YscP, a key player in the type three secretion system of "Yersinia enterocolitica"

The Yersinia injectisome needle has a constant length of around 60nm. Mutants affected in the yscP gene display abnormally long needles, suggesting that needle length is genetically controlled and that YscP is involved in this process. First, both N- and C-term of YscP were shown to be required for proper needle length control. In contrast, the central part could be shortened or lengthened without loss of function, but giving a needle length strictly proportional to the number of residues of YscP. This led us to assign the function of a ruler to YscP. In addition to its role in the determination of the needles length, YscP is required for Yop secretion. The C-term part of YscP, required for proper needle length control, was shown to be also crucial for Yop secretion. Thus it is probably involved in the substrate-specificity switch of the machinery, from the needle component YscF to Yops. The study of this region by Hydrophobic Cluster Analysis led to a characterization of a new domain that we called T3S4 (TypeSecretion SubstrateSpecificity Switch) and which can be found in all counterparts of YscP in other injectisomes but also in flagellum. The conservation of the T3S4 structure suggests a similar function for all these proteins. The T3S4 domains found in injectisomes happened to be partially exchangeable. The overall structure of the domain seems to be the critical point. This is further supported by the fact that single alaninereplacement of the few conserved amino acids not affecting the structure, did not impact the functions. Since YscP is secreted by the injectisome, we analyzed whether its export is necessary for its functions. Two original export signals were found in YscP: one in the N-term (aa 1-35) and one more central (aa 97-137). These two signals exactly correspond to the N-term regions involved in needle length control. When YscP is deprived from its export signals, it still switches the substrate specificity but it cannot control needle length any more. These data suggest a model in which YscP export is linked to needle length control. Having evolved such a complex process for length control stresses the necessity of a certain length. Indeed, the length has evolved to match specific structures at the bacterial and host cell surfaces.