Golgi localization of Hantaan virus glycoproteins requires coexpression of G1 and G2

The membrane glycoproteins G1 and G2 of Hantaan virus (HTNV; family Bunyaviridae) are encoded on the M RNA genome segment as a precursor polypeptide that is cotranslationally cleaved by host proteases. G1 and G2 accumulate in the Golgi complex in cells following either virus infection or transfection with M segment cDNA. However, there are conflicting reports in the literature concerning the Golgi targeting of separately expressed G1. To resolve these differences, a series of M segment and G1 coding region cDNA mutants was constructed containing either C-terminal or internal deletions. The intracellular localization of proteins expressed from these constructs was investigated by using confocal microscopy and double-staining immunofluorescence with G1 and G2 specific monoclonal antibodies and antisera specific for markers of the Golgi complex (GM130 and mannosidase II) and of the ER (calnexin). When expressed individually, G1 and G2 were retained in the ER, whereas when coexpressed from separate plasmids, both proteins localized to the Golgi. A construct expressing the whole G1 coding region and the complete signal sequence of G2 (amino acids 1-648 of the precursor) was found to be the minimal G1 protein competent to rescue G2 to the Golgi. This suggests that the G1 cytoplasmic tail including the downstream G2 signal peptide plays an important role in Golgi localization of HTNV glycoproteins. None of the constructs with internal deletions in the cDNA expressed proteins that localized to the Golgi. Our results indicate that the Golgi retention signal of HTNV glycoproteins may depend on the conformation of oligomerized G1 and G2 complex rather than a precise primary amino acid sequence.