Functional Characterization of Mammalian Inositol 1,4,5-Trisphosphate Receptor Isoforms

Abstract

Inositol 1,4,5-trisphosphate receptors (InsP3R) play a key role in intracellular calcium (Ca2+) signaling. Three mammalian InsP3R isoforms—InsP3R type 1 (InsP3R1), InsP3R type 2 (InsP3R2), and InsP3R type 3 (InsP3R3) are expressed in mammals, but the functional differences between the three mammalian InsP3R isoforms are poorly understood. Here we compared single-channel behavior of the recombinant rat InsP3R1, InsP3R2, and InsP3R3 expressed in Sf9 cells, reconstituted into planar lipid bilayers and recorded with 50mM Ba2+ as a current carrier. We found that: 1), for all three mammalian InsP3R isoforms the size of the unitary current is 1.9 pA and single-channel conductance is 74–80 pS; 2), in optimal recording conditions the maximal single-channel open probability for all three mammalian InsP3R isoforms is in the range 30–40%; 3), in optimal recording conditions the mean open dwell time for all three mammalian InsP3R isoforms is 7–8ms, the mean closed dwell time is ∼10ms; 4), InsP3R2 has the highest apparent affinity for InsP3 (0.10μM), followed by InsP3R1 (0.27μM), and then by InsP3R3 (0.40μM); 5), InsP3R1 has a high-affinity (0.13mM) ATP modulatory site, InsP3R2 gating is ATP independent, and InsP3R3 has a low-affinity (2mM) ATP modulatory site; 6), ATP modulates InsP3R1 gating in a noncooperative manner (nHill=1.3); 7), ATP modulates InsP3R3 gating in a highly cooperative manner (nHill=4.1). Obtained results provide novel information about functional properties of mammalian InsP3R isoforms.