Impact of phosphorylation and phosphorylation-null mutants on the activity and deamination specificity of activation-induced cytidine deaminase.

Activation-induced cytidine deaminase (AID) initiates somatic hypermutation and class switch recombination in B cells by deaminating C --> U on transcribed DNA. Here we analyze the role of phosphorylation and phosphorylation-null mutants on the biochemical behavior of AID, including enzyme specific activity, processivity, deamination spectra, deamination motif specificity, and transcription-dependent deamination in the presence and absence of RPA. We show that a small fraction of recombinant human AID expressed in Sf9 insect cells is phosphorylated at previously identified residues Ser(38) and Thr(27) and also at Ser(41) and Ser(43). S43P AID has been identified in a patient with hyper-IgM immunodeficiency syndrome. Ser-substituted phosphorylation-null mutants (S38A, S41A, S43A, and S43P) exhibit wild type (WT) activity on single-stranded DNA. Deamination of transcribed double-stranded DNA is similar for WT and mutant AID and occurs with or without RPA. Although WT and AID mutants catalyze processive deamination favoring canonical WRC hot spot motifs (where W represents A/T and R is A/G), their deamination spectra differ significantly. The differences between the WT and AID mutants appear to be caused by the replacement of Ser as opposed to an absence of phosphorylation. The spectral differences reflect a marked change in deamination efficiencies in two motifs, GGC and AGC, which are preferred by mutant AID but disfavored by WT AID. Both motifs occur with exceptionally high frequency in human switch regions, suggesting a possible relationship between AID deamination specificity and a loss of antibody diversification.