IL-4-Induced Gene-1 Is a Leukocyte l-Amino Acid Oxidase with an Unusual Acidic pH Preference and Lysosomal Localization

Abstract

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.

Human spermatozoa possess an IL4I1 l-amino acid oxidase with a potential role in sperm function

“…2) in keeping with the known activity of this enzyme in the immune system (Boulland et al 2007). The localization of this enzyme in the sperm acrosome is also consistent with the observation that this enzyme is maximally active at acidic pH and is preferentially targeted to lysosomes (Mason et al 2004); the acrosomal vesicle is known to exhibit a low intraacrosomal pH and shares many features with cellular lysosomes (Working & Meizel 1983, Moreno & Alvarado 2006. Importantly, the anti-IL4I1 antibody used in this study could not discriminate between the two isoforms of this molecule, because the translated protein generated by the two genes represented in Fig.…”

l-Amino acid oxidase of the fungus Hebeloma cylindrosporum displays substrate preference towards glutamate

“…Between two and four putative glycosylation sites were computationally predicted from the basidiomycete Lao1 sequences, although it is not yet known if basidiomycete Lao1s are glycosylated. Most eukaryotic Laos are known or predicted to be processed by the removal of a signal-and/or propeptide (Niedermann & Lerch, 1990;Sun et al, 2002), and vertebrate Laos localize into lysosomes or venom glands (Chavan et al, 2002;Mason et al, 2004). However, sequence comparisons revealed no conserved signal sequences or cleavage sites from the two basidiomycete Lao1 sequences.…”

L-amino acid oxidases: properties and molecular mechanisms of action

“…LAAO королевской кобры (Ophiophagus hannah) в дополнение к гидрофобным аминокислотам активна также по отношению к L-лизину и L-орнитину [55,56]. LAAO лейкоцитов человека (Il4i1) проявляла высокую активность в отношении гидрофобных аминокислот и, в частности, она активно окисляла ароматические L-аминокислоты: фенилаланин, тирозин и триптофан [57].…”