CUSTOMIZED SECRETION CHAPERONES IN PATHOGENIC BACTERIA

Pathogenic yersiniae secrete about a dozen anti-host proteins, the Yop s, by a pathway which does not involve cleavage of a classical signal peptide. The Yop secretory apparatus, called Ysc, for Yop secretion, i s the archetype of type III secretion systems (which serve for the sec retion of virulence proteins by several animal and plant pathogens) an d is related to the flagellar assembly apparatus. The Yop secretion si gnal is N-terminal but has not been defined to date. Apart from the Ys c machinery, secretion of at least four Yops requires cytoplasmic prot eins called Syc (for specific Yop chaperone). Each Syc protein binds t o its cognate Yop. Unlike most cytoplasmic chaperones, these proteins do not have an ATP-binding domain, and are presumably devoid of ATPase activity. They share a few common properties: an acidic pi, a size in the range of 15-20 kDa, and a putative amphipathic a-helix in the C-t erminal portion. They were recently shown to have counterparts in othe r pathogenic bacteria, where they appear to have a similar function.