Vrp1p functions in both actomyosin ring-dependent and Hof1p-dependent pathways of cytokinesis.

Vrp1p/verprolin/End5p is a Saccharomyces cerevisiae proline-rich protein, structurally and functionally related to human Wiskott-Aldrich syndrome protein-interacting protein. Vrp1p is required for viability at 37 degrees C, but not 24 degrees C. Here, we show that loss of Vrp1p (vrp1Delta) leads to a 3-4-fold delay in cytokinesis, wide bud necks, abnormal actomyosin rings, and aberrant septa even at 24 degrees C. Like other mutations affecting the actomyosin ring, vrp1Delta is synthetic lethal with deletion of HOF1 (or CYK2), which encodes a protein related to mammalian proline serine threonine phosphatase-interacting protein and Schizosaccharomyces pombe Cdc15p required for an actomyosin ring-independent pathway of cytokinesis in S. cerevisiae. At 37 degrees C, vrp1Delta cells rapidly cease dividing and exhibit a novel terminal phenotype: a single large bud, two well-separated nuclei, and an interphase microtubule array. The arrested cells have a persistent ring containing both actin and myosin at the bud neck. Many also exhibit some polarisation of cortical actin patches to the bud neck. Vrp1p binds an SH3-domain-containing fragment of Hof1p in vitro. Vrp1p is required in vivo for Hof1p relocalisation to a single ring at the bud neck prior to cytokinesis at 37 degrees C, but not at 24 degrees C. Vrp1p thus acts in both actomyosin ring formation and function, as well as in Hof1p localisation during cytokinesis.