Functional characterization of mammalian inositol 1,4,5-trisphosphate receptor isoforms

¹ UT Southwestern Medical Center at Dallas
² K.U.Leuven, Belgium

^* To whom correspondence should be addressed. E-mail: ilya.bezprozvanny{at}utsouthwestern.edu.

Submitted on July 14, 2004
Revised on August 23, 2004
Accepted on 29 October 2004

	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 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 3 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 probablity for all 3 mammalian InsP3R isoforms is in the range 30-40%; (3) in optimal recording conditions the mean open dwell time for all 3 mammalian InsP3R isoforms is 7-8 ms, the mean closed dwell time is about 10 ms; (4) InsP3R2 has highest apparent affinity for InsP3 (0.10 mM), followed by InsP3R1 (0.27 mM) and then by InsP3R3 (0.40 mM); (5) InsP3R1 has high affinity (0.13 mM) ATP modulatory site, InsP3R2 gating is ATP-independent, and InsP3R3 has low affinity (2 mM) ATP modulatory site; (6) ATP modulates InsP3R1 gating in non-cooperative 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.

Key Words: calcium signaling, inositol (1,4,5)-trisphosphate receptor, planar lipid bilayers, single channels, structure-function

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