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* Gene Therapy Vector Laboratory, and
Laboratory of Gene Activation, North Shore-Long Island Jewish Research Institute, and Departments of Medicine, North Shore University Hospital and New York University School of Medicine, Manhasset, NY 11030
IL-4-induced gene-1 (Il4i1 or Fig1) initially isolated as a gene of unknown function from mouse B lymphocytes, is limited in expression to primarily immune tissues and genetically maps to a region of susceptibility to autoimmune disease. The predicted Il4i1 protein (IL4I1) sequence is most similar to apoptosis-inducing protein and Apoxin I, both L-amino acid oxidases (LAAO; Enzyme Commission 1.4.3.2). We demonstrate that IL4I1 has unique LAAO properties. IL4I1 has preference for aromatic amino acid substrates, having highest specific activity with phenylalanine. In support of this selectivity, IL4I1 is inhibited by aromatic competitors (benzoic acid and para-aminobenzoic acid), but not by nonaromatic LAAO inhibitors. Il4i1 protein and enzyme activity is found in the insoluble fraction of transient transfections, implying an association with cell membrane and possibly intracellular organelles. Indeed, IL4I1 has the unique property of being most active at acidic pH (pH 4), suggesting it may reside preferentially in lysosomes. IL4I1 is N-linked glycosylated, a requirement for lysosomal localization. Confocal microscopy of cells expressing IL4I1 translationally fused to red fluorescent protein demonstrated that IL4I1 colocalized with GFP targeted to lysosomes and with acriflavine, a green fluorescent dye that is taken up into lysosomes. Thus, IL4I1 is a unique mammalian LAAO targeted to lysosomes, an important subcellular compartment involved in Ag processing.
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) and potential tyrosine (Y) phosphorylation sites (
). Predicted FAD cofactor binding domains are indicated by white boxes in the schematic corresponding to the three black lines underneath. The signal peptide and short regions of similarity to many known FAD-binding proteins (MANY) and to specific proteins (phytoene desaturase (PDS), monoamine oxidase (MAO), and tryptophan 2-monooxygenase (TMO)) are shown by the next row of black lines underneath the schematic, corresponding to the gray-spotted boxes. Regions of similarity to bacterial (Bacillus cereus) and eukaryotic (fish (Scomber japonicus) and snake (Crotalus atrox, C. adamanteus)) LAAO are shown by the remaining black lines underneath the schematic of IL4I1.