How endoplasmic reticulum (ER) proteins that are substrates for the ER-associated degradation (ERAD) pathway are recognized for polyubiquitination and proteasomal degradation is largely unresolved. Inositol 1,4,5-trisphosphate receptors (IP 3 Rs) form tetrameric calcium channels in ER membranes, whose primary role is to control the release of ER calcium stores, but whose levels are also regulated, in an activation-dependent manner, by the ERAD pathway. Here we report that the ER membrane protein SPFH1 and its homolog SPFH2 form a heteromeric ϳ2 MDa complex that binds to IP 3 R tetramers immediately after their activation and is required for their processing. The complex is ring-shaped (diameter ϳ250 Å ), and RNA interference-mediated depletion of SPFH1 and SPFH2 blocks IP 3 R polyubiquitination and degradation. We propose that this novel SPFH1/2 complex is a recognition factor that targets IP 3 Rs and perhaps other substrates for ERAD.
The endoplasmic reticulum (ER)2 -associated degradation (ERAD) pathway targets aberrant proteins, including irreversibly misfolded proteins and unassembled subunits of multiprotein complexes, for degradation by the ubiquitin-proteasome system (1). Intriguingly, several ER-resident proteins that are stable under normal conditions are also processed by the ERAD pathway. For example, 3-hydroxy-3-methylglutaryl CoA-reductase, the rate-limiting enzyme in sterol synthesis, is targeted for ERAD when sterols are in excess (2), and inositol 1,4,5-trisphosphate (IP 3 ) receptors (IP 3 Rs), which form tetrameric, IP 3 -and Ca 2ϩ