Contribution of calcium ions to P2X channel responses.
Ca2+ entry through transmitter-gated cation channels, including ATP-gated P2X channels, contributes to an array of physiological processes in excitable and non-excitable cells, but the absolute amount of Ca2+ flowing through P2X channels is unknown. Here we address the issue of precisely how much Ca2+ flows through P2X channels and report the finding that the ATP-gated P2X channel family has remarkably high Ca2+ flux compared with other channels gated by the transmitters ACh, serotonin, protons, and glutamate. Several homomeric and heteromeric P2X channels display fractional Ca2+ currents equivalent to NMDA channels, which hitherto have been thought of as the largest source of transmitter-activated Ca2+ flux. We further suggest that NMDA and P2X channels may use different mechanisms to promote Ca2+ flux across membranes. We find that mutating three critical polar amino acids decreases the Ca2+ flux of P2X2 receptors, suggesting that these residues cluster to form a novel type of Ca2+ selectivity region within the pore. Overall, our data identify P2X channels as a large source of transmitter-activated Ca2+ influx at resting membrane potentials and support the hypothesis that polar amino acids contribute to Ca2+ selection in an ATP-gated ion channel.
Egan
Terrance M
TM
Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 2QH, United Kingdom.
Khakh
Baljit S
BS
D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D018698:Glutamic Acid / Q000378:metabolism; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D007473:Ion Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D007668:Kidney / Q000166:cytology / Q000378:metabolism; D008564:Membrane Potentials / Q000502:physiology; D016297:Mutagenesis, Site-Directed; D018377:Neurotransmitter Agents / Q000378:metabolism / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D018048:Receptors, Purinergic P2 / Q000235:genetics / Q000378:metabolism*; D058469:Receptors, Purinergic P2X; D058476:Receptors, Purinergic P2X2; D058848:Receptors, Purinergic P2X4; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism
D007473:Ion Channels; D018377:Neurotransmitter Agents; C546401:P2RX2 protein, human; C548226:P2RX4 protein, human; D018048:Receptors, Purinergic P2; D058469:Receptors, Purinergic P2X; D058476:Receptors, Purinergic P2X2; D058848:Receptors, Purinergic P2X4; D011994:Recombinant Proteins; D018698:Glutamic Acid; D000255:Adenosine Triphosphate; D002118:Calcium