Nitric oxide is implicated in a variety of signaling pathways in different systems, notably in endothelial cells. Some of its effects can be exerted through covalent modifications of proteins and, among these modifications, increasing attention is being paid to S-nitrosylation as a signaling mechanism. In this work, we show by a variety of methods (ozone chemiluminescence, biotin switch, and mass spectrometry) that the molecular chaperone Hsp90 is a target of S-nitrosylation and identify a susceptible cysteine residue in the region of the C-terminal domain that interacts with endothelial nitric oxide synthase (eNOS). We also show that the modification occurs in endothelial cells when they are treated with S-nitroso-L-cysteine and when they are exposed to eNOS activators. Hsp90 ATPase activity and its positive effect on eNOS activity are both inhibited by S-nitrosylation. Together, these data suggest that S-nitrosylation may functionally regulate the general activities of Hsp90 and provide a feedback mechanism for limiting eNOS activation.atherosclerosis ͉ nitrosation ͉ vascular wall ͉ chaperone R ecent years have witnessed an increasing interest in the roles of nitric oxide (NO) in signal transduction pathways other than its activation of the cGMP pathway. Many of these roles rely on NO's ability to alter protein function through posttranslational modifications. Among these modifications, S-nitrosylation has emerged as a potential and fundamental regulator of protein function. S-nitrosylation is a covalent modification of thiol groups by formation of a thionitrite (-S-NϭO) group, facilitated by the formation of higher nitrogen oxides (1, 2). To date, several dozens of proteins have been shown to become S-nitrosylated and, in many cases, this modification was accompanied by altered function (see table S1 of ref. 1 for review).Nitric oxide, synthesized in the endothelium by endothelial nitric oxide synthase (eNOS), plays crucial roles in the vascular wall, including the maintenance of vascular tone. The possibility that NO might modify eNOS, or elements of the complex system involved in its activation, is an attractive hypothesis, suggesting a potential autoregulatory feedback mechanism. The eNOS enzyme is regulated by several posttranslational modifications including myristoylation, palmitoylation, and phosphorylation (3). This enzyme is also tightly regulated by specific interactions with inhibitory proteins such as caveolin-1 and by positive modulation by the scaffolding protein Hsp90. These interactions have been described in detail, and a relatively complete picture is beginning to emerge (4).We have previously used a proteomic approach to identify several proteins that were S-nitrosylated after exposure of vascular endothelial cells to the physiological nitrosothiol, Snitroso-L-cysteine (CSNO) (5). Further work led to the identification of Hsp90 as a protein susceptible to S-nitrosylation. This chaperone protein, known for its functions in protein folding, degradation, and scaffolding, has attracted renewed ...