The secretory pathway mediates localization of the cell polarity regulatorAip3p/Bud6p

Ai3p/Bud6p is a regulator of cell and cytoskeletal polarity in Saccharomyce s cerevisiae that was previously identified as an actin-interacting protein . Actin-interacting protein 3 (Aip3p) localizes at the cell cortex where cy toskeleton assembly must be achieved to execute polarized cell growth, and deletion of AIP3 causes gross defects in cell and cytoskeletal polarity. We have discovered that Aip3p localization is mediated by the secretory pathw ay. Mutations in early- or late-acting components of the secretory apparatu s lead to Aip3p mislocalization. Biochemical data show that a pool of Aip3p is associated with post-Golgi secretory vesicles. An investigation of the sequences within Aip3p necessary for Aip3p localization has identified a se quence within the N terminus of Aip3p that is sufficient for directing Aip3 p localization. Replacement of the N terminus of Aip3p with a homologous re gion from a Schizosaccharomyces pombe protein allows for normal Aip3p local ization, indicating that the secretory pathway-mediated Aip3p localization pathway is conserved. Delivery of Aip3p also requires the type V myosin mot or Myo2p and its regulatory light-chain calmodulin. These data suggest that one function of calmodulin is to activate Myo2p's activity in the secretor y pathway; this function is likely the polarized movement of late secretory vesicles and associated Aip3p on actin cables.