The anti-HIV-1 editing enzyme APOBEC3G binds HIV-1 RNA and messenger RNAs that shuttle between polysomes and stress granules

Deoxycytidine deaminases APOBEC3G (A3G) and APOBEC3F (A3F) (members of the apolipoprotein B mRNA-editing catalytic polypeptide 3 family) have RNA-binding motifs,i nvade assembling human immunodeficiency virus (HIV-1), and hypermutate reverse transcripts. Antagonistically, HIV-1 viral infectivity factor degrades these enzymes. A3G is enzymatically inhibited by binding RNA within an unidentified large cytosolic ribonucleoprotein, implying that RNA degradation during reverse transcription may activate intravirion A3G at the necessary moment. We purified a biologically active tandem affinity-tagged A3G from human HEK293T cells. Mass spectrometry and coimmunoprecipitation from HEK293T and T lymphocyte extracts identified many RNA-binding proteins specifically associated with A3G and A3F, including poly(A)-binding proteins (PABPs), YB-1, Ro-La, RNA helicases, ribosomal proteins, and Staufen1. Most strikingly, nearly all A3G-associated proteins were known to bind exclusively or intermittently to translating and/or dormant mRNAs. Accordingly, A3G in HEK293T and T lymphocyte extracts was almost completely in A3G-mRNA-PABP complexes that shifted reversibly between polysomes and dormant pools in response to translational inhibitors. For example arsenite, which inhibits 5′-cap-dependent translational initiation, shifted mRNA-A3G-PABP from polysomes into stress granules in a manner that was blocked and reversed by the elongation inhibitor cycloheximide. Immunofluorescence microscopy showed A3G-mRNA-PABP stress granules only partially overlapping with Staufen1. A3G coimmunoprecipitated HIV-1RNA and many mRNAs. Ribonuclease released nearly all A3G-associated proteins, including A3G homo-oligomers and A3G-A3F hetero-oligomers, but the viral infectivity factor remained bound. Many proteins and RNAs associated with A3G are excluded from A3G-containing virions, implying that A3G competitively partitions into virions based on affinity for HIV-1 RNA.