The N-terminal portion of autoinhibitory element modulates human endothelial nitric-oxide synthase activity through coordinated controls of phosphorylation at Thr495 and Ser1177

NO production catalysed by eNOS (endothelial nitric-oxide synthase) plays an important role in the cardiovascular system. A variety of agonists activate eNOS through the Ser(1177) phosphorylation concomitant with Thr(495) dephosphorylation, resulting in increased NO production with a basal level of calcium. To date, the underlying mechanism remains unclear. We have previously demonstrated that perturbation of the AIE (autoinhibitory element) in the FMN-binding subdomain can also lead to eNOS activation with a basal level of calcium, implying that the AIE might regulate eNOS activation through modulating phosphorylation at Thr(495) and Ser(1177). Here we generated stable clones in HEK-293 (human embryonic kidney 293) cells with a series of deletion mutants in both the AIE (Delta 594-604, Delta 605-612 and Delta 626-634) and the C-terminal tail (414; deletion of 1164-1177). The expression of Delta 594-604 and Delta 605-612 mutants in non-stimulated HEK-293 cells substantially increased nitrate/nitrite release into the culture medium; the other two mutants, Delta 626-634 and Delta 1164-1177, displayed no significant difference when compared with WTeNOS (wild-type eNOS). Intriguingly, mutant Delta 594-604 showed close correlation between Ser(1177) phosphorylation and Thr(495) dephosphorylation, and NO production. Our results have indicated that N-terminal portion of AIE (residues 594-604) regulates eNOS activity through coordinated phosphorylation on Ser(1177) and Thr(495).