Type‐III secretion pore formed by flagellar protein FliP

During assembly of the bacterial flagellum, protein subunits that form the exterior structures are exported through a specialized secretion apparatus energized by the proton gradient. This category of protein transport, together with the similar process that occurs in the injectisomes of gram‐negative pathogens, is termed type‐III secretion. The membrane‐embedded part of the flagellar export apparatus contains five essential proteins: FlhA, FlhB, FliP, FliQ and FliR. Here, we have undertaken a variety of experiments that together support the proposal that the protein‐conducting conduit is formed primarily, and possibly entirely, by FliP. Chemical modification experiments demonstrate that positions near the center of certain FliP trans‐membrane (TM) segments are accessible to polar reagents. FliP expression sensitizes cells to a number of chemical agents, and mutations at predicted channel‐facing positions modulate this effect. Multiple assays are used to show that FliP suffices to form a channel that can conduct a variety of medium‐sized, polar molecules. Conductance properties are strongly modulated by mutations in a methionine‐rich loop that is predicted to lie at the inner mouth of the channel, which might form a gasket around cargo molecules undergoing export. The results are discussed in the framework of an hypothesis for the architecture and action of the cargo‐conducting part of the type‐III secretion apparatus.