HETEROTETRAMERIC COMPLEX-FORMATION OF INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR SUBUNITS

The inositol 1,4,5-trisphosphate receptor (IP(3)R) 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 IP(3)R subunits, designated type 1, type 2, and type 3. The levels of expression of IP(3)R subunits in various cell lines were investigated by Western blot analysis using type-specific antibodies against 15 C-terminal amino acids of each IP(3)R subunit. We found that all the three types of IP(3)R subunits were expressed in each cell line examined, but their levels of expression varied. To determine whether IP(3)Rs 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 IP(3)R subunits assemble to form heterotetramers in CHO-K1 cells. We also detected heterotetramers in rat liver, in which IP(3)R type 1 and type 2 are expressed abundantly. Previous studies have shown some functional differences among IP(3)R types, suggesting the possibility that various compositions of subunits show distinct channel properties. The diversity of IP(3)R channels may be further increased by the co-assembly of different IP(3)R subunits to form home- or heterotetramers.