HtpG Is a Metal-Dependent Chaperone Which Assists the DnaK/DnaJ/GrpE Chaperone System of Mycobacterium tuberculosis via Direct Association with DnaJ2.

Protein folding is a crucial process in maintaining protein homeostasis, also known as proteostasis, in the cell. The requirement for the assistance of molecular chaperones in the appropriate folding of several proteins has already called into question the previously held view of spontaneous protein folding. These chaperones are highly ubiquitous cellular proteins, which not only help in mediating the proper folding of other nascent polypeptides but are also involved in refolding of the misfolded or the aggregated proteins. Hsp90 family proteins such as high-temperature protein G (HtpG) are abundant and ubiquitously expressed in both eukaryotic and prokaryotic cells. Although HtpG is known as an ATP-dependent chaperone protein in most organisms, function of this protein remains obscured in mycobacterial pathogens. Here, we aim to investigate significance of HtpG as a chaperone in the physiology of Mycobacterium tuberculosis. We report that M. tuberculosis HtpG (mHtpG) is a metal-dependent ATPase which exhibits chaperonin activity towards denatured proteins in coordination with the DnaK/DnaJ/GrpE chaperone system via direct association with DnaJ2. Increased expression of DnaJ1, DnaJ2, ClpX, and ClpC1 in a Δ mutant strain further suggests cooperativity of mHtpG with various chaperones and proteostasis machinery in M. tuberculosis. M. tuberculosis is exposed to variety of extracellular stressful conditions and has evolved mechanisms to endure and adapt to the adverse conditions for survival. mHtpG, despite being dispensable for M. tuberculosis growth under conditions, exhibits a strong and direct association with DnaJ2 cochaperone and assists the mycobacterial DnaK/DnaJ/GrpE (KJE) chaperone system. These findings suggest the potential role of mHtpG in stress management of the pathogen. Mycobacterial chaperones are responsible for folding of nascent protein as well as reactivation of protein aggregates. M. tuberculosis shows differential adaptive response subject to the availability of mHtpG. While its presence facilitates improved protein refolding via stimulation of the KJE chaperone activity, in the absence of mHtpG, M. tuberculosis enhances expression of DnaJ1/J2 cochaperones as well as Clp protease machinery for maintenance of proteostasis. Overall, this study provides a framework for future investigation to better decipher the mycobacterial proteostasis network in the light of stress adaptability and/or survival.