Structural and binding studies of the mycobacterial heat shock protein reveal a silent state and offer insights into dendritic cell activation.

HtpGMtb of Mycobacterium tuberculosis is an ATP-dependent heat shock protein that assists the correct folding of nascent and stress-accumulated misfolded proteins, in concert with other chaperones. Besides playing a role in stress response, it is able to elicit an immune response against M. tuberculosis infection by activating Dendritic Cells in a Toll Like Receptor 4-mediated manner. However, we lack a full understanding of the molecular determinants of HtpGMtb catalytic activity and Toll Like Receptor 4 activation, due to the lack of structural and biophysical data. Here, we report the first crystal structure of HtpGMtb, in complex with the non-hydrolysable form of ATP. The crystal structure reveals that the HtpGMtb dimer adopts a conformationally silent structure, that precludes the dimerisation of the chaperone catalytic domains needed for ATP hydrolysis. Also, binding studies show that HtpGMtb directly interacts with Toll Like Receptor 4 with a nanomolar affinity, and that this interaction allows HtpGMtb dimer to engage two host receptor molecules. This finding suggests that activation of Toll Like Receptor 4 by HtpGMtb is due to its ability to induce the dimerisation of the host receptor, an essential step for initiating the entire signaling cascade.