The spleen focus-forming virus envelope glycoprotein is defective in oligomerization

The gp52 envelope glycoprotein of Friend spleen focus-forming virus (SFFV) is a recombinant molecule derived from Friend murine leukemia virus (MuLV) by various deletions, insertions, and substitutions. The SFFV gp52 glycoprotein, unlike MuLV envelope glycoproteins, is defective in transport to the cell surface. Only 3-5% of gp52 eventually reaches the cell surface as a processed form (gp65). Although gp52 lacks cytoplasmic tail residues found in MuLV glycoproteins, we have previously shown that this deletion is not responsible for its defective transport. In order to investigate the basis for the defective transport of gp52, we have examined the folding and assembly of gp52 molecules into oligomeric molecules. CV-1 cells infected with vaccinia virus recombinants expressing SFFV gp52 were pulse labeled and the cell extracts were fractionated by velocity centrifugation through sucrose gradients. Immediately after a 10-min pulse, gp52 was detected as a monomer in the upper part of the sucrose gradient (fractions 12 and 14) and it remained as such after a 2-h chase period. However, the processed form, gp65, was found in a lower part of the gradient (fraction 8) after a 2-h chase. The position of gp65 was found to correspond to the position of trimeric influenza hemagglutinin which was analyzed on a parallel sucrose gradient, suggesting that gp65 also exists as a trimer in this fraction. These results indicate that changes in the external domain of gp52 result in improper folding of the glycoprotein molecule, and suggest that this lack of oligomerization is responsible for the defective transport of the molecules. Only those molecules that do form oligomeric structures are transported to the Golgi complex and undergo further oligosaccharide processing, and transport to the cell surface.