Identification of bis-ANS binding sites in Mycobacterium tuberculosis small heat shock protein Hsp16.3: Evidences for a two-step substrate-binding mechanism

Understanding the structural and functional implications of lysine succinylation in Mycobacterium tuberculosis heat shock protein 16.3

Heat shock protein 16.3 (Hsp16.3), a major immunodominant antigen of Mycobacterium tuberculosis, exhibits molecular chaperone function that is essential for pathogen's survival and slow growth inside hosts, as well as for enhancing the efficacy of Bacillus Calmette-Guérin (BCG) vaccine. Proteomic studies revealed that Hsp16.3 undergoes lysine succinylation in vivo at all lysine residues (K47, K64, K78, K85, K114, K119 and K132) except K136. However, the effects of succinylation on its structure and function remain unexplored. This study investigated the impact of succinylation, induced by physiological (succinyl-CoA) and/or non-physiological (succinic anhydride) donors, on the structure, stability and chaperone function of Hsp16.3. Succinylation of all eight lysine residues, affirmed via fluorescamine assay and mass spectrometry, led to structural (secondary and tertiary) alterations, as indicated by circular dichroism (CD), fluorescence and in-silico analyses. Succinylation induced oligomeric dissociation (dodecamer to dimer) and enhanced surface hydrophobicity of Hsp16.3. Moreover, succinylation reduced protein stability, making it more conformationally flexible and less compact, as revealed by urea-denaturation, chymotrypsin-digestion and computational studies. Despite this reduced stability, succinylated Hsp16.3 exhibited enhanced chaperone activity, offering improved protection to stressed-prone client proteins. These findings provide useful insights into this modification, offering potential therapeutic avenues for targeting Hsp16.3 in M. tuberculosis infection.

Lysine acetylation of Hsp16.3: Effect on its structure, chaperone function and influence towards the growth of Mycobacterium tuberculosis

Hsp16.3 plays a vital role in the slow growth of Mycobacterium tuberculosis via its chaperone function. Many secretory proteins, including Hsp16.3 undergo acetylation in vivo. Seven lysine (K) residues (K64, K78, K85, K114, K119, K132 and K136) in Hsp16.3 are acetylated inside pathogen. However, how lysine acetylation affects its structure, chaperone function and pathogen's growth is still elusive. We examined these aspects by executing in vitro chemical acetylation (acetic anhydride modification) and by utilizing a lysine acetylation mimic mutant (Hsp16.3-K64Q/K78Q/K85Q/K114Q/K119Q/K132Q/K136Q). Far- and near-UV CD measurements revealed that the chemically acetylated proteins(s) and acetylation mimic mutant has altered secondary and tertiary structure than unacetylated/wild-type protein. The chemical modification and acetylation mimic mutation also disrupted the oligomeric assembly, increased surface hydrophobicity and reduced stability of Hsp16.3, as revealed by GF-HPLC, 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid binding and urea denaturation experiments, respectively. These structural changes collectively led to an enhancement in chaperone function (aggregation and thermal inactivation prevention ability) of Hsp16.3. Moreover, when the H37Rv strain expressed the acetylation mimic mutant protein, its growth was slower in comparison to the strain expressing the wild-type/unacetylated Hsp16.3. Altogether, these findings indicated that lysine acetylation improves the chaperone function of Hsp16.3 which may influence pathogen's growth in host environment.

