STRUCTURAL INSIGHTS INTO THE HIGH MICROTUBULE BINDING AFFINITY OF KIF14: A MITOTIC KINESIN KNOWN TO PLAY A KEY ROLE IN CYTOKINESIS
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Kinesins use ATP hydrolysis-derived energy to power transport of cellular materials along microtubules and to regulate microtubule organization in eukaryotes. This enables them to serve various cellular functions including organelle and vesicle trafficking, chromosome segregation, and cellular morphogenesis. Association with these roles depends on the structural and mechanochemical properties of the specific kinesin, which can vary widely as the kinesin superfamily is large and structurally diverse compared to other cytoskeletal motors. The mitotic kinesin, KIF14, is an intriguing example of this diversity. It is essential for cytokinesis, and has been implicated in cerebral development and a variety of human cancers. It has unusual biochemical properties in comparison to other kinesins, including extremely tight microtubule binding, insensitivity of this affinity to the identity of the nucleotide bound in its active site, and very slow motility. Upstream of the motor domain is an unusually long N-terminal region that bears no homology to any other protein, yet is important for KIF14 localization to the central spindle and midbody during late anaphase and cytokinesis, respectively. Recent observations of this domain binding and bundling actin have not been previously demonstrated for a kinesin. Using X-ray crystallography and electron microscopy, a better understanding of the basis for these properties, as well as novel activities, is provided here. The crystal structure of the ADP-bound form of the mouse KIF14 motor domain reveals a dramatically opened ATP-binding pocket, as if ready to exchange its bound ADP for Mg·ATP, as well as a much larger twist in the central β-sheet than in other kinesins. This configuration has only been seen in the nucleotide-free states of myosins – known as the ‘rigor’ state – and has never been captured for a kinesin in solution. Moreover, cryo-electron microscopy indicates a distinct binding configuration of the motor domain to microtubules, possibly explaining its high microtubule affinity. Intriguingly, the KIF14 N-terminal extension also binds microtubules, and together with the motor domain, is capable of bundling microtubules. We postulate that these properties of KIF14 relate directly to its unique biochemical activity, and are functionally important for forming and stabilizing midbody microtubules during cytokinesis.