Bacterial type III secretion systems: A complex device for delivery of bacterial effector proteins into eukaryotic host cells

Abstract

Virulence-associated type III secretion systems serve the injection of bacterial effector proteins into eukaryotic host cells. They are able to secrete a great diversity of substrate proteins in order to modulate host cell function, and have evolved to sense host cell contact and to inject their substrates through a translocon pore in the host cell membrane. T3SS substrates contain an N-terminal signal sequence and often also a chaperone-binding domain for cognate T3SS chaperones. These signals guide the substrates to the machine where substrates are unfolded and handed over to the secretion channel formed by the transmembrane domains of the export apparatus components and by the the needle filament. Secretion itself is driven by the proton motif force across the bacterial inner membrane. The needle filament measures 20–150 nm in length and is crowned by a needle tip that mediates host cell sensing. Secretion through T3SS is a highly regulated process with early, intermediate, and late substrates. A strict secretion hierarchy is required to build an injectisome capable of reaching, sensing, and penetrating the host cell membrane, before host cell acting effector proteins are deployed. Here we review the recent progress on elucidating assembly, structure, and function of T3SS injectisomes.