When worlds collide: IP3 receptors and the ERAD pathway

While cell signaling devotees tend to think of the endoplasmic reticulum (ER) as a Ca2+ store, those who study protein synthesis tend to see it more as site for protein maturation, or even degradation when proteins do not fold properly. These two worldviews collide when inositol 1,4,5-trisphosphate (IP3) receptors are activated, since in addition to acting as release channels for stored ER Ca2+, IP3 receptors are rapidly destroyed via the ER-associated degradation (ERAD) pathway, a ubiquitination- and proteasome-dependent mechanism that clears the ER of aberrant proteins. Here we review recent studies showing that activated IP3 receptors are ubiquitinated in an unexpectedly complex manner, and that a novel complex composed of the ER membrane proteins SPFH1 and SPFH2 (erlin 1 and 2) binds to IP3 receptors immediately after they are activated and mediates their ERAD. Remarkably, it seems that the conformational changes that underpin channel opening make IP3 receptors resemble aberrant proteins, which triggers their binding to the SPFH1/2 complex, their ubiquitination and extraction from the ER membrane and finally, their degradation by the proteasome. This degradation of activated IP3 receptors by the ERAD pathway serves to reduce the sensitivity of ER Ca2+ stores to IP3 and may protect cells against deleterious effects of over-activation of Ca2+ signaling pathways.