The organization of endoplasmic reticulum export complexes

XPORT of protein from the ER is the first step in the vectorial movement of cargo through compartments of the secretory pathway of eukaryotic cells. Pioneering studies by Palade (1975) established that the site of exit from the RER in pancreatic acinar cells is through transitional elements, a region of partly rough, partly smooth tubular ER juxtaposed to the cis face of the Golgi stack. Morphologically, ER-derived vesicles are 60-70 nm in diameter and contain an electron-dense coat when viewed using transmission electron microscopy (Ziegel and Dalton, 1962) . This characteristic coat contains COPII components that are assembled in response to the activation of the Sarl GTPase, a machinery now recognized to be evolutionarily conserved in yeast and mammalian cells (for review see Barlowe, 1995) .