Riding Kar9 into the bud

Alignment of the mitotic spindle with the axis of cell division in budding yeast ensures that one and only one chromatid set is pulled into the bud. Microtubule interactions with the cortex set this arrangement by capturing one spindle pole at the bud site. Now, Kimitris Liakopoulos, Justine Kusch, Yves Barral (Swiss Federal Institute of Technology, Zürich, Switzerland), and colleagues find that polarity is achieved because only one set of microtubules is marked for this interaction with the cortex.

The Swiss group identified a protein that distinguishes microtubules from the two spindle poles. This myosin- interacting protein, Kar9, localized specifically to the spindle pole destined for the bud (daughter-bound pole). Kar9 polarity depended on the cell cycle kinase Cdk1. Kar9 phosphorylation correlated with its decreased interaction with the microtubule- and spindle-pole–binding protein, Bim1, thus probably reducing its association with the mother-bound pole.

Meanwhile, microtubules at the daughter-bound pole were loaded with Kar9 and were guided to the bud in a myosin-dependent manner along the polarized actin network. Loss of Kar9 asymmetry (through inhibition of Cdk1) caused both sets of microtubules to be drawn to the bud, indicating that Kar9 distinguishes microtubules from the two poles. “For a long time, people were thinking microtubules were microtubules,” says Barral. “Now, we see they come in different flavors depending on which pole they come from.” ▪

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