Structural Superimposition

Abstact

HtpG(Mtb) 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 HtpG(Mtb) 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 HtpG(Mtb), in complex with the non-hydrolysable form of ATP. The crystal structure reveals that the HtpG(Mtb) dimer adopts a conformationally silent structure, that precludes the dimerisation of the chaperone catalytic domains needed for ATP hydrolysis. Also, binding studies show that HtpG(Mtb) directly interacts with Toll Like Receptor 4 with a nanomolar affinity, and that this interaction allows HtpG(Mtb) dimer to engage two host receptor molecules. This finding suggests that activation of Toll Like Receptor 4 by HtpG(Mtb) is due to its ability to induce the dimerisation of the host receptor, an essential step for initiating the entire signaling cascade.