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Role of Activation-Induced Deaminase Protein Kinase A Phosphorylation Sites in Ig Gene Conversion and Somatic Hypermutation¹

Monalisa Chatterji^*, Shyam Unniraman^*, Kevin M. McBride and David G. Schatz^2,*

^* Howard Hughes Medical Institute, Department of Immunobiology, Yale University School of Medicine, New Haven, CT 06510; and Laboratory of Molecular Immunology, Rockefeller University, New York, NY 10021

Activation-induced deaminase (AID) is thought to initiate somatic hypermutation (SHM), gene conversion (GCV), and class switch recombination (CSR) by the transcription-coupled deamination of cytosine residues in Ig genes. Phosphorylation of AID by protein kinase A (PKA) and subsequent interaction of AID with replication protein A (RPA) have been proposed to play important roles in allowing AID to deaminate DNA during transcription. Serine 38 (S38) of mouse AID is phosphorylated in vivo and lies in a consensus target site for PKA, and mutation of this residue interferes with CSR and SHM. In this study, we demonstrate that S38 in mouse and chicken AID is phosphorylated in chicken DT40 cells and is required for efficient GCV and SHM in these cells. Paradoxically, zebra fish AID, which lacks a serine at the position corresponding to S38, has previously been shown to be active for CSR and we demonstrate that it is active for GCV/SHM. Aspartate 44 (D44) of zebra fish AID has been proposed to compensate for the absence of the S38 phosphorylation site but we demonstrate that mutation of D44 has no effect on GCV/SHM. Some features of zebra fish AID other than D44 might compensate for the absence of S38. Alternatively, the zebra fish protein might function in a manner that is independent of PKA and RPA in DT40 cells, raising the possibility that, under some circumstances, AID mediates efficient Ig gene diversification without the assistance of RPA.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by the Howard Hughes Medical Institute.

² Address correspondence and reprint requests to Dr. David G. Schatz, Howard Hughes Medical Institute, Department of Immunobiology, Yale Medical School, 300 Cedar Street, Box 208011, New Haven, CT 06520-8011. E-mail address: david.schatz{at}yale.edu

³ Abbreviations used in this paper: SHM, somatic hypermutation; CSR, class switch recombination; GCV, gene conversion; AID, activation-induced deaminase; UNG, uracil DNA glycosylase; PKA, protein kinase A; RPA, replication protein A; TPM, templated point mutation; NPM, nontemplated point mutation; WT, wild type; EGFP, enhanced GFP.

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