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Abstract
In the presence of the actin-depolymerizing drug Latrunculin A, Exo70p localizes to
the bud tip in wild-type cells (left) but is mislocalized in cells lacking Bem1p (right).
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.
The exocyst subunit Exo70p is targeted to cortical sites in an actin-independent manner through a direct interaction with the polarity determinant Bem1p.