The ghost in the machine: small GTPases as spatial regulators of exocytosis

Temporal and spatial regulation of membrane-trafficking events is crucial to both membrane identity and overall cell polarity. Small GTPases of the Rab, Rai and Rho protein families have been implicated as important regulators of vesicle docking and fusion events. This review focuses on how these GTPases interact with the exocyst complex, which is a multi-subunit tethering complex involved in the regulation of cell-surface transport and cell polarity. The Rab and Rai GTPases are thought to function in exocyst assembly and vesicle-tethering processes, whereas the Rho family GTPases seem to function in the local activation of the exocyst complex to facilitate downstream vesicle-fusion events. The localized activation of the exocyst by Rho GTPases is likely to have an important role in spatial regulation of exocytosis.