Polar growth and the establishment of polarity in eukaryotic cells is driven by the fusion of post-Golgi vesicles with only particular regions of the plasma membrane. For example, in Saccharomyces cerevisiae, polar growth occurs by directing post-Golgi vesicles to the tip of the cell. The RhoGTPase Cdc42p then leads the cascade of proteins that regulate the polarity of the cell. However, the molecular composition of the link between proteins involved in vesicle tethering, docking and fusion and those proteins involved in generating and maintaining an intracellular gradient of active Cdc42p, is unclear. Here (p. 2996), Nils Johnsson and colleagues identify a new protein complex involved in exocytosis. The proteins Boi1p and Boi2p, two poorly described proteins of the yeast cortex, are the central members of this complex, and the authors show that cells lacking these proteins accumulate post-Golgi vesicles in their buds. The essential PH domains of Boi1p and Boi2p are shown to bind Sec1p, a protein required for SNARE complex formation and vesicle fusion, and help to focus Sec1p to the cell tip. The authors further demonstrate that binding to Cdc42p is not required for the role of Boi1p and Boi2p in vesicle fusion, and although binding to lipid is required, it is not sufficient. Finally, they show that Boi1p and Boi2p bind to the exocyst, the primary tethering complex for vesicles. These data provide a mechanism for how the Boi1p and Boi2p proteins control polarised cell growth, and the authors propose that these proteins form a complex with Cdc42, multiple subunits of the exocyst and Sec1, among other components, that is involved in linking polarity regulators to the exocytic machinery to drive polarized bud growth.
