Regulation of CRAC channel activity by recruitment of silent channels to a high open-probability gating mode.
CRAC (calcium release-activated Ca(2+)) channels attain an extremely high selectivity for Ca(2+) from the blockade of monovalent cation permeation by Ca(2+) within the pore. In this study we have exploited the blockade by Ca(2+) to examine the size of the CRAC channel pore, its unitary conductance for monovalent cations, and channel gating properties. The permeation of a series of methylammonium compounds under divalent cation-free conditions indicates a minimum pore diameter of 3.9 A. Extracellular Ca(2+) blocks monovalent flux in a manner consistent with a single intrapore site having an effective K(i) of 20 microM at -110 mV. Block increases with hyperpolarization, but declines below -100 mV, most likely due to permeation of Ca(2+). Analysis of monovalent current noise induced by increasing levels of block by extracellular Ca(2+) indicates an open probability (P(o)) of approximately 0.8. By extrapolating the variance/mean current ratio to the condition of full blockade (P(o) = 0), we estimate a unitary conductance of approximately 0.7 pS for Na(+), or three to fourfold higher than previous estimates. Removal of extracellular Ca(2+) causes the monovalent current to decline over tens of seconds, a process termed depotentiation. The declining current appears to result from a reduction in the number of active channels without a change in their high open probability. Similarly, low concentrations of 2-APB that enhance I(CRAC) increase the number of active channels while open probability remains constant. We conclude that the slow regulation of whole-cell CRAC current by store depletion, extracellular Ca(2+), and 2-APB involves the stepwise recruitment of silent channels to a high open-probability gating mode.
Prakriya
Murali
M
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA. m-prakriya@northwestern.edu
Lewis
Richard S
RS
D001896:Boron Compounds / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology*; D002414:Cations, Monovalent / Q000378:metabolism; D002463:Cell Membrane Permeability / Q000187:drug effects / Q000502:physiology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D019169:Jurkat Cells; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D021122:Protein Subunits / Q000502:physiology; D012964:Sodium / Q000378:metabolism
D001896:Boron Compounds; D015220:Calcium Channels; D002414:Cations, Monovalent; D021122:Protein Subunits; D012964:Sodium; C109986:2-aminoethoxydiphenyl borate; D002118:Calcium