Transient association of calnexin and calreticulin with newly synthesized G1 and G2 glycoproteins of Uukuniemi virus (family Bunyaviridae)

The membrane glycoproteins G1 and G2 of Uukuniemi virus, a member of the Bu nyaviridae family, are cotranslationally cleaved from a common precursor in the endoplasmic reticulum (ER). Here, we show that newly made G1 and G2 as sociate transiently with calnexin and calreticulin, two lectins involved in glycoprotein folding in the ER. Stable complexes between G1-G2 and calnexi n or calreticulin could be immunoprecipitated after solubilization of virus -infected BHK21 cells with the detergents digitonin or Triton X-100. In add ition, G1-G2-calnexin complexes could be recovered after solubilization wit h CHAPS (3-[(3-cholamidopropyl) -dimethylammonio]-1-propane sulfonate), whi le G1-G2-calreticulin complexes were not readily detected by using this det ergent. Only endoglycosidase H-sensitive forms of G1 were found complexed w ith calnexin. Pulse-chase experiments showed that G1 and G2 associated with both chaperones transiently for up to 120 min. Sequential immunoprecipitat ions with anticalreticulin and anticalnexin antisera indicated that about 5 0% of newly synthesized G1 and G2 was associated with either calnexin or ca lreticulin. Our previous results have shown that newly synthesized G1 and G 2 transiently interact also with the ER chaperone BiP and with protein disu lfide isomerase (R. Persson and R.F. Pettersson, J. Cell Biol; 112:257-266, 1991). Taking all of this into consideration, we conclude that the folding of G1 and G2 in the ER is catalyzed by at least four different folding fac tors.