Interaction of constituents of MDT regimen for leprosy with Mycobacterium leprae HSP18: impact on its structure and function

Mycobacterium leprae, the causative organism of leprosy, harbors many antigenic proteins, and one such protein is the 18-kDa antigen. This protein belongs to the small heat shock protein family and is commonly known as HSP18. Its chaperone function plays an important role in the growth and survival of M. leprae inside infected hosts. HSP18/18-kDa antigen is often used as a diagnostic marker for determining the efficacy of multidrug therapy (MDT) in leprosy. However, whether MDT drugs (dapsone, clofazimine, and rifampicin) do interact with HSP18 and how these interactions affect its structure and chaperone function is still unclear. Here, we report evidence of HSP18-dapsone/clofazimine/rifampicin interaction and its impact on the structure and chaperone function of HSP18. These three drugs interact efficiently with HSP18 (having submicromolar binding affinity) with 1 : 1 stoichiometry. Binding of these MDT drugs to the 'α-crystallin domain' of HSP18 alters its secondary structure and tryptophan micro-environment. Furthermore, surface hydrophobicity, oligomeric size, and thermostability of the protein are reduced upon interaction with these three drugs. Eventually, all these structural alterations synergistically decrease the chaperone function of HSP18. Interestingly, the effect of rifampicin on the structure, stability, and chaperone function of this mycobacterial small heat shock protein is more pronounced than the other two MDT drugs. This reduction in the chaperone function of HSP18 may additionally abate M. leprae survivability during multidrug treatment. Altogether, this study provides a possible foundation for rational designing and development of suitable HSP18 inhibitors in the context of effective treatment of leprosy.

Small but mighty: a functional look at bacterial sHSPs

Small heat shock proteins (sHSPs) are widespread in every kingdom of life, being indispensable for protein quality control networks. Alongside canonical chaperone functions, sHSPs seem to have been a very plastic scaffold for acquiring multiple related functions across evolution. This review aims to summarize what is known about sHSPs functioning in the Bacteria Kingdom.

Insights into Macrophage Autophagy in Latent Tuberculosis Infection: Role of Heat Shock Protein 16.3

Tuberculosis (TB) is a major bacterial infectious disease worldwide that is predominantly caused by Mycobacterium tuberculosis (Mtb). The comorbidity of multiple drug-resistant TB strains with HIV and diabetes is widespread. In the presence of these diseases, host immunity is weakened, allowing the recovery of dormant bacilli and leading to recurrent TB infection. As an important component of the host innate and adaptive immune responses, macrophage autophagy plays a significant role in protecting the host against TB. However, dormant bacilli can escape from autophagosomes and/or suppress autophagy, thus surviving within the host for an extended period of time, although the underlying mechanism remains elusive. Heat shock protein 16.3 (Hsp16.3, HspX, Rv2031c, and Acr) is one of the immunodominant proteins expressed during latent TB infection (LTBI). It may help maintain the protein stability and long-term viability of Mtb by inhibiting macrophage autophagy, resulting in LBTI. In this review, we discuss how dormant bacilli escape from autophagosomes, and we focus on the role of Hsp16.3 in regulating macrophage autophagy in LTBI so as to provide a firm basis for further studies. Hsp16.3 may represent a potential biomarker of LTBI and novel pharmacological target for anti-tubercular drugs.

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.

Structural model of dodecameric heat-shock protein Hsp21: Flexible N-terminal arms interact with client proteins while C-terminal tails maintain the dodecamer and chaperone activity

Small heat-shock proteins (sHsps) prevent aggregation of thermosensitive client proteins in a first line of defense against cellular stress. The mechanisms by which they perform this function have been hard to define due to limited structural information; currently, there is only one high-resolution structure of a plant sHsp published, that of the cytosolic Hsp16.9. We took interest in Hsp21, a chloroplast-localized sHsp crucial for plant stress resistance, which has even longer N-terminal arms than Hsp16.9, with a functionally important and conserved methionine-rich motif. To provide a framework for investigating structure-function relationships of Hsp21 and understanding these sequence variations, we developed a structural model of Hsp21 based on homology modeling, cryo-EM, cross-linking mass spectrometry, NMR, and small-angle X-ray scattering. Our data suggest a dodecameric arrangement of two trimer-of-dimer discs stabilized by the C-terminal tails, possibly through tail-to-tail interactions between the discs, mediated through extended IXVXI motifs. Our model further suggests that six N-terminal arms are located on the outside of the dodecamer, accessible for interaction with client proteins, and distinct from previous undefined or inwardly facing arms. To test the importance of the IXVXI motif, we created the point mutant V181A, which, as expected, disrupts the Hsp21 dodecamer and decreases chaperone activity. Finally, our data emphasize that sHsp chaperone efficiency depends on oligomerization and that client interactions can occur both with and without oligomer dissociation. These results provide a generalizable workflow to explore sHsps, expand our understanding of sHsp structural motifs, and provide a testable Hsp21 structure model to inform future investigations.

