Molecular characterization and assembly of the needle complex of the Salmonella typhimurium type III protein secretion system

Many bacterial pathogens of plants and animals have evolved a specialized p rotein-secretion system termed type III to deliver bacterial proteins into host cells. These proteins stimulate or interfere with host cellular functi ons for the pathogen's benefit. The Salmonella typhimurium pathogenicity is land 1 encodes one of these systems that mediates this bacterium's ability to enter nonphagocytic cells. Several components of this type III secretion system are organized in a supramolecular structure termed the needle compl ex. This structure is made of discrete substructures including a base that spans both membranes and a needle-like projection that extends outward from the bacterial surface. We demonstrate here that the type III secretion exp ort apparatus is required for the assembly of the needle substructure but i s dispensable for the assembly of the base. We show that the length of the needle segment is determined by the type III secretion associated protein I nvJ, We report that InvG, PrgH, and PrgK constitute the base and that Prgl is the main component of the needle of the type III secretion complex. Prgl homologs are present in type III secretion systems from bacteria pathogeni c for animals but are absent from bacteria pathogenic for plants. We hypoth esize that the needle component may establish the specificity of type III s ecretion systems in delivering proteins into either plant or animal cells.