Location of the Permeation Pathway in the Recombinant Type 1 Inositol 1,4,5-Trisphosphate Receptor

Abstract

The inositol 1,4,5-trisphosphate receptor (InsP3R) forms ligand-regulated intracellular Ca2+ release channels in the endoplasmic reticulum of all mammalian cells. The InsP3R has been suggested to have six transmembrane regions (TMRs) near its carboxyl terminus. A TMR-deletion mutation strategy was applied to define the location of the InsP3R pore. Mutant InsP3Rs were expressed in COS-1 cells and single channel function was defined in planar lipid bilayers. Mutants having the fifth and sixth TMR (and the interceding lumenal loop), but missing all other TMRs, formed channels with permeation properties similar to wild-type channels (gCs = 284; gCa = 60 pS; PCa/PCs = 6.3). These mutant channels bound InsP3, but ligand occupancy did not regulate the constitutively open pore (Po > 0.80). We propose that a region of 191 amino acids (including the fifth and sixth TMR, residues 2398–2589) near the COOH terminus of the protein forms the InsP3R pore. Further, we have produced a constitutively open InsP3R pore mutant that is ideal for future site-directed mutagenesis studies of the structure–function relationships that define Ca2+ permeation through the InsP3R channel.