Dodecameric structure of a small Heat Shock Protein from Mycobacterium marinum M.

Small heat shock proteins (sHSPs) are ATP-independent molecular chaperones present ubiquitously in all kingdoms of life. The subunits are low molecular weight species that associate to form higher order structures. Under conditions of stress, sHSPs prevent aggregation of substrate proteins by undergoing rapid changes in their conformation or stoichiometry. Polydispersity and dynamic nature of these proteins have made structural investigations through crystallography a daunting task. In pathogens like Mycobacteria, sHSPs are immuno-dominant antigens, enabling survival of the pathogen within the host and contributing to disease persistence. To understand the mechanism of oligomerisation and mode of action of these proteins, we characterised small heat shock proteins from Mycobacterium marinum M and determined the crystal structure of one of these. The protein crystallised in three different conditions as dodecamers, with dimers arranged in a tetrahedral fashion to form a closed cage-like architecture. The structures illustrate involvement of dimer-dimer associations, mediated via the N- and C-termini, in facilitating oligomer formation and the involvement of dimerisation loop in providing lattice contacts. Interestingly, two of the dodecamers are peptide bound structures where a short pentapeptide corresponding to the amino acid sequence GRLLP is observed in a hydrophobic groove towards the N-terminus of the protein. Hydrophobic and hydrogen bond interactions stabilise the peptide at this interface and provide important insights into the mode of sHSP-substrate interaction. Further, it was observed that in the presence of ATP, the protein undergoes conformational changes leading to a reduction in its molecular chaperoning activity. This article is protected by copyright. All rights reserved.