Catalytic Pocket Inaccessibility of Activation-Induced Cytidine Deaminase Is a Safeguard against Excessive Mutagenic Activity

Abstract

Activation-induced cytidine deaminase (AID) mutates cytidine to uridine at immunoglobulin loci toinitiate secondary antibody diversification but alsocauses genome-wide damage. We previously demonstrated that AID has a relatively low catalytic rate. The structure of AID has not been solved. Thus, to probe the basis for its catalytic lethargy we generated a panel of free or DNA-bound AID models based on eight recently resolved APOBEC structures. Docking revealed that the majority of AID:DNA complexes would be inactive due to substrate binding such that a cytidine is not positioned for deamination. Furthermore, we found that most AID conformations exhibit fully or partially occluded catalytic pockets. We constructed mutant and chimeric AID variants predicted to have altered catalytic pocket accessibility dynamics and observed significant correlation with catalytic rate. Data from modeling simulations and functional tests of AID variants support the notion that catalytic pocket accessibility is an inherent bottleneck for AID activity.