Active nuclear import and cytoplasmic retention of activation-induced deaminase

• Published in: Nature structural & molecular biology Vol. 16; no. 5; pp. 517 - 527
• Main Authors: Orthwein, Alexandre, Campo, Vanina A, Di Noia, Javier M, Hu, Yi, Kavli, Bodil, Buschiazzo, Alejandro, Patenaude, Anne-Marie
• Format: Journal Article
• Language: English
• Published: New York Nature Publishing Group US 01-05-2009; Nature Publishing Group
• Subjects: Active Transport, Cell Nucleus; alpha Karyopherins; alpha Karyopherins - metabolism; Biochemistry; Biochemistry, Molecular Biology; Biological Microscopy; Biomedical and Life Sciences; Cell Nucleus; Cell Nucleus - enzymology; Cellular Biology; Cytidine Deaminase; Cytidine Deaminase - chemistry; Cytidine Deaminase - metabolism; Cytoplasm; Diffusion; Enzymes; Genetic aspects; Health aspects; HeLa Cells; Humans; Immunoglobulins; Life Sciences; Membrane Biology; Models, Molecular; Molecular biology; Mutation; Nuclear Localization Signals; Physiological aspects; Protein Binding; Protein Conformation; Protein Structure; Protein Structure, Tertiary; Retention; Signal transduction; Subcellular Fractions; Subcellular Fractions - enzymology; Canada
• ISSN: 1545-9993; 1545-9985
• DOI: 10.1038/nsmb.1598

The enzyme activation-induced deaminase (AID) promotes antibody diversification after B-cell activation, by causing mutagenic lesions on DNA. Hence, AID's actions must be tightly controlled. AID is found mainly in the cytosolic compartment and contains a known nuclear export sequence. Now a structural nuclear localization sequence and a cytosolic-retention determinant are identified in AID and found to have a role in localization and function.
The enzyme activation-induced deaminase (AID) triggers antibody diversification in B cells by catalyzing deamination and consequently mutation of immunoglobulin genes. To minimize off-target deamination, AID is restrained by several regulatory mechanisms including nuclear exclusion, thought to be mediated exclusively by active nuclear export. Here we identify two other mechanisms involved in controlling AID subcellular localization. AID is unable to passively diffuse into the nucleus, despite its small size, and its nuclear entry requires active import mediated by a conformational nuclear localization signal. We also identify in its C terminus a determinant for AID cytoplasmic retention, which hampers diffusion to the nucleus, competes with nuclear import and is crucial for maintaining the predominantly cytoplasmic localization of AID in steady-state conditions. Blocking nuclear import alters the balance between these processes in favor of cytoplasmic retention, resulting in reduced isotype class switching.