Nα-acetylation of EsxA is required for mycobacterial cytosolic translocation and virulence

Mycobacterium tuberculosis virulence factors, EsxA and EsxB, are secreted as a heterodimer (EsxA:B) and play an essential role in mycobacterial phagosomal escape and cytosolic translocation. Current studies support a model that EsxA must dissociate from its chaperon EsxB at low pH in order for EsxA to interact with host membranes. However, the mechanism of the heterodimer separation is not clear. In the present study, we have obtained evidence that the Nα-acetylation of Thr2 on EsxA, a post-translational modification that is present in mycobacteria, but absent in E. coli, is required for the heterodimer separation. The point mutations at Thr2 without Nα-acetylation inhibited the heterodimer separation and hence prevented EsxA from interacting with the membranes, which resulted in attenuated mycobacterial cytosolic translocation and virulence. Molecular dynamic simulation showed that at low pH the Nα-acetylated Thr2 made direct and frequent bind-and-release contacts with EsxB, which generates a dragging force to pull EsxB away from EsxA.