The APOBEC3 antiviral factors are single-stranded DNA cytosine deaminases that potently restrict HIV replication by hypermutation of viral cDNA replication intermediates. Several of these enzymes also have the capacity to restrict virus replication by a deaminase-independent mechanism that involves direct binding to viral RNA genomes. HIV counteracts this threat by encoding an accessory protein called Vif, which hijacks cellular CBFβ and an E3-ligase complex to bind up to five APOBEC3s and target them for proteasomal degradation (APOBEC3C, APOBEC3D, APOBEC3F, APOBEC3G, and APOBEC3H). Despite considerable progress over the past two decades, a comprehensive biochemical and structural understanding of these APOBEC3-Vif complexes is still lacking. Here, we propose to fill these gaps in knowledge through two specific aims. In Aim 1 we will determine structures of Vif-CBFβ-E3 ubiquitin ligase complexes. In Aim 2 we will determine the atomic details of multiple A3-Vif complexes using x-ray crystallography and/or single-particle cryo-EM. These structures will demonstrate the mechanisms used by Vif to interact with multiple host factors and protect the virus from APOBEC3-mediated restriction. A detailed biochemical and structural understanding of these host-pathogen interactions is anticipated to inform the development of new antiviral strategies such as therapeutic targeting of conserved structural motifs.