Ral: mediator of membrane trafficking

Ral is a multifunctional small GTPase involved in tumorigenesis and in controlling intracellular membrane trafficking. It is mainly activated by factors downstream of Ras, or independently of these factors and operates by protein-protein interactions with an expanding repertoire of partners. RalA is a positive regulator of calcium-evoked exocytosis via binding phospholipase D and is involved in G protein coupled receptor signalling by binding phospholipase C-delta1. The binding of Ral to calmodulin links to intracellular trafficking events. Another link is direct binding of activated Ral (Ral-GTP) to the endocytic and exocytic machineries. Ral-GTP binds RalBP1, which connects to receptor-mediated endocytosis via AP-2. Alternatively, Ral-GTP binds the exocyst complex, which controls secretory vesicle trafficking in regulated secretion and filopodia formation. Thus, Ral-GTP "chooses" between different membrane trafficking pathways. Other Ral partners are still being uncovered that may provide further mechanistic insights into how Ral controls diverse membrane trafficking pathways.