The stability of microtubules during the cell-cycle is regulated by a number of cellular factors, some of which stabilize microtubules and others that promote breakdown. XKCM1 is a kinesin-like protein that induces microtubule depolymerization and is required for mitotic spindle assembly. We have examined the binding and depolymerization effects of XKCM1 on different tubulin polymers in order to learn about its mechanism of action. Zinc-induced tubulin polymers, characterized by an anti-parallel protofilament arrangement, are depolymerized by XKCM1, indicating that this enzyme acts on a single protofilament. GDP-tubulin rings, which correspond to the low-energy state of tubulin, are stable only under conditions that inhibit XKCM1 depolymerizing activity, but can be stabilized by XKCM1 bound to AMPPNP. Tubulin polymers made of subtilisin-treated tubulin (lacking the tubulin C-terminal tail) are resistant to XKCM1-induced depolymerization, suggesting that the interaction of the acidic tail of tubulin with basic residues in XKCM1 unique to Kin I proteins is required for depolymerization.