. Here, we present evidence to show that the Hsp16.3 nonamer dissociates at elevated temperatures, accompanied by a greatly enhanced chaperone-like activity. Moreover, the chaperone-like activity was increased dramatically when the nonameric structure of Hsp16.3 was disturbed by chemical cross-linking, which impeded the correct reassociation of Hsp16.3 nonamer. These suggest that the dissociation of the nonameric structure is a prerequisite for Hsp16.3 to bind to denaturing substrate proteins. On the other hand, our data obtained by using radiolabeled and non-radiolabeled proteins clearly demonstrated that subunit exchange occurs readily between the Hsp16.3 oligomers, even at a temperature as low as 4 degrees C. In light of all these observations, we propose that Hsp16.3, although it appears to be homogeneous when examined at room temperature, actually undertakes rapid dynamic dissociation/reassociation, with the equilibrium, and thus the chaperone-like activities, regulated mainly by the environmental temperature.