The GLN Family of Murine Endogenous Retroviruses Contains an Element Competent for Infectious Viral Particle Formation

Abstract

Several families of endogenous retroviruses (ERVs) have been identified in the mouse genome, in several instances by in silico searches, but for many of them it remains to be determined whether there are elements that can still encode functional retroviral particles. Here, we identify, within the GLN family of highly reiterated ERVs, one, and only one, copy that encodes retroviral particles prone to infection of mouse cells. We show that its envelope protein confers an ecotropic host range and recognizes a receptor different from mCAT1 and mSMIT1, the two previously identified receptors for other ecotropic mouse retroviruses. Electron microscopy disclosed viral particle assembly and budding at the cell membrane, as well as release of mature particles into the extracellular space. These particles are closely related to murine leukemia virus (MLV) particles, with which they have most probably been confused in the past. This study, therefore, identifies a new class of infectious mouse ERVs belonging to the family Gammaretroviridae, with one family member still functional today. This family is in addition to the two MLV and mouse mammary tumor virus families of active mouse ERVs with an extracellular life cycle.Complete sequencing of the mouse genome has led to the identification of multiple families of endogenous retroviruses (ERVs), each with a variable number of elements ranging from a few copies to several hundred (33; reviewed in references 6 and 20). These elements can now be classified according to their pol gene homology, and the resulting phylogenetic analysis recapitulates, in part, the diversity that can be found at the level of the present-day infectious retroviruses of animals. In the cases where functional copies have been identified and characterized, this classification can be further refined according to criteria unrelated to their sequences but involving both the site of assembly and the morphology of the associated virus-like particles. In fact, the latter classification corresponds to the historical one, essentially based on pioneering electron microscopic analyses of the A-, B-, C-, or epsilon-type virus-like particles that can be observed in mouse cells and tissues (5, 36; reviewed in reference 24). Along these lines, and as illustrated in Fig. 1B, the first significant fraction of the mouse ERVs belongs to the family Betaretroviridae and includes the endogenous mouse mammary tumor virus elements (ϳ10 copies in the C57BL/6 genome) and the highly reiterated Mus musculus type D/early transposon (MusD/ETn) ERVs (ϳ350 copies), whose particles have recently been demonstrated to assemble in a strictly intracellular location (28), as observed for type B/D retroviruses. A second large family of ERVs includes the intracisternal A-type particle (more than 1,000 copies) and intracisternal A-type particle-related, envelope-encoding elements (ϳ250 copies), which are also phylogenetically close to betaretroviruses but can be distinguished from the latter by the site of particle assembly, which is not within ...

Endogenous retroviruses

“…One is the GLN family, named for its glutamine tRNA primer-binding site, in contrast to proline tRNA found in many other Class I ERVs [72]. Of the over 80 copies found in the mouse genome, one copy was recently found both to be intact and capable of releasing virus particles [18]. In view of the fact that related sequences have been found in both Mus subspecies and other rodent (but not nonrodent) genomes, it must have entered a common ancestor prior to the Mus/Rattus split (circa 16–23 Mya) [18, 72] (Fig.…”

Transmission, Evolution, and Endogenization: Lessons Learned from Recent Retroviral Invasions

“…The GLN retrovirus is named for the glutamine amino acid (Gln) of the tRNA that is complementary to the primer binding site of the retrovirus. The murine GLN-2 retrovirus is the sole member of the high-copy-number reiterative ERV family present in the C57BL/6 mouse genome and is an infectious retrovirus (64). GLN-2 employs the folate transporter encoded by the murine slc19a1 ortholog to infect mouse cells (Jhen Tsang, David Ribet, Thierry Heidmann, and Marie Dewannieux, submitted for publication).…”

Transposable elements in vertebrates: species evolution and environmental adaptation

“…After the synthesis of cDNA mediated by reverse transcriptase, the integrase inserts the cDNA into a new position of the genome. Unlike retroviruses, LTR are not able to move between cells and to infect them (Malik et al 2000;Ribet et al 2008). The sub-classification of LTR retrotransposons includes three main TE superfamilies in vertebrates: Ty1/Copia (Pseudoviridae), Ty3-gypsy-like (Metaviridae) and BEL/Pao (Chalopin et al 2015).…”