Dissection of a type VI secretion system in Edwardsiella tarda

Bacterial pathogens use different protein secretion systems to deliver virulence factors. Recently, a novel secretion system was discovered in several Gram-negative bacterial pathogens, and was designated as the type VI secretion system (T6SS). In Edwardsiella tarda, a partial E. tarda virulent protein (EVP) gene cluster was implicated in protein secretion. Here, we identified the entire EVP cluster as a T6SS and two additional secreted proteins (EvpI, a homologue of VgrG, and EvpP) were found. We systematically mutagenized all the 16 EVP genes and found that the secretion of EvpP was dependent on 13 EVP proteins including EvpC (a homologue of Hcp) and EvpI but not EvpD and EvpJ. All EVP mutants except Delta evpD were attenuated in blue gourami fish. The 16 EVP proteins can be grouped according to their functions and cellular locations. The first group comprises 11 non-secreted and possibly intracellular apparatus proteins. Among them, EvpO, a putative ATPase which contained a Walker A motif, showed possible interactions with three EVP proteins (EvpA, EvpL and EvpN). The second group includes three secreted proteins (EvpC, EvpI and EvpP). The secretion of EvpC and EvpI is mutually dependent, and they are required for the secretion of EvpP. The interaction between EvpC and EvpP was demonstrated. Lastly, two proteins (EvpD and EvpJ) are not required for the T6SS- dependent secretion.