SPFH1 and SPFH2 mediate the ubiquitination and degradation of inositol 1,4,5-trisphosphate receptors in muscarinic receptor-expressing HeLa cells

Inositol 1,4,5-trisphosphate (IP₃) receptors are endoplasmic reticulum (ER) membrane calcium channels that, upon activation, become substrates for the ER-associated degradation (ERAD) pathway. While it is clear that IP₃ receptors are polyubiquitinated and are transferred to the proteasome by a p97-based complex, currently very little is known about the proteins that initially select activated IP₃ receptors for ERAD. Here, we have transfected HeLa cells to stably express m3 muscarinic receptors to allow for the study of IP₃ receptor ERAD in this cell type, and show that IP₃ receptors are polyubiquitinated and then degraded by the proteasome in response to carbachol, a muscarinic agonist. In seeking to identify proteins that mediate IP₃ receptor ERAD we found that both SPFH1 and SPFH2 (also known as erlin 1 and erlin 2), which exist as a hetero-oligomeric complex, rapidly associate with IP₃ receptors in a manner that precedes polyubiquitination and the association of p97. Suppression of SPFH1 and SPFH2 expression by RNA interference markedly inhibited carbachol-induced IP₃ receptor polyubiquitination and degradation, but did not affect carbachol-induced calcium mobilization or IκBα processing, indicating that the SPFH1/2 complex is a key player in IP₃ receptor ERAD, acting at a step after IP₃ receptor activation, but prior to IP₃ receptor polyubiquitination. Suppression of SPFH1 and SPFH2 expression had only slight effects on the turnover of some exogenous model ERAD substrates, and had no effect on sterol-induced ERAD of endogenous 3-hydroxy-3-methylglutaryl-CoA reductase. Overall, these studies show that m3 receptor-expressing HeLa cells are a valuable system for studying IP₃ receptor ERAD, and suggest that the SPFH1/2 complex is a factor that selectively mediates the ERAD of activated IP₃ receptors.