The cytosolic yeast proteins Secl3-31p, Sec23/24p, and the small GTP binding protein Sarlp form the membrane coat termed COPII. Purified COPII components and GTP form anterograde transport vesicles directly from the endoplasmic reticulum. Cargo capture by COPII proteins is highly selective. Only properly folded forms of secretory and plasma membrane proteins are collected whereas resident ER proteins are ignored. Binding and chemical crosslinking experiments indicate that Sarlp-GTP and Sec23/24p form a non-covalent recruit-me: complex with membrane and lumenal cargo molecules. The recovery of soluble secretory proteins in such a complex re-quires the intervention of a putative membrane receptor or adaptor protein. Coated buds and vesicles form when Sec13/31p is added to membranes that have bound SarI-GTP and Sec23/24p. Sec13-31p may serve to cluster cargo recruitment complexes to produce a membrane patch depleted of ER resident protein:s. No special membrane proteins are required to anchor the formation of a COPII coat. Synthetic liposomes containing PE or PC and a mixture of acidic phospholipids are just as active as native ER membranes in binding COPII proteins in the presence of a non-hydrolyzeble analog of GTP, GMP-PNP.