A Di-Acidic Signal Required for Selective Export

Transport of membrane proteins between intracellular compartments requires specific sequences in the protein cytoplasmic domain to direct packaging into vesicle shuttles. A sequence that mediates export from the endoplasmic reticulum (ER) has proved elusive. A di-acidic signal (Asp-X-Glu, where X represents any amino acid) on the cytoplasmic tail of vesicular stomatitis virus glycoprotein ( VSV-G) and other cargo molecules was required for efficient recruitment to vesicles mediating export from the ER in baby hamster kidney cells. The existence of such a signal provides evidence that export from the ER occurs through a selective mechanism. Vesicular transport of proteins between in-tracellular compartments is coordinated by the activity of coat complexes that direct the selection of cargo through cytoplasmic sig-nals and budding from membranes (1, 2). These coat complexes include clathrin, which mediates sorting of receptors contain