In vitro reconstitution of Type III secretion system - basic and applied research -

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SptP is a virulence effector protein of Salmonella that is involved in bacterial invasion into a host cell. For effective secretion, SptP forms a complex with SptP-specific chaperone SicP through its chaperone-binding domain, residues 35-139. We suggested that residues 106-136 of SptP are important for complex formation with SicP by in vitro reconstitution experiments. The dissociation of the SptP/SicP complex requires ATPase activity of InvC. So, we purified InvC, and then characterized its ATPase activity and tested whether InvC ATPase dissociates the complex. InvC ATPase activity showed positive cooperativity, suggesting that InvC subunits form a homo-oligomer. Furthermore the ATPase activity was stimulated in the presence of liposomes, suggesting that InvC interacts with plasma membrane and the activity is regulated by the interaction. In the presence of liposomes and SptP/SicP complexes, InvC ATPase activity was not increased further and did not dissociate SptP/SicP complexes.