Characterization of differential pore-forming activities of ESAT-6 proteins from Mycobacterium tuberculosis and Mycobacterium smegmatis.

Mycobacterium tuberculosis ESAT-6 (MtbESAT-6) plays essential roles in pathogenesis. MtbESAT-6 exhibits a unique pore-forming activity (PFA) that is not found in its ortholog from non-pathogenic Mycobacterium smegmatis (MsESAT-6). Here, we characterized the differential PFAs and found that exchange of I25-H26/T25-A26 between two proteins reciprocally affected their PFAs. MtbESAT-6(IH/TA) had ~ 40% reduction, while MsESAT-6(TA/IH) fully acquired its activity similar to MtbESAT-6. Mutations of A17E, K38T, N67L or R74Q on MtbESAT-6(IH/TA) further reduced the activity, with MtbESAT-6(IH/TA-17) being the lowest. This study suggests I25-H26 as the pH-sensor essential for MsESAT-6 to fully acquire the activity, while multiple residues contributed to MtbESAT-6 PFA.