Yeast GSK-3 kinase regulates astral microtubule function via phosphorylation of the microtubule-stabilizing kinesin Kip2

The S. cerevisiae kinesin Kip2 stabilises astral microtubules and facilitates spindle positioning through transport of microtubule-associated proteins, such as the yeast CLIP-170 homologue Bik1, dynein and the Adenomatous Polyposis Colirelated protein Kar9 to the plus ends of astral microtubules. Here, we show that Kip2 associates physically with its processivity factor Bim1, the yeast homologue of the EB1 plus end-tracking protein. This interaction requires an EB1-binding motif in the N-terminal extension of Kip2 and is negatively regulated by phosphorylation through Mck1, the yeast Glycogen Synthase Kinase 3. In addition, Mck1-dependent phosphorylation decreases the intrinsic microtubule affinity of Kip2. Reduction in Kip2 phosphorylation leads to stabilisation of astral microtubules and accumulation of Kip2, dynein and Kar9 at microtubule plus ends, while loss of Mck1 function leads to defects in spindle positioning. Furthermore, we provide evidence that a subpopulation of Mck1 at the bud-cortex phosphorylates Kip2. We propose that yeast GSK-3 spatially controls astral microtubule dynamics and the loading of dynein and Kar9 on astral microtubule plus ends by regulating Kip2 interactions with Bim1 and microtubules. Jo ur na l o f C el l S ci en ce • A dv an ce a rt ic le