Down-regulation of types I, II and III inositol 1,4,5-trisphosphate receptors is mediated by the ubiquitin/proteasome pathway

Activation of certain phosphoinositidase-C-linked cell-surface receptors is known to cause an acceleration of the proteolysis of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] receptors and, thus, lead to Ins(1,4,5)P₃-receptor down-regulation. In the current study we have sought to determine whether the ubiquitin/proteasome pathway is involved in this adaptive response. The data presented show (i) that activation of phosphoinositidase-C-linked receptors causes Ins(1,4,5)P₃-receptor ubiquitination in a range of cell types (AR4-2J cells, INS-1 cells and rat cerebellar granule cells), (ii) that the Ins(1,4,5)P₃-receptor down-regulation induced by activation of these receptors is blocked by proteasome inhibitors, (iii) that all known Ins(1,4,5)P₃ receptors (types I, II and III) are substrates for ubiquitination, (iv) that ubiquitination occurs while Ins(1,4,5)P₃ receptors are membrane-bound, (v) that Ins(1,4,5)P₃-receptor ubiquitination and down-regulation are stimulated only by those agonists that elevate Ins(1,4,5)P₃ concentration persistently, and (vi) that a portion of cellular Ins(1,4,5)P₃ receptors (those that are not type-I-receptor-associated) can be resistant to ubiquitination and degradation. In total these data indicate that the ubiquitin/proteasome pathway mediates Ins(1,4,5)P₃-receptor down-regulation and suggest that ubiquitination is stimulated by the binding of Ins(1,4,5)P₃ to its receptor.