| Hsp90 is a well conserved chaperone which plays a role in a wide range of cellular processes including gene regulation, telomere maintenance, protein trafficking, cell signaling, and protein synthesis. Hsp90 functions with other co-chaperones including Cdc37, which interacts specifically with kinases to regulate stability and activity of various kinases. Hsp90 also serves as a scaffold protein to facilitate signaling pathways. Here, we have isolated Hsp90 and Cdc37 from cardiac p38alpha MAPK complex, and we showed p38alpha directly interacts with Cdc37 to form a complex with Hsp90. Interestingly, pharmacological inhibition of Hsp90 led to a robust increase in autophosphorylation of p38alpha both in vivo and in vitro. Furthermore, we found that Cdc37 is both necessary and sufficient to regulate the basal activity of p38 through autophosphorylation. To determine whether the regulation of autophosphorylation by Hsp90-Cdc37 complex is unique to p38alpha MAPK, IRE1alpha, an ER resident protein which can also autophosphorylate, was also investigated. We identified IRE1alpha not only interacts with Hsp90-Cdc37 complex, but it is also activated in response to Hsp90 inhibition or Cdc37 knockdown. This IRE1alpha activation was reversed by kinase-dead mutant of IRE1alpha (IRE1alphaKD), suggesting that Hsp90-Cdc37 complex also regulates IRE1alpha activity through autophosphorylation. Thus, we have demonstrated here a novel role of Hsp90-Cdc37 complex as a negative regulator of autophosphorylation. To examine the relevance of p38 activity at a physiological level, its role in cardiac contractility was also investigated. The result indicated p38 activation results in decreased contractility mediated by depression in maximal tension and maximal ATPase activity associated with phosphorylation of alpha-Tm. Since Hsp90 is associated with many other proteins including receptors, signaling proteins, and cytoskeleton, it is speculated that Hsp90 may function as a hub in different signaling networks to sustain cellular homeostasis. |
