Spontaneous Channel Activity of the Inositol 1,4,5-Trisphosphate (InsP3) Receptor (InsP3R). Application of Allosteric Modeling to Calcium and InsP3 Regulation of InsP3R Single-channel Gating

• DOI: 10.1085/jgp.200308809
• Publication Date: 2003-10-28T07:43:22Z
• Authors: Don-On Daniel Mak; Sean M.J. McBride; J. Kevin Foskett
• Keywords: N-type calcium channel

The InsP3R Ca2+ release channel has a biphasic dependence on cytoplasmic free concentration ([Ca2+]i). InsP3 activates gating primarily by reducing the sensitivity of to inhibition high [Ca2+]i. To determine if relieving is sufficient for activation, we examined single-channel activities in low [Ca2+]i absence InsP3, patch clamping isolated Xenopus oocyte nuclei. For both endogenous type 1 and recombinant rat 3 channels, spontaneous InsP3-independent with open probability Po (∼0.03) were observed < 5 nM same frequency as presence whereas no 25 Ca2+. These results establish half-maximal inhibitory be 1.2–4.0 demonstrate that can active when all its ligand-binding sites (including InsP3) are unoccupied. In simplest allosteric model fits observations nuclear patch-clamp studies regulation steady-state behavior types isoforms, including activities, tetrameric adopt six different conformations, equilibria among which controlled two one activating Ca2+-binding InsP3-binding manner outlined Monod-Wyman-Changeux model. binding affecting affinities high-affinity transforming it into an site. InsP3-liganded channels mediated low-affinity also suggests besides ligand-regulated mechanism, ligand-independent mechanism responsible maximum being less than unity. validity this was established successful quantitative prediction after had been exposed ultra-low bath [Ca2+].