ABSTRACT
The innate antiviral factor APOBEC3G (A3G) possesses RNA binding activity and deaminates HIV-1 DNA. High-molecular mass forms of A3G can be isolated from a variety of cell types but exhibit limited deaminase activity relative to low-molecular mass species prepared under RNA-depleted conditions. To investigate the fundamental oligomeric state and shape of A3G, we conducted sedimentation velocity analyses of the pure enzyme under RNA-deficient conditions. The results reveal a predominant dimer in equilibrium with minor monomeric and tetrameric species. Hydrodynamic modeling of the dimer supports an extended cylindrical shape that assembles into an elongated tetramer. Overall, the results provide physical restraints for the A3G quaternary structure that have implications for modulating antiviral function.
Subject(s)
Anti-HIV Agents/pharmacology , Biopolymers/chemistry , Cytidine Deaminase/chemistry , APOBEC-3G Deaminase , Models, Molecular , Molecular WeightABSTRACT
Human APOBEC3G (hA3G) is a cytidine deaminase that restricts human immunodeficiency virus (HIV)-1 infection in a vif (the virion infectivity factor from HIV)-dependent manner. hA3G from HIV-permissive activated CD4+ T-cells exists as an inactive, high molecular mass (HMM) complex that can be transformed in vitro into an active, low molecular mass (LMM) variant comparable with that of HIV-non-permissive CD4+ T-cells. Here we present low resolution structures of hA3G in HMM and LMM forms determined by small angle x-ray scattering and advanced shape reconstruction methods. The results show that LMM particles have an extended shape, dissimilar to known cytidine deaminases, featuring novel tail-to-tail dimerization. Shape analysis of LMM and HMM structures revealed how symmetric association of dimers could lead to minimal HMM variants. These observations imply that the disruption of cellular HMM particles may require regulation of protein-RNA, as well as protein-protein interactions, which has implications for therapeutic development.
