Kinesin-8s are dual-activity motor proteins that can move processively on microtubules and depolymerize microtubule plus-ends, but their mechanism of combining these distinct activities remains unclear. We addressed this by obtaining cryo-EM structures (2.6–3.9 Å) of Candida albicans Kip3 in different catalytic states on the microtubule lattice and on a curved microtubule end mimic. We also determined a crystal structure of microtubule-unbound CaKip3-ADP (2.0 Å) and analyzed the biochemical activity of CaKip3 and kinesin-1 mutants. These data reveal that the microtubule depolymerization activity of kinesin-8 originates from conformational changes of its motor core that are amplified by dynamic contacts between its extended loop-2 and tubulin. On curved microtubule ends, loop-1 inserts into preceding motor domains, forming head-to-tail arrays of kinesin-8s that complement loop-2 contacts with curved tubulin and assist depolymerization. On straight tubulin protofilaments in the microtubule lattice, loop-2-tubulin contacts inhibit conformational changes in the motor core, but in the ADP-Pi state these contacts are relaxed, allowing neck-linker docking for motility. We propose that these tubulin shape-induced alternations between pro-microtubule-depolymerization and pro-motility kinesin states, regulated by loop-2, are the key to the dual activity of kinesin-8 motors.

Kinesin-8 是双活性运动蛋白，可以在微管上持续移动并使微管正端解聚，但其结合这些不同活性的机制仍不清楚。我们通过在微管晶格和弯曲微管末端模拟物上获得不同催化状态的白色念珠菌Kip3 的冷冻电镜结构 (2.6–3.9 Å) 来解决这个问题。我们还确定了未结合微管的Ca Kip3-ADP (2.0 Å) 的晶体结构，并分析了Ca Kip3 和驱动蛋白-1 突变体的生化活性。这些数据表明，kinesin-8 的微管解聚活性源自其运动核心的构象变化，这种变化通过其延伸的 Loop-2 和微管蛋白之间的动态接触而放大。在弯曲的微管末端，loop-1 插入前面的运动域，形成头尾相连的驱动蛋白 8 阵列，补充 Loop-2 与弯曲微管蛋白的接触并协助解聚。在微管晶格中的直微管蛋白原丝上，环 2-微管蛋白接触抑制运动核心的构象变化，但在 ADP-Pi 状态下，这些接触松弛，允许颈连接器对接以实现运动。我们提出，这些由微管蛋白形状诱导的促微管解聚和促运动驱动蛋白状态之间的交替（由loop-2调节）是驱动蛋白8马达双重活性的关键。