Structural characterization of protein transport by the Type 9 Secretion System

Abstract Secretion Systems are protein export machines that enable bacteria to exploit their environment through the release of protein effectors. The Type 9 Secretion System (T9SS) is an essential virulence determinant in the oral pathogens responsible for periodontitis and immune dysfunction. We have trapped and structurally characterised the Type 9 translocon in the process of substrate transport. The active translocon is augmented by a large periplasmic structure incorporating a homologue of the canonical periplasmic chaperone Skp. Substrate proteins bind to a carrier protein within the translocon pore and energy is required to release this substrate-carrier protein complex from the translocon. We infer a unique protein transport mechanism in which a carrier protein is used to pull substrate proteins through the translocon using energy from a trans-envelope power chain.