Polymorphisms in human APOBEC3H differentially regulate ubiquitination and antiviral activity

The APOBEC3 family of cytidine deaminases are an important part of the host innate immune defense against endogenous retroelements and retroviruses like human immunodeficiency virus (HIV). APOBEC3H (A3H) is the most polymorphic of the human APOBEC3 genes, with four major haplotypes circulating in the population. Haplotype II is the only antivirally-active variant of A3H, while the majority of the population possess independently destabilizing polymorphisms present in haplotype I (R105G) and haplotypes III and IV (N15del). Here, we show that instability introduced by either polymorphism is positively correlated with degradative ubiquitination, while haplotype II is protected from this modification. Inhibiting ubiquitination by mutating all of the A3H lysines increased expression of haplotypes III and IV, but these stabilized forms of haplotype III and IV had a strict nuclear localization, and did not incorporate into virions, nor exhibit antiviral activity, thus separating stabilization from function. On the other hand, the instability and functional deficiencies of haplotype III could be rescued by fusion to haplotype II, supporting a model by which antiviral A3H is actively stabilized through a cytoplasmic retention mechanism. Thus, the evolutionary loss of A3H activity in many humans involves functional deficiencies independent of protein stability.