Probing the roles of the only universally conserved leucine residue (Leu122) in the oligomerization and chaperone-like activity of Mycobacterium tuberculosis small heat shock protein Hsp16.3

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.

Temperature and concentration-controlled dynamics of rhizobial small heat shock proteins

A hallmark of alpha-crystallin-type small heat shock proteins (sHsps) is their highly dynamic oligomeric structure which promotes intermolecular interactions involved in subunit exchange and substrate binding (chaperone-like activity). We studied the oligomeric features of two classes of bacterial sHsps by size exclusion chromatography and nanoelectrospray mass spectrometry. Proteins of both classes formed large complexes that rapidly dissociated upon dilution and at physiologically relevant heat shock temperatures. As the secondary structure was not perturbed, temperature- and concentration-dependent dissociations were fully reversible. Complexes formed between sHsps and the model substrate citrate synthase were stable and exceeded the size of sHsp oligomers. Small Hsps, mutated in a highly conserved glycine residue at the C-terminal end of the alpha-crystallin domain, formed labile complexes that disassembled more readily than the corresponding wild-type proteins. Reduced complex stability coincided with reduced chaperone activity.

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.

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.

The reassembling process of the nonameric Mycobacterium tuberculosis small heat-shock protein Hsp16.3 occurs via a stepwise mechanism

Conditions are reported under which the reassembled intermediates of the heat-shock protein Hsp16.3 after being denatured in 8 M urea were detected by mainly using urea-gradient PAGE (with modifications) and urea-denaturing pore-gradient PAGE. Hsp16.3 is the small heat-shock protein from Mycobacterium tuberculosis, which exists as a specific nonamer and was proposed to form a trimer-of-trimers structure. The refolding and reassembling of this protein was achieved rapidly by dilution or dialysis, suggesting an effectively spontaneous recovery of quaternary structure. Data presented in this report demonstrate that the in vitro reassembling process of Hsp16.3 protein occurs through a spontaneous and effective stepwise mechanism. Modified urea-gradient PAGE may provide a general method for studying the reassembling processes of other oligomeric proteins.

Functional similarities between the small heat shock proteins Mycobacterium tuberculosis HSP 16.3 and human alphaB-crystallin

Mycobacterium tuberculosis heat shock protein 16.3 (MTB HSP 16.3) accumulates as the dominant protein in the latent stationary phase of tuberculosis infection. MTB HSP 16.3 displays several characteristics of small heat shock proteins (sHsps): its expression is increased in response to stress, it protects against protein aggregation in vitro, and it contains the core 'alpha-crystallin' domain found in all sHsps. In this study we characterized the chaperone activity of recombinant MTB HSP 16.3 in several different assays and compared the results to those obtained with recombinant human alphaB-crystallin, a well characterized member of the sHsp family. Recombinant MTB HSP 16.3 was expressed in Escherichia coli and purified to apparent homogeneity. Similar to alphaB-crystallin, MTB HSP16.3 suppressed citrate synthase aggregation and in the presence of 3.5 mm ATP the chaperone activity was enhanced by twofold. ATP stabilized MTB HSP 16.3 against proteolysis by chymotrypsin, and no effect was observed with ATPgammaS, a nonhydrolyzable analog of ATP. Increased expression of MTB HSP 16.3 resulted in protection against thermal killing in E. coli at 48 degrees C. While the sequence similarity between human alphaB-crystallin and MTB HSP 16.3 is only 18%, these results suggest that the functional similarities between these proteins containing the core 'alpha-crystallin' domain are much closer.

Site-directed mutation on the only universally conserved residue Leu122 of small heat shock protein Hsp16.3

Hsp16.3 from Mycobacterium tuberculosis belongs to the small heat shock protein family and has chaperone-like activity in vitro. The only universally conserved hydrophobic residue Leu122 was substituted by Val and Ala, respectively. The mutations on the Leu122 of Hsp16.3 have resulted in much lower structural stability in vivo and in vitro. Both mutant proteins exhibited much weaker chaperone-like activities than the Hsp16.3 WT under heat shock conditions. Taken together, the highly hydrophobic residue L122 of Hsp16.3 was suggested to play a very important role in maintaining not only the structural stability but also the chaperone-like activity.