Medical College of Wisconsin
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Cooperative activation of IP3 receptors by sequential binding of IP3 and Ca2+ safeguards against spontaneous activity. Curr Biol 1997 Jul 01;7(7):510-8 PMID: 9210378

Abstract

BACKGROUND: Ca2+ waves allow effective delivery of intracellular Ca2+ signals to cytosolic targets. Propagation of these regenerative Ca2+ signals probably results from the activation of intracellular Ca2+ channels by the increase in cytosolic [Ca2+] that follows the opening of these channels. Such positive feedback is potentially explosive. Mechanisms that limit the spontaneous opening of intracellular Ca2+ channels are therefore likely to have evolved in parallel with the mechanism of Ca2+-induced Ca2+ release.

RESULTS: Maximal rates of 45Ca2+ efflux from permeabilised hepatocytes superfused with medium in which the [Ca2+] was clamped were cooperatively stimulated by inositol 1,4,5-trisphosphate (IP3). A minimal interval of approximately 400 msec between IP3 addition and the peak rate of Ca2+ mobilisation indicate that channel opening does not immediately follow binding of IP3. Although the absolute latency of Ca2+ release was unaffected by further increasing the IP3 concentration, it was reduced by increased [Ca2+].

CONCLUSIONS: We propose that the closed conformation of the IP3 receptor is very stable and therefore minimally susceptible to spontaneous activation; at least three (probably four) IP3 molecules may be required to provide enough binding energy to drive the receptor into a stable open conformation. We suggest that a further defence from noise is provided by an extreme form of coincidence detection. Binding of IP3 to each of its four receptor subunits unmasks a site to which Ca2+ must bind before the channel can open. As IP3 binding may also initiate receptor inactivation, there may be only a narrow temporal window during which each receptor subunit must bind both of its agonists if the channel is to open rather than inactivate.

Author List

Marchant JS, Taylor CW

Author

Jonathan S. Marchant PhD Chair, Professor in the Cell Biology, Neurobiology and Anatomy department at Medical College of Wisconsin

MESH terms used to index this publication - Major topics in bold

Binding Sites
Calcium
Calcium Channels
Cell Membrane
Inositol 1,4,5-Trisphosphate
Inositol 1,4,5-Trisphosphate Receptors
Ion Channel Gating
Models, Molecular
Receptors, Cytoplasmic and Nuclear
Signal Transduction



View this publication's entry at the Pubmed website PMID: 9210378
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