We tested the hypothesis that key residues in a putative intraluminal loop contribute to determination of ion permeation through the intracellular Ca 2+ release channel (inositol 1,4,5-trisphosphate receptors (IP 3Rs)) that is gated by the second messenger inositol 1,4,5-trisphophate (IP3). To accomplish this, we mutated residues within the putative pore forming region of the channel and analyzed the functional properties of mutant channels using a 45Ca2+ flux assay and single channel electrophysiological analyses. Two IP 3R mutations, V2548I and D2550E, retained the ability to release 45Ca2+ in response to IP3. When analyzed at the single channel level; both recombinant channels had IP 3-dependent open probabilities similar to those observed in wild-type channels. The mutation V2548I resulted in channels that exhibited a larger K+ conductance (489 ± 13 picosiemens (pS) for V2548I versus 364 ± 5 pS for wild-type), but retained a Ca2+ selectivity similar to wild-type channels (PCa2+:PK+ ∼ 4:1). Conversely, D2550E channels were nonselective for Ca2+ over K+ (PCa2+:PK+ ∼ 0.6:1), while the K+ conductance was effectively unchanged (391 ± 4 pS). These results suggest that amino acid residues Val2548 and Asp 2550 contribute to the ion conduction pathway. We propose that the pore of IP3R channels has two distinct sites that control monovalent cation permeation (Val2548) and Ca2+ selectivity (Asp 2550).
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology