COPII vesicle coat protein required for ER transport vesicle budding%3B essential factor in endoplasmic reticulum exit site (ERES) formation%2C as well as in COPII-mediated ER-to-Golgi traffic%3B bound to periphery of ER membranes and may act to stabilize initial COPII complexes%3B interacts with Sec23p%2C Sec24p and Sec31p