Kinesins are microtubule-dependent motor proteins, some of which moonlight as microtubule polymerases, such as the yeast protein Kip2. Here, we show that the CLIP-170 ortholog Bik1 stabilizes Kip2 at microtubule ends where the motor domain of Kip2 promotes microtubule polymerization. Live-cell imaging and mathematical estimation of Kip2 dynamics reveal that disrupting the Kip2–Bik1 interaction aborts Kip2 dwelling at microtubule ends and abrogates its microtubule polymerization activity. Structural modeling and biochemical experiments identify a patch of positively charged residues that enables the motor domain to bind free tubulin dimers alternatively to the microtubule shaft. Neutralizing this patch abolished the ability of Kip2 to promote microtubule growth both in vivo and in vitro without affecting its ability to walk along microtubules. Our studies suggest that Kip2 utilizes Bik1 as a cofactor to track microtubule tips, where its motor domain then recruits free tubulin and catalyzes microtubule assembly.

中文翻译：

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驱动蛋白 Kip2 的运动结构域促进微管尖端的微管聚合

驱动蛋白是微管依赖性运动蛋白，其中一些作为微管聚合酶兼职，例如酵母蛋白 Kip2。在这里，我们证明 CLIP-170 直系同源物 Bik1 可以稳定 Kip2 的微管末端，其中 Kip2 的运动结构域促进微管聚合。 Kip2 动力学的活细胞成像和数学估计表明，破坏 Kip2-Bik1 相互作用会中止 Kip2 在微管末端的驻留，并消除其微管聚合活性。结构建模和生化实验确定了一片带正电的残基，使运动结构域能够将游离的微管蛋白二聚体选择性地结合到微管轴上。中和这个补丁消除了 Kip2 在体内和体外促进微管生长的能力，而不影响其沿着微管行走的能力。我们的研究表明，Kip2 利用 Bik1 作为辅助因子来追踪微管尖端，然后其运动域招募游离微管蛋白并催化微管组装。