S-nitrosylation of the scaffold protein STRAP enhances oxidative stress-induced apoptosis.

Serine-threonine kinase receptor-associated protein (STRAP) functions as a negative regulator of apoptosis by inhibiting apoptosis signal-regulating kinase 1 (ASK1) activity. STRAP is consistently present in the inducible nitric oxide synthase (iNOS) interactome and contains two essential cysteine residues, Cys152 and Cys270, which are required for its interaction with ASK1. However, the role of the STRAP-iNOS interaction remains unclear. In this study, we found that STRAP specifically interacts with iNOS but not with endothelial NOS or neuronal NOS. iNOS mediates the S-nitrosylation of STRAP, which disrupts the STRAP-ASK1 interaction, increases ASK1 activity, activates the mitogen-activated protein kinase kinase 3 (MKK3) and mitogen-activated protein kinase (p38) pathway, and enhances hydrogen peroxide-induced apoptosis. Notably, Cys152 and Cys270 are also the primary S-nitrosylation sites of STRAP. Mutation of these residues to serine (STRAP-C152/270S) abolishes the STRAP-ASK1 interaction, constitutively activates the ASK1-MKK3-p38 pathway, and increases apoptosis. Moreover, iNOS overexpression fails to promote hydrogen peroxide-induced apoptosis in STRAP-C152/270S-expressing cells, underscoring the essential role of STRAP S-nitrosylation in iNOS-mediated cell death. This study provides the first evidence that S-nitrosylation of STRAP is critical for the regulation of apoptosis and uncovers a novel cell survival mechanism mediated by the iNOS-SNO-STRAP-ASK1 signaling axis.