Location of the permeation pathway in the recombinant type 1 inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) forms ligand-regulated intracellular Ca2+ release channels in the endoplasmic reticulum of all ma mmalian cells. The InsP(3)R has been suggested to have six transmembrane re gions (TMRs) near its carboxyl terminus. A TMR-deletion mutation strategy w as applied to define the location of the InsP(3)R pore. Mutant InsP(3)Rs we re expressed in COS-1 cells and single channel function was defined in plan ar lipid bilayers. Mutants having the fifth and sixth TMR (and the interced ing lumenal loop), but missing all other TMRs, formed channels with permeat ion properties similar to wild-type channels (gCs = 284; gCa = 60 pS; P-Ca/ P-Cs = 6.3). These mutant channels bound InsP(3), but ligand occupancy did not regulate the constitutively open pore (P-o > 0.80). We propose that a r egion of 191 amino acids (including the fifth and sixth TMR, residues 2398- 2589) near the COOH terminus of the protein forms the InsP(3)R pore. Furthe r we have produced a constitutively open InsP(3)R pore mutant that is ideal for future site-directed mutagenesis studies of the structure-function rel ationships that define Ca2+ permeation through the InsP(3)R channel.