Down-modulation of primate lentiviral receptors by Nef proteins of simian immunodeficiency virus (SIV) of chimpanzees (SIVcpz) and related SIVs: implication for the evolutionary event at the emergence of SIVcpz

Phylogenetic Reconstruction and Functional Characterization of the Ancestral Nef Protein of Primate Lentiviruses

Nef is an accessory protein unique to the primate HIV-1, HIV-2, and SIV lentiviruses. During infection, Nef functions by interacting with multiple host proteins within infected cells to evade the immune response and enhance virion infectivity. Notably, Nef can counter immune regulators such as CD4 and MHC-I, as well as the SERINC5 restriction factor in infected cells. In this study, we generated a posterior sample of time-scaled phylogenies relating SIV and HIV Nef sequences, followed by reconstruction of ancestral sequences at the root and internal nodes of the sampled trees up to the HIV-1 Group M ancestor. Upon expression of the ancestral primate lentivirus Nef protein within CD4+ HeLa cells, flow cytometry analysis revealed that the primate lentivirus Nef ancestor robustly downregulated cell-surface SERINC5, yet only partially downregulated CD4 from the cell surface. Further analysis revealed that the Nef-mediated CD4 downregulation ability evolved gradually, while Nef-mediated SERINC5 downregulation was recovered abruptly in the HIV-1/M ancestor. Overall, this study provides a framework to reconstruct ancestral viral proteins and enable the functional characterization of these proteins to delineate how functions could have changed throughout evolutionary history.

A naturally occurring feline APOBEC3 variant that loses anti-lentiviral activity by lacking two amino acid residues

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) is a mammalian protein that restricts lentiviral replication. Various polymorphisms of mammalian APOBEC3 genes have been observed in humans, Old World monkeys and domestic cats; however, the genetic diversity of APOBEC3 genes in other mammals remains unaddressed. Here we identify a novel haplotype of the feline APOBEC3Z3 gene, an APOBEC3 gene that restricts feline immunodeficiency virus (FIV) replication, in a Eurasian lynx (Lynx lynx). Compared to the previously identified lynx APOBEC3Z3 (haplotype I), the new sequence (haplotype II) harbours two amino acid deletions (Q16 and H17) and a nonsynonymous substitution (R68Q). Interestingly, lynx APOBEC3Z3 haplotype II does not suppress FIV infectivity, whereas haplotype I does. Mutagenesis experiments further revealed that deleting two amino acids (Q16 and H17) causes anti-FIV activity loss. This report demonstrates that a naturally occurring APOBEC3 variant loses anti-lentiviral activity through the deletion of two amino acid residues.

Species-specific differences in the ability of feline lentiviral Vif to degrade feline APOBEC3 proteins

How host-virus co-evolutionary relationships manifest is one of the most intriguing issues in virology. To address this topic, the mammal-lentivirus relationship can be considered as an interplay of cellular and viral proteins, particularly apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3) and viral infectivity factor (Vif). APOBEC3s enzymatically restrict lentivirus replication, whereas Vif antagonizes the host anti-viral action mediated by APOBEC3. In this study, the focus was on the interplay between feline APOBEC3 proteins and two feline immunodeficiency viruses in cats and pumas. To our knowledge, this study provides the first evidence of non-primate lentiviral Vif being incapable of counteracting a natural host's anti-viral activity mediated via APOBEC3 protein.