A NONSTRUCTURAL GAG-ENCODED GLYCOPROTEIN PRECURSOR IS NECESSARY FOR EFFICIENT SPREADING AND PATHOGENESIS OF MURINE LEUKEMIA VIRUSES

In addition to the Gag-Pol and Env precursors whose translation initia tes at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag). glycoGag translati on is initiated at CUG codons located upstream of the Gag AUG initiati on codon. In contrast to Gag, glycoGag is translocated into the endopl asmic reticulum and is absent from virions. Since glycoGag has been de scribed to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag(-) mutation in the Friend murine leukemia virus (F-MuLV ). F-MuLV induces severe early hemolytic anemia and subsequent erythro leukemia within 2 months after inoculation of newborn mice. We obtaine d a glycoGag(-) F-MuLV, strain H5, by inserting an octanucleotide link er downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induc e severe early hemolytic anemia, and latency of erythroleukemia was si gnificantly increased most likely because of an approximately 1-week d elay in the in vivo spreading. Accordingly, induction of recombinant p olytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-Mu LV H5. Western blot (immunoblot) performed after inoculation of newbor n mice with this glycoGag(-) virus indicated the emergence of new glyc oGag(+) viruses. PCR analyses with F-MuLV-specific primers demonstrate d in vivo pseudoreversions restoring the glycoGag reading frame. Our r esults demonstrated that glycoGag expression is positively selected an d essential for full spreading and pathogenic abilities.