Subcellular targeting and agonist-induced site-specific phosphorylation of endothelial nitric-oxide synthase

The endothelial isoform of nitric-oxide synthase (eNOS) undergoes a complex pattern of covalent modifications, including acylation with the fatty acids myristate and palmitate as well as phosphorylation on multiple sites. eNOS acylation is a key determinant for the reversible subcellular targeting of the enzyme to plasmalemmal caveolae. We transfected a series of hemagglutinin epitope-tagged eNOS mutant cDNAs deficient in palmitoylation (palm(-)) and/or myristoylation (myr(-)) into bovine aortic endothelial cells; after treatment with the eNOS agonists sphingosine 1-phosphate or vascular endothelial growth factor, the recombinant eNOS was immunoprecipitated using an antibody directed against the epitope tag, and patterns of eNOS phosphorylation were analyzed in immunoblots probed with phosphorylation state-specific eNOS antibodies. The wild-type eNOS underwent agonist-induced phosphorylation at serine 1179 (a putative site for phosphorylation by kinase Akt), but phosphorylation of the myr(-) eNOS at this residue was nearly abrogated; the palm(-) eNOS exhibited an intermediate phenotype. The addition of the CD8 transmembrane domain to the amino terminus of eNOS acylation-deficient mutants rescued the wild-type phenotype of robust agonist-induced serine 1179 phosphorylation. Thus, membrane targeting, but not necessarily acylation, is the critical determinant for agonist-promoted eNOS phosphorylation at serine 1179. In striking contrast to serine 1179, phosphorylation of eNOS at serine 116 was enhanced in the myr(-) eNOS mutant and was markedly attenuated in the CD8-eNOS membrane-targeted fusion protein. We conclude that eNOS targeting differentially affects eNOS phosphorylation at distinct sites in the protein and suggest that the inter-relationships of eNOS acylation and phosphorylation may modulate eNOS localization and activity and thereby influence NO signaling pathways in the vessel wall.