Murine Leukemia Virus P50 Protein Counteracts APOBEC3 by Blocking Its Packaging

Abstract

<jats:p>Apolipoprotein B editing, catalytic subunit 3 (APOBEC3) family members are cytidine deaminases that play important roles in intrinsic responses to retrovirus infection. Complex retroviruses like HIV-1 encode the viral infectivity factor (Vif) protein to counteract APOBEC3 proteins. Vif induces degradation of APOBEC3G and other APOBEC3 proteins and thereby prevents their packaging into virions. It is not known if murine leukemia virus (MLV) encodes a Vif-like protein. Here we show that the MLV P50 protein, produced from an alternatively spliced <jats:italic>gag</jats:italic> RNA, interacts with the C-terminus of mouse APOBEC3 and prevents its packaging without causing its degradation. By infecting APOBEC3 knockout and wild type mice with Friend or Moloney MLV P50-deficient viruses, we found that APOBEC3 restricts the mutant viruses more than WT viruses <jats:italic>in vivo</jats:italic>. Replication of P50-mutant viruses in an APOBEC3-expressing stable cell line was also much slower than WT viruses and overexpressing P50 in this cell line enhanced mutant virus replication. Thus, MLV encodes a protein, P50, that overcomes APOBEC3 restriction by preventing its packaging into virions.</jats:p> <jats:p><jats:bold>IMPORTANCE</jats:bold> MLV has existed in mice for at least a million years, in spite of the existence of host restriction factors that block infection. Although MLV is considered a simple retrovirus when compared to lentiviruses, it does encode proteins generated from alternatively spliced RNAs. Here we show that P50, generated from an alternatively spliced RNA encoded in <jats:italic>gag</jats:italic>, counteracts APOBEC3 by blocking its packaging. MLV also encodes a protein, glycoGag, that increases capsid stability and limits APOBEC3 access to the reverse transcription complex (RTC). Thus, MLV has evolved multiple means of preventing APOBEC3 from blocking infection, explaining its survival as an infectious pathogen in mice.</jats:p>

The Role of APOBECs in Viral Replication

“…A recent study by this group also suggested that M-MLV and F-MLV encode a second viral protein that antagonizes APOBEC restriction [312]. Previous experiments suggested that alternative splicing of genomic MLV RNA leads to two proteins, p50 and p60, which are translated from the Gag AUG and CUG initiation codons, respectively [313].…”

Degradation-Independent Inhibition of APOBEC3G by the HIV-1 Vif Protein

“…Murine leukemia virus (MLV) possesses two mechanisms to overcome the restriction by mouse A3. On the one hand, its P50 protein, produced from an alternatively spliced gag RNA [ 145 , 146 ], prevents mouse A3 packaging by interacting with its C-terminus CD2 domain, but impacts neither its degradation nor its deaminase activity [ 147 ]. Considering that the C-terminus of mouse A3 is necessary for its incorporation into viruses, it is not surprising that P50 binding to this domain affects A3 packaging.…”

Retroviral Restriction Factors and Their Viral Targets: Restriction Strategies and Evolutionary Adaptations

“…Unlike the degradative impact of Vif on A3G, g-gag antagonizes mA3 indirectly by stabilizing the viral core and preventing A3 access to the viral DNA/RNA by shielding the viral reverse transcriptase complex [ 167 ]. Apart from g-gag, mA3-mediated restriction of MLV replication is also be counteracted by the viral p50 protein, produced by alternative splicing of gag , which interacts with mA3 and prevents its packaging into newly produced virions [ 168 , 169 ].…”