FIGURE 18-15 Proteins that destabilize the ends of microtubules. (a) A member of the kinesin-13 family enriched at a microtubule end can enhance the disassembly of that end. [Although depolymerization of the (+) end is shown, kinesin-13 can also depolymerize the (−) end.] These proteins are ATPases, and ATP enhances their activity by dissociating them from the αβ-tubulin dimer. (b) Op18/stathmin binds selectively to curved protofilaments and enhances their dissociation from a microtubule end. This protein’s activity is inhibited by phosphorylation.