A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

Abstract

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described 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 erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker 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 induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

A single retroviral gag precursor signal peptide recognized by FBL-3 tumor-specific cytotoxic T lymphocytes

“…The biological role of glycosylated Gag-Pol precursor Pr75 gag initiated at an alternative CTG codon(s) located upstream of the usual ATG initiation codon for Pr65 gag has long been puzzling. Recently, Corbin et al (8) demonstrated that glycosylated Gag expression is essential for complete in vivo spreading and for pathogenicity of FV. If the expression of glycosylated Gag products on FV-induced tumor cell surfaces is advantageous for in vivo cell proliferation, a highly immunogenic portion of the leader peptide inevitably induces effective CTL responses which may be critical for disease control.…”

Translational control of retroviruses

“…Instead, the protein is exposed on the extra-cellular side of the plasma membrane, where it displays potential mitogenic activity 26,27 . In the case of the Friend MLV (FMLV), GCSA is not necessary for virus replication in cell culture but is required for virus pathogenesis, that is, haemolytic anaemia and erythroleukaemia in mice 27,28 .…”

The antiviral activity of antibodies in vitro and in vivo

“…An antibody against gag also delayed disease onset as a Fab fragment (albeit less effectively), indicating that clearance of infected cells by Fc-mediated effector functions cannot wholly explain the protective effect. It has been shown that MuLV glycogag plays an important role in viral spread and pathogenesis (Corbin et al, 1994;Portis et al, 1994). Occupancy of glyco-gag on the cell surface by antibody in the form of whole IgG as well as Fab fragment may therefore be able to interfere with glyco-gag function, thereby providing protection by limiting virus spread without directly clearing infected cells.…”