Acentrosomal spindle assembly and stability in C. elegans oocytes requires a kinesin-12 non-motor microtubule interaction domain

During the meiotic divisions in oocytes, microtubules are sorted and organized by motor proteins to generate a bipolar spindle in the absence of centrosomes [1]. In most organisms, kinesin-5 family members crosslink and slide microtubules to generate outward force that promotes acentrosomal spindle bipolarity [2-7]. However, the mechanistic basis for how other kinesin families act on acentrosomal spindles has not been explored. We investigated this question in C. elegans oocytes, where kinesin-5 is not required to generate outward force [8]. Instead, the kinesin-12 family motor KLP-18 performs this function [9-12]. KLP-18 acts with adaptor protein MESP-1 (meiotic spindle 1) to sort microtubule minus ends to the periphery of a microtubule array, where they coalesce into spindle poles [12]. If either of these proteins is depleted, outward sorting of microtubules is lost and minus ends converge to form a monoaster. Here we use a combination of in vitro biochemical assays and in vivo mutant analysis to provide insight into the mechanism by which these proteins collaborate to promote acentrosomal spindle assembly. We identify a microtubule binding site on the C-terminal stalk of KLP-18 and demonstrate that a direct interaction between the KLP-18 stalk and MESP-1 activates non-motor microtubule binding. We also provide evidence that this C-terminal domain is required for KLP-18 activity during spindle assembly and show that KLP-18 is continuously required to maintain spindle bipolarity. This study thus provides new insight into the construction and maintenance of the oocyte acentrosomal spindle as well as into kinesin-12 mechanism and regulation.