Formation of heterotrimers between the membrane-integrated and the soluble glycoproteins of vesicular stomatitis virus leads to their intracellular cotransport.

BHK cells infected with vesicular stomatitis virus serotype Indiana generate intracellularly two different types of glycoproteins: the authentic membrane-integrated G protein of virions and a smaller soluble Gs protein lacking the transmembrane and cytoplasmic domains which is secreted into the growth medium. A Gs1 protein species which is formed during or shortly after translation in the endoplasmic reticulum lumen is modified in the same way as the G1 protein by endoglycosidase H-sensitive oligosaccharides of the high-mannose type. Both G1 and Gs1 are almost simultaneously transported, trimmed, and processed into G2 and Gs2 species which possess carbohydrate side chains of the complex type, making both glycoproteins resistant to endoglycosidase H cleavage. Secretion of Gs2 protein into the growth medium and arrival of G2 protein on the cell surface occur concomitantly. Membrane-integrated G protein and the soluble Gs protein molecules oligomerize intracellularly into heterotrimers which can be immunoprecipitated after chemical cross-linking. Gs protein seems to contain sufficient structural information for the formation of heterotrimers which are efficiently transported to the cell surface. Heterotrimer formation between G and Gs proteins explains the rapid secretion of Gs molecules.