Length control of the "Yersinia" injectisome

Many pathogenic bacteria harbor a type III secretion system to translocate effector proteins from the bacterium into the host cell cytosol. This one-step translocation requires a nanomachinery which was termed injectisome. It consists of a basal body, spanning the two bacterial membranes, and a needle-like structure, bridging the distance between the bacterium and the target cell. Control of the length of the type III secretion injectisome needle is crucial for a correct function. In Yersinia, the YscP protein is involved in needle length control: the number of YscP residues directly correlates with needle length. In this thesis, this correlation was shown to be dependent on the secondary structure of YscP. By substitution of individual residues, needle length could be altered without changing the number of residues in YscP. The molecular ruler model was proposed for length control of the Yersinia injectisome needle. There are, however, two possibilities for the molecular ruler model regarding the amount of YscP needed for regulation of the needle length of one injectisome. In the static model, only one molecule of YscP and in a more dynamic model, several proteins are required for length control of one needle. Here, it was demonstrated that partially diploid bacteria, expressing a short and a long YscP simultaneously assemble distinct sets of short and long needles. These results suggest that only one YscP molecule is required for length control of one needle. In Yersinia, the YscU protein (a member of the export machinery) was suggested to be involved in the substrate specificity switch. Here, YscU was demonstrated to play a role in substrate recognition but not in substrate switching. Taken together, a refined model for length control of the Yersinia injectisome needle is proposed in this thesis, confirming the role of YscP as a molecular ruler.