3.A.7.17.1
The EsxA/B protein secretion pathway system (EPSP) (Anderson et al. 2011; Burts et al. 2005; Chen et al. 2012; Sundaramoorthy et al. 2008).  EssC is an integral component of the bacterial """"Type VII"""" secretion system (T7SS). The protein is predicted to consist of an intracellular repeat of forkhead-associated (FHA) domains at the N-terminus, two transmembrane helices and three P-loop containing ATPase-type domains, D1-3, forming the C-terminal intracellular segment. Zoltner et al. 2016 presented crystal structures of the N-terminal FHA domains (EssC-N) and a C-terminal fragment EssC-C from Geobacillus thermodenitrificans, encompassing two of the ATPase-type modules, D2 and D3. Module D2 binds ATP with high affinity whereas D3 does not. The EssC-N and EssC-C constructs are monomeric in solution but the full-length recombinant protein, with molecular weight about 169 kDa, forms a multimer of approximately 1 MDa. The observation of protomer contacts in the crystal structure of EssC-C together with similarity to the DNA translocase FtsK, suggests a model for a hexameric EssC assembly. Such an observation potentially identifies the key component of pore formation required for secretion. The extreme C-terminal ATPase domain appears essential for EssC activity as a key part of the T7SS whilst D2 and FHA domains are required for the production of a stable and functional protein (Zoltner et al. 2016). TheTMSs of EssA and EssB interact, regulating secretion (Ahmed et al. 2018). This system resembles the ESX systems of Mycobacterium tuberculosis (TC# 3.A.24).