Heterotetrameric complex formation of inositol 1,4,5-trisphosphate receptor subunits

The inositol 1,4,5-trisphosphate receptor (IP₃R) exists as a tetrameric complex to form a functional inositol 1,4,5-trisphosphate-gated Ca²⁺ channel. Molecular cloning studies have shown that there are at least three types of IP₃R subunits, designated type 1, type 2, and type 3. The levels of expression of IP₃R subunits in various cell lines were investigated by Western blot analysis using type-specific antibodies against 15 C-terminal amino acids of each IP₃R subunit. We found that all the three types of IP₃R subunits were expressed in each cell line examined, but their levels of expression varied. To determine whether IP₃Rs form heterotetramers, we employed immunoprecipitation experiments using Chinese hamster ovary cells (CHO-K1 cells), in which all three types are abundantly expressed. Each type- specific antibody immunoprecipitated not only the respective cognate type but also the other two types. This result suggests that distinct types of IP₃R subunits assemble to form heterotetramers in CHO-K1 cells. We also detected heterotetramers in rat liver, in which IP₃R type 1 and type 2 are expressed abundantly. Previous studies have shown some functional differences among IP₃R types, suggesting the possibility that various compositions of subunits show distinct channel properties. The diversity of IP₃R channels may be further increased by the co-assembly of different IP₃R subunits to form homo- or heterotetramers.