Controlling lentiviruses: single amino acid changes can determine specificity.

T he human immunodeficiency virus (HIV) is a member of the lentivirus family of retroviruses that we share with other primates. The known strains of primate lentiviruses fall into six major lineages identified from 25 species of African primates (1), and it is clear from phylogenetic analysis of lentiviral sequences that their evolutionary history involves a number of cross-species transmissions. Indeed, HIV-1 is a close relative of a virus found in chimpanzees (simian immunodeficiency virus from chimpanzees, or SIVcpz), and HIV-2 is a relative of a virus found in sooty mangabeys (SIVsmm). However, it is not clear whether other members of the primate lentivirus family could spread to humans under the right transmission conditions. Thus, it is important to understand the potential barriers that govern crossspecies transmissions from animals to human. Two articles in this issue of PNAS (2, 3) and also an article in press at PNAS (4) indicate that a single amino acid position in a host antiviral enzyme called Apobec3G (5) may be one such barrier that protects humans from primate lentiviruses found in the widely distributed family of AGMs (Chlorocebus aethiops) (Fig. 1). Innate cellular antiviral responses limit the damage that viruses inflict on their hosts. However, such defensive measures create strong selective pressures leading to the evolution of viral countermeasures. The study of the viral strategies to evade host antiviral responses has often led to better understanding of underlying cellular processes, for example, control of IFN responses, antigen processing, antiapoptotic responses, and gene silencing. A more recently described example of an innate antiviral response overcome by the acquisition of a specific viral countermeasure is the Apobec3G protein. Apobec3G is a cellular protein that is incorporated into the virion of retroviruses and acts during the process of reverse transcription (copying viral RNA into DNA) to direct the deamination of cytidine to uridine on the minus strand of viral DNA. This results in massive mutation of guanidine to adenines on the coding strand (6–9) and or degradation of the viral genome (10). Consequently, lentiviruses (with one known exception) have acquired a gene called vif, whose gene product prevents the incorporation and subsequent antiviral activity of Apobec3G by binding and targeting it down a proteasomedependent degradation pathway (11–13). Apobec3G belongs to a superfamily of nucleic acid deaminases that includes Apobec1, activation induced deaminase (AID), and Escherichia coli cytidine deaminase (ECCDA). Although the substrates for these deaminases vary, the amino acid sequence and catalytic mechanism for cytidine deamination is largely conserved across the family. The apobec3 locus shows evidence of relatively recent expansion and tandem duplication resulting in five genes and two probable pseudogenes (Apobec3A– Apobec3G) (14) with distinct patterns of expression, although their normal functions are unknown (15). Previous studies suggested that the Vif protein of various primate lentiviruses typically functioned only in the host species from which the virus was derived (16), and the species-specific exclusion of Apobec3G in virions by Vif (17) accounted for this host specificity. That is, human Apobec3G was inhibited by Vif from HIV-1 but not by Vif from the SIV of AGMs (SIVagm), whereas AGM apobec3G was inhibited by Vif from SIVagm but not by Vif of HIV-1 (Fig. 1). This finding raised the possibility that each lentivirus has a Vif protein suitably evolved for evading the innate antiretroviral defense strategy by the given Apobec3G of its usual host. Humans encode an aspartate at amino acid 128 (D128) in Apobec3G, whereas AGM encodes a lysine at that position (K128) (Fig. 1). Remarkably, swapping the two amino acids reverses the species specificity: human Apobec3G (D128K) is no longer inhibited by HIV Vif but is inhibited by SIVagm Vif, and vice versa (Fig. 1). Therefore, WT HIV has the same phenotype as HIV lacking the vif gene if it infects a human cell harboring the D128K mutation in Apobec3G. These results suggest that a single amino acid is the sole determinant of the differential interaction of the two Vif proteins with the Apobec3G proteins of their respective hosts. An obvious difference between the amino acids positioned at 128 in AGM and humans is a net charge of two. By making several different mutations at this position, two groups concluded that human Apobec3G must be neutral or negatively charged at amino acid 128 to functionally interact with HIV Vif, whereas AGM Apobec3G must be neutral or positively charged at amino acid 128 to functionally interact with SIVagm Vif (2, 3). Based on homology modeling to E. coli cytidine deaminase, amino acid 128 lies in a loop between the catalytic domain and the pseudocatalytic domain (2). This predicted solvent-