Eukaryotic protein trafficking involves packaging of target molecules into membranous vesicles that bud from a donor compartment, travel to a specific destination, fuse, and release their contents into an acceptor compartment. Vesicles that bud from the Golgi cisternae and from the ER contain a non-clathrin based coat. This coat is an oligomeric complex whose subunits are termed COPs (coatomer proteins). COPI is the golgi- and endosome-associated COP complex, while COPII is the ER-associated coat complex. In yeast, the COPII complex has a cargo binding subcomplex that includes Sar1p, Sec23p, and Sec24p, which recruits Sec13p and Sec31p leading to vesicle formation. The human homologues of Sec31p are Sec31A and Sec31B. Sec31A has 40% identity with Sec31B, and contains an N-terminal WD-40 domain and a C-terminal proline-rich region. Sec31A mRNA is widely expressed, while Sec31B is found only in testis and thymus. In HeLa, Sec31A localizes to vesicular structures in the perinuclear region of the cell, and co-localizes with the COPII component Sec13. Antibodies against Sec31A inhibit ER to Golgi transport of vesicular stomatitis G protein. Thus, Sec31 may be an important COPII component involved in ER to Golgi transport, as well as a marker for COPII vesicles.