Differential contribution of Bud6p and Kar9p to microtubule capture and spindle orientation in S. cerevisiae

In Saccharomyces cerevisiae, spindle orientation is controlled by a temporal and spatial program of microtubule (MT)–cortex interactions. This program requires Bud6p/Aip3p to direct the old pole to the bud and confine the new pole to the mother cell. Bud6p function has been linked to Kar9p, a protein guiding MTs along actin cables. Here, we show that Kar9p does not mediate Bud6p functions in spindle orientation. Based on live microscopy analysis, kar9Δ cells maintained Bud6p-dependent MT capture. Conversely, bud6Δ cells supported Kar9p-associated MT delivery to the bud. Moreover, additive phenotypes in bud6Δ kar9Δ or bud6Δ dyn1Δ mutants underscored the separate contributions of Bud6p, Kar9p, and dynein to spindle positioning. Finally, tub2C354S, a mutation decreasing MT dynamics, suppressed a kar9Δ mutation in a BUD6-dependent manner. Thus, Kar9p-independent capture at Bud6p sites can effect spindle orientation provided MT turnover is reduced. Together, these results demonstrate Bud6p function in MT capture at the cell cortex, independent of Kar9p-mediated MT delivery along actin cables.