Analysis of cysteine mutations on the transmembrane protein of Moloney murine leukemia virus.

Earlier studies of murine leukemia viruses (MuLVs) have reported that a percentage of surface protein (SU) remains covalently associated with transmembrane protein (TM) through formation of disulfide bonds. Among MuLVs, there are three conserved cysteine residues within the extracellular domain of TM. These cysteine residues were substituted individually with serines to define their function and possible role in disulfide bonding with SU. Using oligonucleotide-directed mutagenesis, seven mutant constructs were generated with individual as well as multiple cysteine mutations. Transient transfection of all seven cysteine mutations resulted in nonviable virus. Analysis of intracellular proteins of producer mutant cell lines have demonstrated that precursor envelope protein (gPr80env; SU/TM) is being synthesized, but transport and processing of gPr80env is blocked in the endoplasmic reticulum. Two independent reversions of one cysteine mutation have been isolated and characterized.