Overexpressed PRC1 bundles microtubules in interphase cells.

New results from Mollinari et al. (page 1175) reveal that cytokinesis requires microtubule bundles to be formed at the spindle midzone and that the mitotic spindle–associated protein PRC1 does this bundling.

PRC1 is a cyclin-dependent kinase substrate known to be localized to the central spindle midzone during mitosis, where antiparallel over- lapping microtubules are the site of accumulation of many proteins required for cytokinesis. With their new results, the authors have found that the structure of the spindle midzone is maintained by PRC1 microtubule bundling activity.

Mollinari et al. found that PRC1 could form bundles of microtubules both in vitro and in vivo, where high levels of exogenously expressed PRC1 caused unusual cytoplasmic arrays of microtubules in interphase cells. Bundling was required for cytokinesis; in the absence of PRC1, chromosomes separated normally along a disorganized anaphase spindle, but furrowing during cytokinesis stalled, and daughter cells failed to separate. The bundles may therefore be important for the coordinated passage of proteins at anaphase from kinetochores along microtubules to the cell cortex, where they initiate assembly of the acto-myosin ring and the onset of cytokinesis.

As other stages of the cell cycle were unaffected by the lack of PRC1 activity, bundling appears to be important only during late stages of mitosis. Mutation of the phosphorylation motif enhanced the bundling activity of PRC1, suggesting that Cdc2 inhibits the protein during early mitosis. â–ª