Inositol 1,4,5-trisphosphate [correction of tris-phosphate] activation of inositol trisphosphate [correction of tris-phosphate] receptor Ca2+ channel by ligand tuning of Ca2+ inhibition.
Inositol 1,4,5-trisphosphate (IP3) [corrected] binding to its receptors (IP3R) in the endoplasmic reticulum (ER) activates Ca2+ release from the ER lumen to the cytoplasm, generating complex cytoplasmic Ca2+ concentration signals including temporal oscillations and propagating waves. IP3-mediated Ca2+ release is also controlled by cytoplasmic Ca2+ concentration with both positive and negative feedback. Single-channel properties of the IP3R in its native ER membrane were investigated by patch clamp electrophysiology of isolated Xenopus oocyte nuclei to determine the dependencies of IP3R on cytoplasmic Ca2+ and IP3 concentrations under rigorously defined conditions. Instead of the expected narrow bell-shaped cytoplasmic free Ca2+ concentration ([Ca2+]i) response centered at approximately 300 nM-1 microM, the open probability remained elevated (approximately 0.8) in the presence of saturating levels (10 microM) of IP3, even as [Ca2+]i was raised to high concentrations, displaying two distinct types of functional Ca2+ binding sites: activating sites with half-maximal activating [Ca2+]i (Kact) of 210 nM and Hill coefficient (Hact) approximately 2; and inhibitory sites with half-maximal inhibitory [Ca2+]i (Kinh) of 54 microM and Hill coefficient (Hinh) approximately 4. Lowering IP3 concentration was without effect on Ca2+ activation parameters or Hinh, but decreased Kinh with a functional half-maximal activating IP3 concentration (KIP3) of 50 nM and Hill coefficient (HIP3) of 4 for IP3. These results demonstrate that Ca2+ is a true receptor agonist, whereas the sole function of IP3 is to relieve Ca2+ inhibition of IP3R. Allosteric tuning of Ca2+ inhibition by IP3 enables the individual IP3R Ca2+ channel to respond in a graded fashion, which has implications for localized and global cytoplasmic Ca2+ concentration signaling and quantal Ca2+ release.
Mak
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DO
Department of Physiology, University of Pennsylvania, Philadelphia, PA 19104-6100, USA.
McBride
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Foskett
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JK
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005246:Feedback; D005260:Female; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D015640:Ion Channel Gating; D007700:Kinetics; D008564:Membrane Potentials; D009865:Oocytes / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000502:physiology*; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction; D014982:Xenopus laevis
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D011994:Recombinant Proteins; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium