FUNCTIONAL-ANALYSIS OF THE CYTOPLASMIC TAIL OF MOLONEY MURINE LEUKEMIA-VIRUS ENVELOPE PROTEIN

The cytoplasmic tail of the immature Moloney murine leukemia virus (Mo MuLV) envelope protein is approximately 32 amino acids long, During vi ral maturation, the viral protease cleaves this tail to release a 16-a mino acid R peptide, thereby rendering the envelope protein fusion com petent. A series of truncations, deletions, and amino acid substitutio ns were constructed in this cytoplasmic tail to examine its role in fu sion and viral transduction, Sequential truncation of the cytoplasmic tail revealed that removal of as few as 11 amino acids resulted in sig nificant fusion when the envelope protein was expressed in NIH 3T3 cel ls, similar to that seen following expression of an R-less envelope (t runcation of 16 amino acids). Further truncation of the cytoplasmic ta il beyond the R-peptide cleavage site toward the membrane-spanning reg ion had no additional effect on the level of fusion observed, In contr ast, some deletions and nonconservative amino acid substitutions in th e membrane-proximal region of the cytoplasmic tail (residues L602 to F 605) reduced the amount of fusion observed in XC cell cocultivation as says, suggesting that this region influences the fusogenicity of full- length envelope protein, Expression of the mutant envelope proteins in a retroviral vector system revealed that decreased envelope-mediated cell-cell fusion correlated with a decrease in infectivity of the resu lting virions. Additionally, some mutant envelope proteins which were capable of mediating cell-cell fusion were not efficiently incorporate d into retroviral particles, resulting in defective virions. The cytop lasmic tail of MoMuLV envelope protein therefore influences both the f usogenicity of the envelope protein and its incorporation into virions .