Structural studies of mycobacterial HptG reveal a silent state and offer insights into TLR4 activation

Abstract

HtpG of Mycobacterium tuberculosis (HtpGMtb) is an ATP-dependent chaperone that assists the correct folding of nascent and stress-accumulated misfolded proteins, in concert with other chaperones. Beside playing a role in stress response, it is able to elicit an immune response against M. tuberculosis infection by activating Dendritic Cells in a TLR4-mediated manner. However, we lack a full understanding of the molecular determinants of HtpGMtb catalytic activity and TLR4 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, AMPPNP. 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 TLR4 with a nanomolar affinity, and that this interaction allows HtpGMtb dimer to engage two TLR4 molecules. This finding suggests that activation of TLR4 by HtpGMtb is due to its ability to induce intra-cellular receptor dimerisation in an LPS-like mode.