Figure 7. Speculative ‘ratcheting’ model for processivity.

The model was inspired by the fact that the TOG:tubulin engagement status regulates the lattice-binding activity of the basic region. We assume two limiting states for Stu2 on the microtubule end: a ‘loaded’ state wherein the basic region binds weakly to the lattice because at least two TOGs are engaged with tubulins, and an ‘empty’ state wherein the basic region binds strongly to the lattice because the TOGs have released their tubulins to the microtubule. In this ‘post-release’ state, strong interactions between the basic region and the lattice help retain the empty polymerase near the growing end where it is poised to capture ‘fresh’ unpolymerized tubulins for another round of activity. Capture and incorporation of fresh tubulins drives movement with the growing end.