N -Linked Glycosylation Protects Gammaretroviruses against Deamination by APOBEC3 Proteins

Abstract

Retroviruses are pathogens with rapid infection cycles that can be a source of disease, genome instability, and tumor development in their hosts. Host intrinsic restriction factors, such as APOBEC3 (A3) proteins, are constitutively expressed and dedicated to interfering with the replication cycle of retroviruses. To survive, propagate, and persist, retroviruses must counteract these restriction factors, often by way of virus genome-encoded accessory proteins. Glycosylated Gag, also called glycosylated Pr80 Gag (gPr80), is a gammaretrovirus genome-encoded protein that inhibits the antiretroviral activity of mouse A3 (mA3). Here we show that gPr80 exerts two distinct inhibitory effects on mA3: one that antagonizes deamination-independent restriction and another one that inhibits its deaminase activity. More specifically, we find that the number of N-glycosylated residues in gPr80 inversely correlates with the sensitivity of a gammaretrovirus to deamination by mouse A3 and also, surprisingly, by human A3G. Finally, our work highlights that retroviruses which have successfully integrated into the mouse germ line generally express a gPr80 with fewer glycosylated sites than exogenous retroviruses. This observation supports the suggestion that modulation of A3 deamination intensity could be a desirable attribute for retroviruses to increase genetic diversification and avoid immune detection. Overall, we present here the first description of how gammaretroviruses employ posttranslational modification to antagonize and modulate the activity of a host genome-encoded retroviral restriction factor. IMPORTANCEAPOBEC3 proteins are host factors that have a major role in protecting humans and other mammals against retroviruses. These enzymes hinder their replication and intensely mutate their DNA, thereby inactivating viral progeny and the spread of infection. Here we describe a newly recognized way in which some retroviruses protect themselves against the mutator activity of APOBEC3 proteins. We show that gammaretroviruses expressing an accessory protein called glycosylated Gag, or gPr80, use the host's posttranslational machinery and, more specifically, N-linked glycosylation as a way to modulate their sensitivity to mutations by APOBEC3 proteins. By carefully controlling the amount of mutations caused by APOBEC3 proteins, gammaretroviruses can find a balance that helps them evolve and persist.

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

“…Among the functions that have been attributed to glycogag is protection of MLVs against restriction by mA3 (7, 10, 30, 31). We have tested for this activity in our experimental system.…”

APOBECs and virus restriction

“…Moreover, glyco-Gag-defective viruses reverted to wild-type function during infections of A3-expressing animals, but not A3-null animals, demonstrating the importance of glyco-Gag in antagonizing A3-dependent restriction (Stavrou et al, 2013). Recent data have also indicated that loss of N-linked glycosylation sites in glyco-Gag result in increased hypermutation by A3 (Rosales Gerpe et al, 2015). Interestingly, glyco-Gag-mutant virions are less stable than wild-type particles during ultracentrifugation with detergent (Stavrou et al, 2013).…”

Feline Immunodeficiency Virus Vif N-Terminal Residues Selectively Counteract Feline APOBEC3s

“…Vifs directly bind A3s often in a species-specific manner and recruit them to an E3 ubiquitin ligase complex containing Cullin5 (Cul5), elongin B/C (EloB/C), and RING-box protein RBX2 to induce polyubiquitination and degradation of the A3s by the proteasome (17,18). Other retroviral proteins that counteract A3s are Bet of foamy viruses, the nucleocapsid of human T cell leukemia virus type 1 and the glycosylated (glyco)-Gag of murine leukemia virus (19)(20)(21)(22)(23)(24).…”

“…Primates have seven genes (A3A to A3D, A3F to A3H), whereas cats encode four genes (A3Z2a to A3Z2c, A3Z3) (3)(4)(5). A3 proteins contain either one or two zinc (Z)-binding domains with the conserved motif of HxE(x) [23][24][25][26][27][28] CxxC (where "x" can be any residue) (4). These proteins can target retroviruses and inhibit viral replication by deamination of cytidines in viral single-strand DNA that forms during reverse transcription, introducing G-to-A hypermutations in the coding strand (6)(7)(8)(9)(10).…”