The anchoring of microtubules to subcellular structures is critical for cell polarity and motility. Although the process of anchoring cytoplasmic microtubules to the centrosome has been studied in some detail1-4, it is not known how spindle microtubules are anchored to the mitotic centrosome and, particularly, whether anchoring and nucleation of mitotic spindles are functionally separate. Here, we show that a fission yeast coiled-coil protein, Msd1, is required for anchoring the minus end of spindle microtubules to the centrosome equivalent, the spindle-pole body (SPB). msd1 deletion causes spindle microtubules to abnormally extend beyond SPBs, which results in chromosome missegregation. Importantly, this protruding spindle is phenocopied by the amino-terminal deletion mutant of Alp4, a component of the γ-tubulin complex5 (γ-TuC), which lacks the potential Msd1-interacting domain. We propose that Msd1 interacts with γ-TuC, thereby specifically anchoring the minus end of microtubules to SPBs without affecting microtubule nucleation.
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