Inositol 1,4,5-trisphosphate (InsP(3)) receptors are down-regulated in response to chronic activation of certain cell surface receptors because their degradation is accelerated, Studies on the nature of the down-regulatory process and the protease(s) responsible for receptor degradation are described here, InsP(3) receptor down-regulation was not accompanied by parallel changes in the concentrations of several other relevant proteins (endoplasmic reticulum Ca2+-ATPase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and protein kinases alpha and epsilon). Thus, the down-regulatory process selectively targets InsP(3) receptors for degradation, Furthermore, down-regulation was unaffected by brefeldin A and NH4Cl, indicating that InsP(3) receptor degradation occurs without removal of receptors from the endoplasmic reticulum and independently of functional lysosomes, Analysis of InsP(3) receptor immunofluorescence confirmed that the receptors are not redistributed prior to or during down-regulation, Finally, of a range of protease inhibitors tested, only N-acetyl-Leu-Leu-norleucinal blocked down-regulation. Thus, cysteine protease activity accounts for InsP(3) receptor degradation and analysis of proteolysis in permeabilized cells indicates that this activity is calpain, Thus, InsP(3) receptor downregulation appears to result from the highly selective calpain-mediated degradation of InsP(3) receptors, Calpain activity may be stimulated by the high concentrations of Ca2+ that are thought to be found in the vicinity of activated InsP(3) receptors.
