Although several reports have indicated that eNOS is a highly sensitive calpain substrate, the occurrence of a concomitant Ca2+-dependent activation of the synthase and of the protease has never been analyzed in specific direct experiments. In this study, we have explored in vivo how eNOS can undergo Ca2+-dependent translocation and activation, protected against degradation by activated calpain. Here we demonstrate that following a brief exposure to Ca2+-loading, the cytosolic eNOS-HSP90 complex recruits calpain in a form in which the chaperone and the synthase are almost completely resistant to digestion by the protease. Furthermore, in the presence of the HSP90 inhibitor geldanamycin, a significant decrease in NO production and an extensive degradation of eNOS protein occurs, indicating that dissociation from membranes and association with the chaperone is correlated to the protection of the synthase. Experiments with isolated membrane preparations confirm the primary role of HSP90 in dissociation of eNOS from caveolae. Prolonged exposure of cells to Ca2+-loading resulted in an extensive degradation of both eNOS and HSP90, accompanied by a large suppression of NO production. We propose that the protective effect exerted by HSP90 on eNOS degradation mediated by calpain represents a novel and critical mechanism that assures the reversibility of the intracellular trafficking and activation of the synthase
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