A Helical Transport Mechanism for Type III Secretion

Abstract

Gram-negative pathogenic bacteria, like Salmonella typhimurium and Shigella flexneri, employ the Type III Secretion System (T3SS) to infect human cells. The T3SS is a large protein secretion channel that assembles to span a ∼50nm gap between the bacterial and target cell walls. A key component of the S. typhimurium SPI-1 T3SS is the 80 residue needle subunit PrgI which polymerizes to form a 25 A wide channel through which proteins are transported. During needle assembly, the PrgI subunits pass through the nascent channel before attaching to the tip.We have studied the mechanism of PrgI transport using near-atomistic molecular dynamics simulations. We found that the channel's inward facing amino acids and its helical symmetry direct PrgI diffusion along a helical pathway (the i+1 crystallographic axis) with 2.4nm axial displacement per 360 degrees rotation. In vivo assays have shown that mutations of channel residues inhibit the subunit secretion required for needle self-assembly.Our combined studies evidence that the channel surface plays an active role in substrate secretion, rather than being a passive corridor for linear diffusion. Our evidence of rotation-translation coupling suggests the that the T3S needle might rotate during effector secretion.