Fusion activity of virions of murine leukemia virus

The fusion activity associated with many strains of murine leukemia virus (MLV) is commonly used to measure MLV infectivity. Other strains of MLV lack fusion activity and some of these can produce variants which have fusion activity. The SC-1/uv-XC fusion assay (Rowe et al., 1970) measures the infectivity of MLV grown in SC-1 cells by the XC cell fusion activity of the progeny virus. The fusion activity of parental MLV in XC cells can be measured directly within a few hours after infection. No new macromolecular synthesis (early after infection) is required for MLV-induced fusion. Treatment of MLV virions with proteolytic enzymes destroys their ability to fuse XC cells. The fusion activity of MLV, like the infectivity of MLV, is specifically inhibited by anti-MLV gp70 serum but not by antiserum against other MLV polypeptides. The infectivity of MLV is much more susceptible to uv irradiation than viral fusion activity and noninfectious (uv irradiated) MLV can cause XC cell fusion. When MLV virions are disrupted with NP-40 detergent, the fusion activity can be restored after high-speed centrifugation and dialysis of the detergent. The fusion activity of NP-40 disrupted MLV (after high-speed centrifugation and dialysis) is inhibited by antiserum against MLV gp70. Thus, the fusion activity of MLV is associated with the virion envelope glycoprotein.