Structure of the cytoplasmic domain of FlhA and implication for flagellar type III protein export.

Summary FlhA is the largest integral membrane component of the flagellar type III protein export apparatus of Salmonella and is composed of an N-terminal transmembrane domain (FlhATM) and a C-terminal cytoplasmic domain (FlhAC). FlhAC is thought to form a platform of the export gate for the soluble components to bind to for efficient delivery of export substrates to the gate. Here, we report a structure of FlhAC at 2.8 A resolution. FlhAC consists of four subdomains (ACD1, ACD2, ACD3 and ACD4) and a linker connecting FlhAC to FlhATM. The sites of temperature-sensitive (ts) mutations that impair protein export are distributed to all four domains, with half of them at subdomain interfaces. Analyses of the ts mutations and four suppressor mutations to the G368C ts mutation suggested that FlhAC changes its conformation for its function. Molecular dynamics simulation demonstrated an open-close motion with a 5–10 ns oscillation in the distance between ACD2 and ACD4. These results along with further mutation analyses suggest that a dynamic domain motion of FlhAC is essential for protein export.