Dissociation of in vitro DNA deamination activity and physiological functions of AID mutants.

Activation-induced cytidine deaminase (AID) is essential for the DNA cleavage that initiates both somatic hypermutation (SHM) and class switch recombination (CSR) of the Ig gene. Two alternative mechanisms of DNA cleavage by AID have been proposed: RNA editing and DNA deamination. In support of the latter, AID has DNA deamination activity in cell-free systems that is assumed to represent its physiological function. To test this hypothesis, we generated various mouse AID mutants and compared their DNA deamination, CSR, and SHM activities. Here, we compared DNA deamination, CSR, and SHM activities of various AID mutants and found that most of their CSR or SHM activities were disproportionate with their DNA deamination activities. Specifically, we identified a cluster of mutants (H48A, L49A, R50A, and N51A) with low DNA deamination activity but relatively intact CSR activity. Of note is an AID mutant (N51A) that retained CSR function but lost DNA deamination activity. In addition, an APOBEC1 mutation at N57, homologous to N51 of AID, also abolished DNA deamination activity but retained RNA editing activity. These results indicate that DNA deamination activity does not represent the physiological function of AID.