Protonation of histidine groups inhibits gating of the quisqualate/kainate channel protein in isolated catfish cone horizontal cells.
Increases in the extracellular hydrogen ion concentration ([H+]o) but not the intracellular concentration ([H+]i) antagonized the inward going membrane currents recorded from isolated cone horizontal cells during application of quisqualate, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid, and kainate. The pK determined from a titration curve was 6.5 with a slope greater than 1, indicating protonation of several histidines. The reduction in membrane current was voltage-independent. The affinity of the agonist for the receptor, the single-channel conductance, and the open time were unaffected by [H+]o. [H+]o antagonism was not the result of charge neutralization such as screening surface charge. Diethylpyrocarbonate, a histidine-modifying reagent, reduced the agonist-induced current, but disulfide- and sulfhydryl-modifying reagents were ineffective. These results suggest that histidine groups on the external face of the channel protein provide a functional site regulating channel gating.
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550.
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BN
Christensen
E
E
Hida
D000818:Animals; D002397:Catfishes; D002469:Cell Separation; D006639:Histidine / Q000378:metabolism*; D006859:Hydrogen / Q000037:antagonists & inhibitors; D015640:Ion Channel Gating; D007473:Ion Channels / Q000378:metabolism*; D007608:Kainic Acid / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D010786:Photoreceptor Cells / Q000166:cytology / Q000378:metabolism*; D011522:Protons; D016318:Quisqualic Acid / Q000378:metabolism*
D007473:Ion Channels; D009419:Nerve Tissue Proteins; D011522:Protons; D006639:Histidine; D006859:Hydrogen; D016318:Quisqualic Acid; D007608:Kainic Acid