The formation of two daughter cells requires accurate cytokinesis. The mitotic spindle marks the division plane in animal cells and delivers signals to the equatorial cortex through central spindle and astral microtubules. In anaphase, the centralspindlin complex accumulates on overlapping antiparallel microtubules of the central spindle. Centralspindlin comprises the mitotic kinesin-like protein 1 (MKLP1) and MgcRacGAP; the latter interacts with the RhoA guanine nucleotide exchange factor Ect2 to control RhoA activity, which is essential for cytokinesis. On page 1809, Ann Miller and co-workers find that MgcRacGAP also localises to astral microtubule plus ends at the equatorial cell cortex in Xenopus laevis embryos. Within MgcRacGAP, the authors identify an SxIP motif to which EB proteins that track the plus end of microtubules can bind. Mutations in the SxIP motif lead to the loss of both MgcRacGAP tracking with EB3 and equatorial accumulation of MgcRacGAP. Furthermore, astral microtubules are disorganised when the MgcRacGAP SxIP motif is mutated, and the authors observe failure in RhoA localisation and cytokinesis under these conditions. In addition, MgcRacGAP is also found at cell–cell contact sites, and the mutation in the SxIP motif perturbs the morphology of adherens junction. Taken together, this work establishes an important role for the SxIP motif of MgcRacGAP for its function in interphase and cytokinesis.
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