Molecular linkage underlying microtubule orientation toward cortical sites in yeast.

Selective microtubule orientation toward spatially defined cortical sites is 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 localizes to the bud tip and is required for the orientation of microtubules toward this region. Here, we show that Kar9 directs microtubule orientation by acting through Bim1, a conserved microtubule-binding protein. Bim1 homolog EB1 was originally identified through its interaction with adenomatous polyposis coli (APC) tumor suppressor, raising the possibility that an APC-EB1 linkage orients microtubules in higher cells.

Cyk3, a novel SH3-domain protein, affects cytokinesis in yeast.

Cytokinesis requires the wholesale reorganization of the cytoskeleton and secretion to complete the division of one cell into two. In the budding yeast Saccharomyces cerevisiae, the IQGAP-related protein Iqg1 (Cyk1) promotes cytokinetic actin ring formation and is required for cytokinesis and viability [1-3]. As the actin ring is not essential for cytokinesis or viability, Iqg1 must act by another mechanism [4]. To uncover this mechanism, a screen for high-copy suppressors of the iqg1 lethal phenotype was performed. CYK3 suppressed the requirement for IQG1 in viability and cytokinesis without restoration of the actin ring, demonstrating that CYK3 promotes cytokinesis through an actomyosin-ring-independent pathway. CYK3 encodes a novel SH3-domain protein that was found in association with the actin ring and the mother-bud neck. cyk3 null cells had misshapen mother-bud necks and were deficient in cytokinesis. In the cyk3 null strain, actin rearrangements associated with cytokinesis appeared normal, suggesting that the phenotype reflects a defect in secretory targeting or septal synthesis. Deletion of either cyk3 or hof1 alone results in a mild cytokinetic phenotype [5-7], but deletion of both genes resulted in lethality and a complete cytokinetic block, suggesting overlapping function. Thus, Cyk3 appears to be important for cytokinesis and acts potentially downstream of Iqg1.