Type III InsP3 receptor channel stays open in the presence of increased calcium.
The inositol 1,4,5-trisphosphate receptor (InsP3R) is the main calcium(Ca2+) release channel in most tissues. Three isoforms have been identified, but only types I and II InsP3R have been characterized. Here we examine the functional properties of the type III InsP3R because this receptor is restricted to the trigger zone from which Ca2+ waves originate and it has distinctive InsP3-binding properties. We find that type III InsP3R forms Ca2+ channels with single-channel currents that are similar to those of type I InsP3R; however, the open probability of type III InsP3R isoform increases monotonically with increased cytoplasmic Ca2+ concentration, whereas the type I isoform has a bell-shaped dependence on cytoplasmic Ca2+. The properties of type III InsP3R provide positive feedback as Ca2+ is released; the lack of negative feedback allows complete Ca2+ release from intracellular stores. Thus, activation of type III InsP3R in cells that express only this isoform results in a single transient, but global, increase in the concentration of cytosolic Ca2+. The bell-shaped Ca2+-dependence curve of type I InsP3R is ideal for supporting Ca2+ oscillations, whereas the properties of type III InsP3R are better suited to signal initiation.
Hagar
R E
RE
Department of Physiology, University of Connecticut Health Center, Farmington 06030, USA. rob@hermen.med.yale.edu
Burgstahler
A D
AD
Nathanson
M H
MH
Ehrlich
B E
BE
D000255:Adenosine Triphosphate / Q000378:metabolism; D000494:Allosteric Regulation; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D020013:Calcium Signaling; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D002118:Calcium