The human APOBEC3 (A3) cytidine deaminases, such as APOBEC3G (A3G) and APOBEC3F 
(A3F), are potent inhibitors of Vif-deficient human immunodeficiency virus type 
1 (HIV-1). HIV-1 Vif (viral infectivity factor) binds A3 proteins and targets 
these proteins for ubiquitination and proteasomal degradation. As such, the 
therapeutic blockage of Vif-A3 interaction is predicted to stimulate natural 
antiviral activity by rescuing APOBEC expression and virion packaging. In this 
study, we describe a successful application of the Protein Fragment 
Complementation Assay (PCA) based on the enzyme TEM-1 β-lactamase to study 
Vif-A3 interactions. PCA is based on the interaction between two protein binding 
partners (e.g., Vif and A3G), which are fused to the two halves of a dissected 
marker protein (β-lactamase). Binding of the two partners reassembles 
β-lactamase and hence reconstitutes its activity. To validate our assay, we 
studied the effect of well-described Vif (DRMR, YRHHY) and A3G (D128K) mutations 
on the interaction between the two proteins. Additionally, we studied the 
interaction of human Vif with other members of the A3 family: A3F and APOBEC3C 
(A3C). Our results demonstrate the applicability of PCA as a simple and reliable 
technique for the assessment of Vif-A3 interactions. Furthermore, when compared 
with co-immunoprecipitation assays, PCA appeared to be a more sensitive 
technique for the quantitative assessment of Vif-A3 interactions. Thus, with our 
results, we conclude that PCA could be used to quantitatively study specific 
domains that may be involved in the interaction between Vif and APOBEC proteins.