Nitric Oxide-stimulated Guanine Nucleotide Exchange on p21

The protooncogene p21 , a monomeric G protein family member, plays a critical role in converting extracellular signals into intracellular biochemical events. Here, we report that nitric oxide (NO) activates p21 in human T cells as evidenced by an increase in GTP-bound p21 . In vitro studies using pure recombinant p21 demonstrate that the activation is direct and reversible. Circular dichroism analysis reveals that NO induces a profound conformational change in p21 in association with GDP/GTP exchange. The mechanism of activation is due to S -nitrosylation of a critical cysteine residue which stimulates guanine nucleotide exchange. Furthermore, we demonstrate that p21 is essential for NO-induced downstream signaling, such as NF-κB activation, and that endogenous NO can activate p21 in the same cell. These studies identify p21 as a target of NO in T cells and suggest that NO activates p21 by an action which mimics that of guanine nucleotide exchange factors.