KRAS-induced actin-interacting protein is required for the proper localization of inositol 1,4,5-trisphosphate receptor in the epithelial cells

Three inositol 1,4,5-trisphosphate receptor (IP₃R) subtypes are differentially expressed among tissues and function as the Ca²⁺ release channel on specialized endoplasmic reticulum (ER) membranes. The proper subcellular localization of IP₃R is crucial for its proper function, but this molecular mechanism is unclear. KRAS-induced actin-interacting protein (KRAP) was originally identified as a cancer-related molecule, and is involved in the regulation of whole-body energy homeostasis and pancreatic exocrine system. We herein identified IP₃R as an associated molecule with KRAP in vivo, and the association was validated by the co-immunoprecipitation and confocal immunostaining studies in mouse tissues including liver and pancreas. The association of KRAP with IP₃R was also observed in the human epithelial cell lines including HCT116, HeLa and HEK293 cells. Intriguingly, KRAP interacts with distinct subtypes of IP₃R in a tissue-dependent manner, i.e. IP₃R1 and IP₃R2 in the liver and IP₃R2 and IP₃R3 in the pancreas. The NH₂-terminal amino acid residues 1–610 of IP₃R are critical for the association with KRAP and KRAP–IP₃R complex resides in a specialized ER but not a typical reticular ER. Furthermore, the localization of particular IP₃R subtypes in tissues from KRAP-deficient mice is obviously disturbed, i.e. IP₃R1 and IP₃R2 in the liver and IP₃R2 and IP₃R3 in the pancreas. These findings demonstrate that KRAP physically associates with IP₃R and regulates the proper localization of IP₃R in the epithelial cells in vivo and cultured cells, and might shed light on the Ca²⁺ signaling underlying physiological cellular programs, cancer development and metabolism-related diseases.