Revisiting ankyrin-InsP3 receptor interactions: ankyrin-B associates with the cytoplasmic N-terminus of the InsP3 receptor.

Inositol 1,4,5-trisphosphate (InsP(3)) receptors are calcium-release channels found in the endoplasmic/sarcoplasmic reticulum (ER/SR) membrane of diverse cell types. InsP(3) receptors release Ca(2+) from ER/SR lumenal stores in response to InsP(3) generated from various stimuli. The complex spatial and temporal patterns of InsP(3) receptor-mediated Ca(2+) release regulate many cellular processes, ranging from gene transcription to memory. Ankyrins are adaptor proteins implicated in the targeting of ion channels and transporters to specialized membrane domains. Multiple independent studies have documented in vitro and in vivo interactions between ankyrin polypeptides and the InsP(3) receptor. Moreover, loss of ankyrin-B leads to loss of InsP(3) receptor membrane expression and stability in cardiomyocytes. Despite extensive biochemical and functional data, the validity of in vivo ankyrin-InsP(3) receptor interactions remains controversial. This controversy is based on inconsistencies between a previously identified ankyrin-binding region on the InsP(3) receptor and InsP(3) receptor topology data that demonstrate the inaccessibility of this lumenal binding site on the InsP(3) receptor to cytosolic ankyrin polypeptides. Here we use two methods to revisit the requirements on InsP(3) receptor for ankyrin binding. We demonstrate that ankyrin-B interacts with the cytoplasmic N-terminal domain of InsP(3) receptor. In summary, our findings demonstrate that the ankyrin-binding site is located on the cytoplasmic face of the InsP(3) receptor, thus validating the feasibility of in vivo ankyrin-InsP(3) receptor interactions.