MDM2 can interact with the C-terminus of AID but it is inessential for antibody diversification in DT40 B cells

Activation-induced deaminase (AID) is essential for immunoglobulin gene diversification by the distinct processes of class switch recombination, somatic hypermutation and gene conversion. Most evidence indicates that AID triggers these reactions through the direct deamination of cytosine residues in the DNA. However, AID is predominantly cytoplasmic and the mechanism that directs it to the immunoglobulin loci remains elusive. Like its homolog APOBEC1, which requires at least one additional factor to efficiently edit APOB RNA, other proteins are likely to be required for the proper targeting of AID to the immunoglobulin loci. Here, we show that AID can interact with MDM2, an oncoprotein that shuttles between the nucleus and the cytoplasm and targets p53 for nuclear export and degradation. This interaction mapped to the carboxy-terminal region of AID that harbors a nuclear export sequence, suggesting that MDM2 may be involved in the nucleo-cytoplasmic trafficking of AID. We therefore assessed the role of MDM2 in immunoglobulin gene diversification by disrupting MDM2 in DT40, an avian B cell line that constitutively undergoes AID-dependent immunoglobulin gene diversification. The subcellular localization of AID was unaffected in MDM2-deficient DT40 cells. However, slight hyper-and hypo-conversion phenotypes were caused by MDM2-abrogation and overexpression, respectively. These observations suggested that MDM2 has the capacity to negatively regulate AID. Intriguingly, the same carboxy-terminal residues of AID were recently shown to be inessential for somatic hypermutation and immunoglobulin gene conversion but they were strictly required for class switch recombination.