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Probing the recognition signal for ubiquitination of suicide‐inactivated neuronal nitric oxide synthase with a C331A mutant of the enzyme

Nitric oxide synthase (NOS), a cytochrome P450‐like hemoprotein enzyme, catalyzes the synthesis of nitric oxide, a critical signaling molecule in a variety of physiological processes. Our lab has discovered that certain drugs suicide‐inactivate neuronal NOS (nNOS) and lead to the preferential ubiquitination of the inactivated nNOS. We wish to understand how drugs that covalently alter the active site of nNOS labilize nNOS for ubiquitination. A C331A mutant of nNOS was shown to have partially disrupted tetrahydrobiopterin and L‐arginine binding, perhaps similar to a suicide‐inactivated nNOS. We show here that the C331A nNOS was preferentially ubiquitinated by DE52‐retained fraction of reticulocyte lysate, which contains all ubiquitinating enzymes. Moreover, we found that the slowly reversible inhibitor, NG‐nitro‐L‐arginine, but not the inactive D‐isomer, stabilizes and protects both the wild‐type and mutant nNOS from ubiquitination with an IC50 of 0.2 μM and 7.7 μM, respectively. Highly similar results to the in vitro studies were obtained in HEK293T cells after transient transfection of C331A nNOS. The in vitro ubiquitination of nNOS was found to depend on CHIP, an E3 ligase, and hsp70. Thus, it appears that hsp70•CHIP can sense structural changes, most likely small perturbations around the active site, and directs the selective ubiquitination of nNOS. Supported in part by NIH grants GM077430 and DA022354.