Long-term activation of inositol 1,4,5-trisphosphate receptors (IPRs) leads to their degradation by the ubiquitin-proteasome pathway. The first and rate-limiting step in this process is thought to be the association of conformationally active IPRs with the erlin1/2 complex, an endoplasmic reticulum-located oligomer of erlin1 and erlin2 that recruits the E3 ubiquitin ligase RNF170, but the molecular determinants of this interaction remain unknown. Here, through mutation of IPR1, we show that the erlin1/2 complex interacts with the IPR1 intralumenal loop 3 (IL3), the loop between transmembrane (TM) helices 5 and 6, and in particular, with a region close to TM5, since mutation of amino acids D-2471 and R-2472 can specifically block erlin1/2 complex association. Surprisingly, we found that additional mutations in IL3 immediately adjacent to TM5 (e.g., D2465N) almost completely abolish IPR1 Ca channel activity, indicating that the integrity of this region is critical to IPR1 function. Finally, we demonstrate that inhibition of the ubiquitin-activating enzyme UBE1 by the small-molecule inhibitor TAK-243 completely blocked IPR1 ubiquitination and degradation without altering erlin1/2 complex association, confirming that association of the erlin1/2 complex is the primary event that initiates IPR1 processing and that IPR1 ubiquitination mediates IPR1 degradation. Overall, these data localize the erlin1/2 complex-binding site on IPR1 to IL3 and show that the region immediately adjacent to TM5 is key to the events that facilitate channel opening.