Role of DnaK in HspR-HAIR interaction of Mycobacterium tuberculosis

Heat shock proteins (Hsps) are a highly conserved family of proteins. The regulation of expression of Hsps in Mycobacterium tuberculosis, is regulated both positively and negatively by alternate sigma factors and transcriptional DNA repressors, respectively. HspR is a negative regulator of expression of hsps, DnaK, ClpB, and Acr2 in M. tuberculosis. In this study, we expressed the M. tuberculosis HspR (MtHspR) in E. coli, and functionally characterized it. MtHspR independently bound to its putative cognate DNA, the HAIR element. MtHspR was found to exist in a dynamic mixture of dimeric and monomeric protein and presence of salt led to the formation of trimers which lacked the DNA binding activity. MtHspR was found to be heat stable with a Tm of 66°C. HspR-HAIR binding was stable upto 60°C suggesting that MtHspR is not the heat stress sensor. Mycobacterial DnaK was found to interact directly with MtHspR-HAIR complex in vitro in an ATP independent manner. The DnaK-HspR-HAIR binding pattern altered at high temperatures in the presence of aggregated α-casein substrate, suggesting that DnaK may indirectly be responding to heat stress in a feedback loop mechanism.