ANALYSIS OF THE MURINE LEUKEMIA-VIRUS R-PEPTIDE - DELINEATION OF THE MOLECULAR DETERMINANTS WHICH ARE IMPORTANT FOR ITS FUSION INHIBITION-ACTIVITY

In previous studies, the C-terminal R peptide of the murine leukemia v irus (MuLV) Env protein was shown to be a potent inhibitor of viral fu sion activity. In the present study, we investigated the molecular det erminants in the MuLV Env protein cytoplasmic tail which are important for the fusion inhibition activity of the R peptide. We constructed a series of mutant MuLV env genes which express Env proteins with seria l truncations, internal deletions, or amino acid substitutions in the cytoplasmic tail. To analyze their cell fusion activity, we employed a quantitative fusion assay. We found that truncations of up to 7 amino acids from the C terminus of the cytoplasmic tail had no detectable e ffect on the lack of fusion activity of the full-length Env protein; h owever, further truncations resulted in a progressive increase in cell fusion activity. Studies of mutant proteins with amino acid substitut ions in the cytoplasmic tail showed that Leu-627 plays an important ro le in fusion inhibition by the R peptide, while most of the other amin o acids in the R peptide were not essential for fusion inhibition. Stu dies of mutant proteins with internal deletions upstream of the cleava ge site in the cytoplasmic tail showed that this region is also involv ed in fusion inhibition by the R peptide, although only to a limited e xtent. The results are consistent,vith a model in which the MuLV R pep tide exhibits its fusion inhibition activity through interaction with a cellular factor(s).