Formation and intracellular transport of a heterodimeric viral spike protein complex.

We have analyzed the heterodimerization and intracellular transport from the ER to the Golgi complex (GC) of two membrane glycoproteins of a bunyavirus (Uukuniemi virus) that matures by a budding process in the GC. The glycoproteins G1 and G2, which form the viral spikes, are cotranslationally cleaved in the ER from a 110,000-D precursor. Newly synthesized G1 was transported to the GC and incorporated into virus particles about 30-45 min faster than newly synthesized G2. Analysis of the kinetics of intrachain disulfide bond formation showed that G1 acquired its mature form within 10 min, while completion of disulfide bond formation of G2 required a considerably longer time (up to 60 min). During the maturation process, G2 was transiently associated with the IgG heavy chain binding protein for a longer time than G1. Protein disulfide isomerase also coprecipitated with antibodies against G1 and G2. In virus particles, G1 and G2 were present exclusively as heterodimers. Immunoprecipitation with monoclonal antibodies showed that heterodimerization occurred rapidly, probably in the ER, between newly made G1 and mature, dimerization competent G2. Taken together, our results show that these two viral glycoproteins have different maturation kinetics in the ER. We conclude that the apparent different kinetics of ER to GC transport of G1 and G2 is due to the different rates by which these proteins fold and become competent to enter into heterodimeric complexes prior to exit from the ER.