Exocyst finds way without Rho

Polarized exocytosis leads to polarization of the exocytic machinery. The counterintuitive findings from Roumanie et al. (page 583) place Rho GTPases at the top of this polarization ladder.In the past, Rho GTPases were thought to polarize secretion by recruiting the exocyst. This hypothesis stemmed mainly from the fact that exocyst proteins bind directly to the GTPases.

Roumanie and colleagues now show that the exocyst is polarized even when its interaction with Rho is impaired. They identified yeast strains with Rho GTPase mutations that impair exocytosis at the bud but not the localization of the important players. The exocyst is therefore at the bud site, yet not fully active.

Secretion defects were rescued by GTP-bound forms of Rho, but rescue did not require GTP hydrolysis. Based on other...