HIV-1 Vif requires core binding factor Beta to degrade the APOBEC3 restriction factors and facilitate viral replication

Abstract

While there are a number of antiretroviral drugs for the treatment of Human Immunodeficiency Virus (HIV), they are all expensive, invasive, susceptible to resistance, and are not curative. One potential future drug target is the interaction between the human antiviral APOBEC3 proteins and the HIV counterdefense protein, Vif. Vif binds to and neutralizes the DNA-mutating APOBEC3 proteins by recruitment of an E3 ubiquitin ligase complex that targets them for degradation. Design of small molecule therapeutics to disrupt this interaction and free the antiviral APOBEC3 proteins has been hampered by an incomplete understanding of the Vif E3 ubiquitin ligase complex and conflicting reports as to which of the seven different APOBEC3 proteins contribute to HIV restriction in vivo. To determine which APOBEC3 proteins contribute to HIV restriction, we performed a comprehensive analysis of both human and rhesus macaque APOBEC3 families in T cells. Based on six criteria (expression, virion incorporation, HIV restriction, viral genome mutation, neutralization by Vif, and conservation), we found that four APOBEC3 proteins have the potential to restrict HIV replication. To better understand the Vif E3 ligase complex responsible for neutralizing these proteins, we performed extensive purification experiments with HIV Vif and discovered that Vif interacts with the cellular transcription factor Core Binding Factor Beta (CBFB). We discovered that CBFB not only allows for reconstitution of the Vif E3 ligase complex in vitro, but also stabilizes Vif in vivo, subsequently facilitating ligase assembly and allowing for APOBEC3 degradation. This functional interaction is highly conserved, being required to enhance the steady-state levels of Vif proteins from all tested HIV subtypes and required for the degradation of all restrictive human and rhesus APOBEC3 proteins by their respective lentiviral Vif proteins. Mutagenesis screening revealed that CBFB interacts with Vif and its normal RUNX transcription partners on genetically separable interfaces, indicating this essential virus-host interaction may serve as a viable drug target with minimal off-target effects. Disruption of this newly identified and highly conserved CBFB-Vif interaction would release the entire multitude of restrictive APOBEC3 proteins and significantly inhibit HIV infection, making this interaction a promising new target for small molecule therapeutics.