Control of microtubule dynamics by Stu2p is essential for spindle orientation and metaphase chromosome alignment in yeast

Stu2p is a member of a conserved family of microtubule-binding proteins and an essential protein in yeast. Here, we report the first in vivo analysis of microtubule dynamics in cells lacking a member of this protein family. F or these studies, we have used a conditional Stu2p depletion strain express ing alpha -tubulin fused to green fluorescent protein. Depletion of Stu2p l eads to fewer and less dynamic cytoplasmic microtubules in both GI and prea naphase cells. The reduction in cytoplasmic microtubule dynamics is due pri marily to decreases in both the catastrophe and rescue frequencies and an i ncrease in the fraction of time microtubules spend pausing. These changes h ave significant consequences for the cell because they impede the ability o f cytoplasmic microtubules to orient the spindle. In addition, recovery of fluorescence after photobleaching indicates that kinetochore microtubules a re no longer dynamic in the absence of Stu2p. This deficiency is correlated with a failure to properly align chromosomes at metaphase. Overall, we pro vide evidence that Stu2p promotes the dynamics of microtubule plus-ends in vivo and that these dynamics are critical for microtubule interactions with kinetochores and cortical sites in the cytoplasm.