YscP, a Yersinia protein required for Yop secretion that is surface exposed, and released in low Ca2+

Yersinia Ysc apparatus is made of more than 20 proteins, 11 of which have h omologues in many type III systems. Here, we characterize YscP from Yersini a enterocolitica, This 515-residue protein has a high proline content, a la rge tandem repetition and a slow migration in SDS-PAGE, Unlike the products of neighbouring genes, it has a counterpart only in Pseudomonas aeruginosa and it varies even between Yersinia Ysc machineries. An yscP(Delta 97-465) mutant was unable to secrete any Yop, even under conditions overcoming fee dback inhibition of Yop synthesis. Interestingly, a cloned yscP(Delta 57-32 4) from Yersinia pestis introduced in the yscP(Delta 97-465) mutant can sus tain a significant Yop secretion and thus partially complemented the mutati on. This explains the leaky phenotype observed with the yscP mutant of Y, p estle, In accordance with this secretion deficiency, YscP is required for t he delivery of Yop effecters into macrophages. Mechanical shearing, immunol abelling and electron microscopy experiments showed that YscP is exposed at the bacterial surface when bacteria are incubated at 37 degrees C in the p resence of Ca2+ and thus do not secrete Yops, At 37 degrees C, when Ca2+ io ns are chelated, YscP is released like a Yop protein. We conclude that YscP is a part of the Ysc injectisome which is localized at the bacterial surfa ce and is destabilized by Ca2+ chelation.