FIG. 7.

Evidence that MLV NH-mediated inhibition of fusion involves hetero-oligomerization with envelope protein. (A) Mutation that disrupts the alpha-helical structure of MLV NH causes the loss of its inhibitory activity. HEK293 cells were cotransfected with 1.5 μg each of expression vectors for fusogenic MLV envelope protein (pCEETr), pTet-Off, and either wild-type NH-YFPgpi, the leucine-to-proline mutant NH(L-P)-YFPgpi, or the parental-vector YFPgpi. Twenty four hours later, the transfected cells were cocultured with U2OSLucCATGFP for 16 h and assayed for luciferase activity. (B) The expression of MLV NH in receptor-bearing cells does not inhibit MLV envelope protein-mediated fusion. U2OSLuc cells were cotransfected with 1.6 μg of mCAT1 and 0.0, 1.6, or 3.2 μg of NH-YFPgpi plus 3.2, 1.6, or 0.0 μg of YFPgpi, respectively, and cocultivated with HEK293 cells cotransfected with 2 μg each of pCEETR and pTet-Off. Error bars indicate the standard deviations of results with triplicate samples in a representative experiment of two performed. RLU, relative light units.