Role of a membrane glycoprotein in friend virus erythroleukemia: Nucleotide sequences of nonleukemogenic mutant and spontaneous revertant viruses

We previously isolated spontaneous env gene mutants of Friend spleen focus-forming virus that are nonleukemongenic in adult mice but form leikemogenic revertants in newborns; we found that the revertants contain secondary env mutations. To identify sites in the encoded membrane glycoprotein that are important for its pathogenic function, we molecularly cloned and partially sequenced the env genes of two mutant viruses (clone 63 and clone 40 and one revertant (clone 4(rev)). Clone 63 contained three noncontiguous point mutations that caused nonconservative amino acid substitutions of Gly-119 → ARg-119, Cys-180 → Tyr-180, and Gly-203 → Arg-203 in the xenotrophic-related domain of the env glycoprotein. These substitutions were presumably responsible for the altered electrophoretic and pathogenic properties of the mutant glycoprotein. The presence of these and several other G-A nucleotide substitutions at different sites in one spontaneous mutant provided striking evidence that error-rich proviruses can form during retroviral replication. Clone 4 contained a point mutation that generated a premature termination condon at amino acid residue 304 (Gln-304 → Ochre-304). This termination codon was located immediately after the proposed xenotropic-ecotropic recombination site and eliminated the electropic-related domain, including the putative membrane anchor of the glycoprotein. Clone 4(rev) was a true revertant derived from clone 4 in which the premature termination codon had back-mutated to re-form the wild-type sequence. These results confirm an essential role for the env gene in Friend spleen focus-forming virus pathogenesis and suggest that the encoded membrane glycoprotein contains different domains that contribute to its pathogenic function.