Polymorphisms in Human APOBEC3H Differentially Regulate Ubiquitination and Antiviral Activity.

Nicholas M Chesarino,Michael Emerman

doi:10.3390/v12040378

Abstract

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). In this paper, 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 the 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. Fusion chimeras with haplotype II allowed for stabilization, cytoplasmic retention, and packaging of the N15del-containing haplotype III, but the haplotype III component of these chimeras was unable to restrict HIV-1 on its own. Thus, the evolutionary loss of A3H activity in many humans involves functional deficiencies independent of protein stability.

Highlights

Restriction factors are a critical component of the innate immune response to cellular infection by viruses by inhibiting various stages of the viral replication cycle
We found that the rates of ubiquitination were greater in unstable haplotypes I, III, and IV, while the stable haplotype II was largely protected from this modification
We found that viral incorporation of our chimeras matched their respective Human Immunodeficiency Virus (HIV)-1 restriction with the surprising exception of haplotype III-containing chimeras (Figure 5B)

Summary

Introduction

Restriction factors are a critical component of the innate immune response to cellular infection by viruses by inhibiting various stages of the viral replication cycle. The APOBEC3 (A3) family of cytidine deaminases are potent restriction factors against endogenous retroelements and retroviruses, with four members of this family (A3D, A3F, A3G, and A3H) demonstrating considerable antiviral activity against lentiviruses such as Simian Immunodeficiency Virus (SIV) and Human Immunodeficiency. The antiviral A3 proteins incorporate into nascent virions and act during the reverse transcription stage of retroviral infection to convert cytosines to uracils in viral single-stranded DNA [2]. Vif hijacks a host cellular E3 ubiquitin ligase complex and acts as a substrate receptor for A3 proteins, promoting their ubiquitination and subsequent degradation via the proteasome [8,9,10,11]. The direct antagonistic relationship between host A3 and lentiviral Vif underlies an evolutionary arms race that selects for mutations in A3 that can escape Vif antagonism, and mutations

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