The inner rod protein controls substrate switching and needle length in a Salmonella type III secretion system

Abstract

Type III secretion machines are essential for the biology of manybacteria that are pathogenic or symbiotic for animals, plants, orinsects. They exert their function by delivering bacterial effectorproteins into target eukaryotic cells. The core component of thesemachines is the needle complex, a multiprotein structure thatspans the bacterial envelope and serves as a conduit for proteinsthat transit this secretion pathway. The needle complex is composed of a multiring base embedded in the bacterial envelope anda filament-like structure, the needle, that projects from the bacterial surface and is linked to the base by the inner rod. Assembly ofthe needle complex proceeds in a step-wise fashion that is initiated by the assembly of the base and is followed by the export ofthe building subunits for the needle and inner rod substructures.Once assembled, the needle complex reprograms its specificity andbecomes competent for the secretion of effector proteins. Herethrough genetic, biochemical, and electron microscopy analysesof the Salmonella inner rod protein subunit PrgJ we present evidence that the assembly of the inner rod dictates the timing ofsubstrate switching and needle length. Furthermore, the identification of mutations in PrgJ that specifically alter the hierarchy ofprotein secretion provides additional support for a complex role ofthe inner rod substructure in type III secretion.