Functional Interplay Between Murine Leukemia Virus Glycogag, Serinc5, and Surface Glycoprotein Governs Virus Entry, with Opposite Effects on Gammaretroviral and Ebolavirus Glycoproteins

Abstract

Gammaretroviruses, such as murine leukemia viruses (MLVs), encode, in addition to the canonical Gag, Pol, and Env proteins that will form progeny virus particles, a protein called “glycogag” (glycosylated Gag). MLV glycogag contains the entire Gag sequence plus an 88-residue N-terminal extension. It has recently been reported that glycogag, like the Nef protein of HIV-1, counteracts the antiviral effects of the cellular protein Serinc5. We have found, in agreement with prior work, that glycogag strongly enhances the infectivity of MLVs with some Env proteins but not those with others. In contrast, however, glycogag was detrimental to MLVs carrying Ebolavirus glycoprotein. Glycogag could be replaced, with respect to viral infectivity, by the unrelated S2 protein of equine infectious anemia virus. We devised an assay for viral entry in which virus particles deliver the Cre recombinase into cells, leading to the expression of a reporter. Data from this assay showed that both the positive and the negative effects of glycogag and S2 upon MLV infectivity are exerted at the level of virus entry. Moreover, transfection of the virus-producing cells with a Serinc5 expression plasmid reduced the infectivity and entry capability of MLV carrying xenotropic MLV Env, particularly in the absence of glycogag. Conversely, Serinc5 expression abrogated the negative effects of glycogag upon the infectivity and entry capability of MLV carrying Ebolavirus glycoprotein. As Serinc5 may influence cellular phospholipid metabolism, it seems possible that all of these effects on virus entry derive from changes in the lipid composition of viral membranes.

SERINC5 protein inhibits HIV-1 fusion pore formation by promoting functional inactivation of envelope glycoproteins

“…Whereas this model could reconcile discrepant findings regarding the effect of SER5 on binding of anti-MPER antibodies (16), the lack of effect on binding to HXB2 pseudoviruses (Fig. 5H) and the apparently similar levels of PS in control and SER5ϩ particles (15) are inconsistent with this possibility. We surmised that the similar extents of 4E10 binding to control and SER5ϩ HXB2pp may be caused by the excessive HXB2 gp41 MPER exposure in the absence of the restriction factor and/or from the functional inactivation of this Env on SER5ϩ virions, which may sequester the MPER domains (30).…”

“…Whereas MLV infection is also antagonized by SER5, fusion of HIV-1 particles pseudotyped with vesicular stomatitis virus (VSV)-G or Ebola virus glycoproteins is relatively resistant to this factor (10,11,14). In fact, Ebola pseudovirus infectivity has been reported to be enhanced by incorporation of SER5 (15).…”

Influence of different glycoproteins and of the virion core on SERINC5 antiviral activity

“…Two examples of convergent evolution of anti-SERINC function by virally encoded proteins are found outside of primate immunodeficiency viruses. Murine leukemia virus (MLV) Glycogag and equine infectious anemia virus (EIAV) S2 are viral antagonists of SERINC5 activity, and neither share sequence or structural homology with Nef, nor to each other (17,(22)(23)(24).…”

“…SERINC5 increases HIV-1 sensitivity to antibodies and peptides targeting the membrane-proximal external region of gp41, suggesting that it somehow alters the conformation of the HIV-1 glycoprotein near the virion membrane (19,25). Importantly, HIV-1 particles pseudotyped with vesicular stomatitis virus (VSV) G or Ebola virus glycoprotein are resistant to SERINC5 antiviral activity (17,18,24). These initial observations suggest a correlation between the location of viral fusion and sensitivity to SERINC5 activity, with glycoproteins that mediate fusion at the cell surface (Env from HIV-1 and amphotropic MLV [A-MLV]) being sensitive and those that mediate fusion in endo-lysosomal compartments (VSV-G and Ebola GP) being resistant (17,24).…”

“…Importantly, HIV-1 particles pseudotyped with vesicular stomatitis virus (VSV) G or Ebola virus glycoprotein are resistant to SERINC5 antiviral activity (17,18,24). These initial observations suggest a correlation between the location of viral fusion and sensitivity to SERINC5 activity, with glycoproteins that mediate fusion at the cell surface (Env from HIV-1 and amphotropic MLV [A-MLV]) being sensitive and those that mediate fusion in endo-lysosomal compartments (VSV-G and Ebola GP) being resistant (17,24). Taken together these results indicate that the virion glycoprotein is a viral determinant of sensitivity to SERINC5.…”