Identification of Two APOBEC3F Splice Variants Displaying HIV-1 Antiviral Activity and Contrasting Sensitivity to Vif*

Abstract

Approximately half of all human genes undergo alternative mRNA splicing. This process often yields homologous gene products exhibiting diverse functions. Alternative splicing of APOBEC3G (A3G) and APOBEC3F (A3F), the major host resistance factors targeted by the HIV-1 protein Vif, has not been explored. We investigated the effects of alternative splicing on A3G/A3F gene expression and antiviral activity. Three alternatively spliced A3G mRNAs and two alternatively spliced A3F mRNAs were detected in peripheral blood mononuclear cells in each of 10 uninfected, healthy donors. Expression of these splice variants was altered in different cell subsets and in response to cellular stimulation. Alternatively spliced A3G variants were insensitive to degradation by Vif but displayed no antiviral activity against HIV-1. Conversely, alternative splicing of A3F produced a 37-kDa variant lacking exon 2 (A3F⌬2) that was prominently expressed in macrophages and monocytes and was resistant to Vif-mediated degradation. Alternative splicing also produced a 24-kDa variant of A3F lacking exons 2-4 (A3F⌬2-4) that was highly sensitive to Vif. Both A3F⌬2 and A3F⌬2-4 displayed reduced cytidine deaminase activity and moderate antiviral activity. These alternatively spliced A3F gene products, particularly A3F⌬2, were incorporated into HIV virions, albeit at levels less than wild-type A3F. Thus, alternative splicing of A3F mRNA generates truncated antiviral proteins that differ sharply in their sensitivity to Vif.Human A3G and A3F belong to the family of APOBEC3 cytidine deaminases that exert antiviral activity against diverse exogenous viruses, including HIV-1 (reviewed in Ref. 1). In humans, seven A3 proteins (designated A3A, A3B, A3C, A3DE, A3F, A3G, and A3H) are tandemly arrayed on chromosome 22 (2, 3). In contrast, mice have only a single A3 gene (mA3) located on a syntenic region of chromosome 15 (4, 5). Despite extensive evolutionary expansion of this locus, expression of human A3 genes remains tightly regulated and tissue-specific (3). Alternative splicing might represent one mechanism whereby the level and action of the A3 family is modulated.Alternative splicing of pre-mRNA represents a major mechanism for expanding gene function and diversity (6, 7). Approximately 40 -60% of human genes are alternatively spliced (8 -13), and about one-quarter of this alternative splicing is conserved across species (11). Read-through alternative splicing of feline A3 genes generates a novel functional isoform (4), whereas alternative splicing and/or diminished expression of mA3 impairs the ability of certain strains of mice to recover from Friend virus complex infection (Rfv3) (14, 15). In humans, gene polymorphisms and alternative splicing of A3H result in diverse effects on antiviral activity (16). However, it is not known whether A3G and A3F, the major anti-HIV A3 enzymes and targets of HIV Vif, are functionally regulated by alternative splicing.A3G and A3F contain dual conserved catalytic domains located in the N-and C-terminal regions termed...

A Single Amino Acid in Human APOBEC3F Alters Susceptibility to HIV-1 Vif

“…1). An additional recent report on the existence of a Vif-susceptible splice variant of huA3F composed largely of the CTD is also consistent with these data (36).…”

Identification of Specific Determinants of Human APOBEC3F, APOBEC3C, and APOBEC3DE and African Green Monkey APOBEC3F That Interact with HIV-1 Vif

“…Thus, it is certainly possible that A3F has multiple determinants that contribute to its interaction with Vif. In agreement with this, Lassen et al recently reported that the N terminus of A3F has domains that are involved in regulating Vif sensitivity (23).…”

Isolation of a TRAIL Antagonist from the Serum of HIV-infected Patients

“…Inactivation of the host restriction factors encoded by the apoplipoprotein B mRNA editing, enzyme catalytic polypeptide family of proteins by Vif (57), and the host counter-evasion of this strategy by producing APOBEC3F splice variants that are Vif resistant but that maintain antiviral activity (58) are prime examples. Because the nature of the HIV life cycle is such that multiple transcriptional splice variants are required for productive HIV replication, the ability of HIV to impact splicing of host transcripts is not surprising (59).…”