The evolution of the molecular response to stress and its relevance to trauma and stressor-related disorders

The experience of "stress", in its broadest meaning, is an inevitable part of life. All living creatures have evolved multiple mechanisms to deal with such threats and challenges and to avoid damage to the organism that may be incurred from these stress responses. Trauma and stressor-related disorders are psychiatric conditions that are caused specifically by the experience of stress, though depression, anxiety and some other disorders may also be unleashed by stress. Stress, however, is not a mandatory criterion of these diagnoses. This article focuses on the evolution of the neurochemicals involved in the response to stress and the systems in which they function. This includes the skin and gut, and the immune system. Evidence suggests that responses to stress are evolutionarily highly conserved, have wider involvement than the hypothalamic pituitary adrenal stress axis alone, and that excessive stress responses can produce stressor-related disorders in both humans and animals.

Chaperone function and mechanism of small heat-shock proteins

Small heat-shock proteins (sHSPs) are ubiquitous ATP-independent molecular chaperones that play crucial roles in protein quality control in cells. They are able to prevent the aggregation and/or inactivation of various non-native substrate proteins and assist the refolding of these substrates independently or under the help of other ATP-dependent chaperones. Substrate recognition and binding by sHSPs are essential for their chaperone functions. This review focuses on what natural substrate proteins an sHSP protects and how it binds the substrates in cells under fluctuating conditions. It appears that sHSPs of prokaryotes, although being able to bind a wide range of cellular proteins, preferentially protect certain classes of functional proteins, such as translation-related proteins and metabolic enzymes, which may well explain why they could increase the resistance of host cells against various stresses. Mechanistically, the sHSPs of prokaryotes appear to possess numerous multi-type substrate-binding residues and are able to hierarchically activate these residues in a temperature-dependent manner, and thus act as temperature-regulated chaperones. The mechanism of hierarchical activation of substrate-binding residues is also discussed regarding its implication for eukaryotic sHSPs.

Multilevel structural characteristics for the natural substrate proteins of bacterial small heat shock proteins

Small heat shock proteins (sHSPs) are ubiquitous molecular chaperones that prevent the aggregation of various non-native proteins and play crucial roles for protein quality control in cells. It is poorly understood what natural substrate proteins, with respect to structural characteristics, are preferentially bound by sHSPs in cells. Here we compared the structural characteristics for the natural substrate proteins of Escherichia coli IbpB and Deinococcus radiodurans Hsp20.2 with the respective bacterial proteome at multiple levels, mainly by using bioinformatics analysis. Data indicate that both IbpB and Hsp20.2 preferentially bind to substrates of high molecular weight or moderate acidity. Surprisingly, their substrates contain abundant charged residues but not abundant hydrophobic residues, thus strongly indicating that ionic interactions other than hydrophobic interactions also play crucial roles for the substrate recognition and binding of sHSPs. Further, secondary structure prediction analysis indicates that the substrates of low percentage of β-sheets or coils but high percentage of α-helices are un-favored by both IbpB and Hsp20.2. In addition, IbpB preferentially interacts with multi-domain proteins but unfavorably with α + β proteins as revealed by SCOP analysis. Together, our data suggest that bacterial sHSPs, though having broad substrate spectrums, selectively bind to substrates of certain structural features. These structural characteristic elements may substantially participate in the sHSP-substrate interaction and/or increase the aggregation tendency of the substrates, thus making the substrates more preferentially bound by sHSPs.

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.