Kinesin-8/Kip3 requires beta tubulin tail for depolymerase activity

Carboxy-terminal tails (CTTs) of tubulin proteins are sites of regulating microtubule function. We previously conducted a genetic interaction screen and identified Kip3, a kinesin-8 motor, as potentially requiring the β-tubulin CTT (β-CTT) for function. Here we use budding yeast to define how β-CTT promotes Kip3 function and the features of β-CTT that are important for this mechanism. We find that β-CTT is necessary for Kip3 depolymerase activity, but not for microtubule binding and motility. Mutant yeast cells lacking β-CTT show increased accumulation of Kip3 at plus ends and along microtubules, but no increase in catastrophe when Kip3 is overexpressed. In vitro experiments show that the β-CTT is necessary for Kip3 to form a tight complex with soluble tubulin but is unnecessary for Kip3 to bind tubulin in the microtubule lattice. These results suggest a model in which β-CTT promotes Kip3 depolymerase activity by supporting a Kip3-tubulin binding state that is only accessible at the microtubule plus end or in solution.