Bem1p directs vesicular traffic

The polarity protein Bem1p recruits the exocyst subunit Exo70p to the site of polarized exocytosis, Liu and Novick reveal.

Budding yeast direct secretory vesicles to the sites of polarized bud growth, where they are tethered to the cell cortex by an octameric complex called the exocyst. The exocyst assembles on vesicles as they move along actin cables into the bud, but two of its subunits—Exo70p and Sec3p—are also recruited directly to the sites of exocytosis by an actin-independent mechanism. Sec3p is recruited by members of the Rho GTPase family, including the polarity determinant Cdc42p, and the phospholipid PI(4,5)P₂. Although Exo70p also binds to PI(4,5)P₂, the mechanism underlying its recruitment to exocytic sites remains unclear.

Liu and Novick found that the polarity determinant Bem1p—a scaffold protein that binds to both Cdc42p and its activating protein Cdc24p—was required for the actin-independent localization of Exo70p to polarized exocytic sites. Bem1p and Exo70p colocalized throughout the cell cycle and bound directly to one another in vitro. The researchers identified point mutations in Exo70p that specifically disrupted its interaction with Bem1p without affecting the protein’s association with other known binding partners. These mutations disrupted Exo70p’s actin-independent localization to exocytic sites, which, when combined with mutations in other secretory proteins, inhibited cell growth.

A combination of Bem1p and PI(4,5)P₂ therefore recruits Exo70p to exocytic sites. Senior author Peter Novick now wants to investigate how the small GTPase Rho3p—another protein that binds to Exo70p—contributes to the exocyst subunit’s function.