TI - Phosphorylation by protein kinase a inhibits nuclear import of 5-lipoxygenase . AB - The enzyme 5-lipoxygenase initiates the synthesis of leukotrienes from arachidonic/ acid . Protein kinase A phosphorylates 5-lipoxygenase on Ser(523) , and this reduces its activity . We report here that phosphorylation of Ser(523) also shifts the subcellular distribution of 5-lipoxygenase from the nucleus to the cytoplasm . Phosphorylation and redistribution of 5-lipoxygenase could be produced by overexpression of the protein kinase A catalytic subunit alpha , by pharmacological activators of protein kinase A , and by prostaglandin E(2) . Mimicking phosphorylation by replacing Ser(523) with glutamic acid caused cytoplasmic localization ; replacement of Ser(523) with alanine prevented phosphorylation and redistribution in response to protein kinase A activation . Because Ser(523) is positioned within the nuclear localization sequence -518 of 5-lipoxygenase , the ability of protein kinase A to phosphorylate and alter the localization of green fluorescent protein fused to the nuclear localization sequence -518 peptide was also tested . Site -directed replacement of Ser(523) with glutamic acid within the peptide impaired nuclear accumulation ; overexpression of the protein kinase A catalytic subunit alpha and pharmacological activation of protein kinase caused phosphorylation of the fusion protein at Ser(523) , and the phosphorylated protein was found chiefly in the cytoplasm . Taken together , these results indicate that phosphorylation of Ser(523) inhibits the nuclear import function of a nuclear localization sequence , resulting in the accumulation of 5-lipoxygenase enzyme in the cytoplasm . As cytoplasmic localization can be associated with reduced leukotriene synthetic capacity , phosphorylation of Ser(523) serves to inhibit leukotriene production by both impairing catalytic activity and by placing the enzyme in a site that is unfavorable for action .