Model for the proposed mechanisms that promote MSHA extension and surface piliation.

In wildtype cells (left), PilT promotes initiation of pilus extension, potentially by maturing the minor pilins into an initiation complex. Then, MshE can promote processive extension of the pilus in a c-di-GMP-dependent manner. The PilT-dependent accessory retraction ATPase motor PilU is dispensable for promoting pilus extension, but it can facilitate extension if PilT’s ATPase activity is inactivated. If PilT is absent (middle), initiation is inhibited, so processive extension is greatly diminished. Finally, mutations to the MshA major pilin (right), like MshAP31S, can promote initiation of MSHA extension even in the absence of PilT, suggesting that the structure of the major pilin is an important determinant of pilus extension.