Mammalian homologues of yeast Sec31p - An ubiquitously expressed form is localized to endoplasmic reticulum (ER) exit sites and is essential for ER-Golgi transport

The yeast coat protein II (COPII) is responsible for vesicle budding from t he endoplasmic reticulum (ER), Mammalian functional homologues for all yeas t COPII components, except for Sec31p, have been reported. We have cloned a mammalian cDNA whose product (Sec31A) is about 26% identical to Saccharomy ces cerevisiae Sec31p, Data base searches also revealed another partial seq uence encoding a polypeptide (Sec31B) that is 40% identical to Sec31A North ern analysis revealed that Sec31A transcripts are ubiquitously and abundant ly expressed, while Sec31B transcripts are particularly enriched in the tes tis and thymus, but present in very low levels in other tissues. Sec31A is localized to vesicular structures that scatter throughout the cell but are concentrated at the perinuclear region. The structures marked by Sec31A con tain Sec13, a component of COPII that is well characterized to mark the ER exit sites. Immunoelectron microscopy revealed that Sec31A colocalizes with Sec13 in structures with extensive vesicular-tubular profiles. Antibodies raised against a C-terminal portion of Sec31A co-precipitate Sec13 and inhi bit ER-Golgi transport of temperature-arrested vesicular stomatitis G prote in in a semi-intact cell assay. Cytosol immunodepleted of Sec31A failed to support vesicular stomatitis G protein transport, which can be rescued by a high molecular weight fraction of the cytosol containing both Sec31A and S ec13, We conclude that Sec31A represents a functional mammalian homologue o f yeast Sec31p.