Characterizing the interaction of a type VII-secreted antimycobacterial toxin with its small helical partner proteins

The type VII secretion system (T7SS) is a membrane-embedded protein export pathway found in mycobacteria and Gram-positive bacteria. Recently it was shown that Mycobacterium abscessus uses its ESX-4 variant of the T7SS to secrete a toxin, EatA, which targets arabinogalactan present in the mycobacterial cell envelope. Prior to its export, EatA forms a complex with a pair of small proteins from the WXG100 family, TapA1 and TapA2. Here we investigated a structural model of the EatA N-terminal domain in complex with TapA1 and TapA2 using site-directed mutagenesis and bacterial 2-hybrid assays. Our results are consistent with the three proteins forming a stacked bundle of alpha-helices. Structural modelling also predicted an interaction of the EatA-TapA1-TapA2 complex with EsxT-EsxU, a second pair of WXG100-family proteins that are likely required for the mechanistic operation of ESX-4. Whilst we could demonstrate a potential interaction between TapA2 and EsxT by bacterial 2-hybrid analysis, we were not able to purify a complex of all five proteins.