Molecular linkage underlying microtubule orientation toward cortical sitesin yeast

Selective microtubule orientation-toward spatially defined cortical sites i s critical to polarized cellular processes as diverse as axon outgrowth and T cell cytotoxicity. In yeast, oriented cytoplasmic microtubules align the mitotic spindle between mother and bud. The cortical marker protein Kar9 L ocalizes to the bud tip and is required for the orientation of microtubules toward this region. Here, we show that Kar9 directs microtubule orientatio n by acting through Bim1, a conserved microtubule-binding protein. Bim1 hom olog EB1 was originally identified through its interaction with adenomatous polyposis coil (APC) tumor suppressor, raising the possibility that an APC -EB1 linkage orients microtubules in higher cells.