Activation-induced cytidine deaminase (AID) is a member of the apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) family of cytidine deaminases. AID mutates immunoglobulin loci to initiate secondary antibody diversification. The APOBEC3 (A3) sub-branch mutates viral pathogens in the cytosol and acidic endosomal compartments. Accordingly, AID functions optimally near-neutral pH, while most A3s are acid-adapted (optimal pH 5.5–6.5). To gain a structural understanding for this pH disparity, we constructed high-resolution maps of AID catalytic activity vs pH. We found AID's optimal pH was 7.3 but it retained most (>70%) of the activity at pH 8. Probing of ssDNA-binding residues near the catalytic pocket, key for bending ssDNA into the pocket (e.g. R25) yielded mutants with altered pH preference, corroborating previous findings that the equivalent residue in APOBEC3G (H216) underlies its acidic pH preference. AID from bony fish exhibited more basic optimal pH (pH 7.5–8.1) and several R25-equivalent mutants altered pH preference. Comparison of pH optima across the AID/APOBEC3 family revealed an inverse correlation between positive surface charge and overall catalysis. The paralogue with the most robust catalytic activity (APOBEC3A) has the lowest surface charge and most acidic pH preference, while the paralogue with the most lethargic catalytic rate (AID) has the most positive surface charge and highest optimal pH. We suggest one possible mechanism is through surface charge dictating an overall optimal pH that is different from the optimal pH of the catalytic pocket microenvironment. These findings illuminate an additional structural mechanism that regulates AID/APOBEC3 mutagenesis.
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Cover Image
Cover Image
In this issue Oldham and colleagues (pp. 57–74) have performed extensive structural and kinetic characterization of the SAT enzyme from the antibiotic-resistant pathogen Neisseria gonorrhoeae. Using X-ray crystallography, we have solved the structures of NgSAT with the non-natural ligand, l-malate. They present data which could be explored to combat the rise of antimicrobial resistant N. gonorrhoeae. The cover image shows a ribbon diagram of the NgSAT trimer illustrating the position and formation of the active site between two adjacent monomers. The image is courtesy of Joanna Hicks.
The optimal pH of AID is skewed from that of its catalytic pocket by DNA-binding residues and surface charge
Atefeh Ghorbani, Justin J. King, Mani Larijani; The optimal pH of AID is skewed from that of its catalytic pocket by DNA-binding residues and surface charge. Biochem J 14 January 2022; 479 (1): 39–55. doi: https://doi.org/10.1042/BCJ20210529
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