Functional characterization of mammalian inositol 1,4,5-trisphosphate receptor isoforms.
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 50 mM 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 7-8 ms, the mean closed dwell time is approximately 10 ms; 4), InsP3R2 has the highest apparent affinity for InsP(3) (0.10 microM), followed by InsP3R1 (0.27 microM), and then by InsP3R3 (0.40 microM); 5), InsP3R1 has a high-affinity (0.13 mM) ATP modulatory site, InsP3R2 gating is ATP independent, and InsP3R3 has a low-affinity (2 mM) ATP modulatory site; 6), ATP modulates InsP3R1 gating in a noncooperative manner (n(Hill) = 1.3); 7), ATP modulates InsP3R3 gating in a highly cooperative manner (n(Hill) = 4.1). Obtained results provide novel information about functional properties of mammalian InsP3R isoforms.
Tu
Huiping
H
Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390, USA.
Wang
Zhengnan
Z
Nosyreva
Elena
E
De Smedt
Humbert
H
Bezprozvanny
Ilya
I
D000255:Adenosine Triphosphate / Q000737:chemistry*; D000818:Animals; D004553:Electric Conductivity; D015544:Inositol 1,4,5-Trisphosphate / Q000032:analysis / Q000737:chemistry*; D015640:Ion Channel Gating; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D008564:Membrane Potentials; D020033:Protein Isoforms / Q000032:analysis / Q000737:chemistry; D051381:Rats; D011994:Recombinant Proteins / Q000032:analysis / Q000737:chemistry; D013329:Structure-Activity Relationship
D008051:Lipid Bilayers; D020033:Protein Isoforms; D011994:Recombinant Proteins; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate