Combinatorial mechanisms regulating AID-dependent DNA deamination: interacting proteins and post-translational modifications

Abstract

Protective humoral immune responses result from immunoglobulin (Ig) diversification reactions that proceed through programmed DNA double-strand breaks and mutations in developing or mature B cells. While primary Ig diversity is dependent on V(D)J recombination and the RAG proteins, secondary diversification is achieved through class switch recombination (CSR) and somatic hypermutation (SHM), which require AID (activation induced deaminase). Because aberrant AID activity can result in mutations in non-Ig loci and DNA translocations between the Ig locus and non-Ig genes, the activity of AID must be stringently regulated. AID mRNA expression is regulated transcriptionally by cytokine stimulation and post-transcriptionally by miRNAs. AID activity is regulated by post-translational modifications, subcellular localization, and interaction with other proteins. All of these molecular mechanisms have evolved to specifically induce AID-dependent mutations and DNA double-strand breaks at the Ig loci to promote maximal Ig gene diversification while limiting the access of this mutator to non-Ig regions.

Figure 1

Mechanisms regulating AID protein levels and subcellular localization. Cytoplasmic AID is stabilized by Hsp90, whereas nuclear AID is degraded by ubiquitylation or interaction with REG-γ. AID enters the nucleus passively through nuclear pores or actively through its putative N-terminal NLS (nuclear localization signal). AID is actively exported from the nucleus through its C-terminal NES sequence, which binds to Exportin1.

Figure 2

Proteins interacting with AID at S regions during CSR

Figure 3

AID phosphorylation sites. The primary structure of AID is shown with the numbers below indicating the amino acid residues that demarcate the putative NLS, the cytidine deaminase catalytic domain, and the NES (nuclear export sequence). Sites of AID phosphorylation are indicated with vertical lines above the primary structure, where boxed amino acids indicate AID phosphorylation sites that were identified in B cells undergoing CSR.

Figure 4

Model of AID activation and RPA recruitment during class switch recombination. Activation of CSR induces the recruitment of PKA and AID either independently or as a complex through a putative, unidentified targeting factor X. The PKA regulatory subunits (R) inhibit AID phosphorylation and keep AID inactive for CSR until a burst of cAMP induces the release of the catalytic subunits (C), which phosphorylate AID. Phosphorylated AID recruits RPA to S regions to activate the CSR cascade.