The gene, APOBEC3G, belongs to a family of primate genes that produce enzymes (in this case, APOBEC3G) that "edit" DNA and RNA, by slipping into viral particles and inducing mutations that replace one base (cytosine) with another (uracil) as the virus undergoes reverse transcription in the host cell's cytoplasm. The edited virus fails to replicate. HIV, in turn, generates a protein called Vif that binds to the APOBEC3G enzyme and targets it for degradation, thereby eliminating its antiviral activity.
Since the protein-binding regions that govern these interactions have a direct effect on the fitness of both virus and host, one would expect to see the proteins angling for advantage, with Vif maximizing its ability to recognize APOBEC3G and APOBEC3G doing its best to evade Vif. Such battles are thought to result in frequent mutations that alter the amino acids involved in the interaction; the perpetuation of such advantageous mutations is called positive selection.
As predicted, the APOBEC3G gene is under strong positive selection. But that selection characterized by an analysis of twelve primates species spanning 33 million years of evolution - appears to predate the existence of HIV-type viruses. So, what has been fueling APOBEC3G's rapid evolution?
APOBEC3G and Vif interact in T-cells, but the fact that selective pressure on APOBEC3G has been constant over the course of primate evolution suggests that another force is also acting on the gene. Sawyer et al. propose that this force is exerted by human endogenous retroviruses, which act in many ways like foreign retroviruses. These mobile genetic elements emanating from one's own genome pose less of an immediate threat than a retrovirus like
Contact: Catriona MacCallum
Public Library of Science