Gating kinetics of single large-conductance Ca2+-activated K+ channels in high Ca2+ suggest a two-tiered allosteric gating mechanism

Brad S. Rothberg, Karl L. Magleby

Research output: Contribution to journalArticle

104 Scopus citations

Abstract

The Ca2+-dependent gating mechanism of large-conductance calcium- activated K+ (BK) channels from cultured rat skeletal muscle was examined from low (4 μM) to high (1,024 μM) intracellular concentrations of calcium (Ca2+(i)) using single-channel recording. Open probability (P(o)) increased with increasing Ca2+(i) (K0.5 11.2 ± 0.3 μM at +30 mV, Hill coefficient of 3.5 ± 0.3), reaching a maximum of ~0.97 for Ca2+(i) ~ 100 μM. Increasing Ca2+(i) further to 1,024 μM had little additional effect on either P(o) or the single-channel kinetics. The channels gated among at least three to four open and four to five closed states at high levels of Ca2+i (> 100 μM), compared with three to four open and five to seven closed states at lower Ca2+(i). The ability of kinetic schemes to account for the single-channel kinetics was examined with simultaneous maximum likelihood fitting of two-dimensional (2-D) dwell-time distributions obtained from low to high Ca2+(i). Kinetic schemes drawn from the 10-state Monod- Wyman-Changeux model could not describe the dwell-time distributions from low to high Ca2+(i). Kinetic schemes drawn from Eigen's general model for a ligand-activated tetrameric protein could approximate the dwell-time distributions but not the dependency (correlations) between adjacent intervals at high Ca2+(i). However, models drawn from a general 50 state two-tiered scheme, in which there were 25 closed states on the upper tier and 25 open states on the lower tier, could approximate both the dwell-time distributions and the dependency from low to high Ca2+(i). In the two- tiered model, the BK channel can open directly from each closed state, and a minimum of five open and five closed states are available for gating at any given Ca2+(i). A model that assumed that the apparent Ca2+-binding steps can reach a maximum rate at high Ca2+(i) could also approximate the gating from low to high Ca2+(i). The considered models can serve as working hypotheses for the gating of BK channels.

Original languageEnglish (US)
Pages (from-to)93-124
Number of pages32
JournalJournal of General Physiology
Volume114
Issue number1
DOIs
StatePublished - Jul 1 1999

Keywords

  • BK channel
  • Eigen
  • K(ca) channel
  • Markov
  • Monod-Wyman-Changeux

ASJC Scopus subject areas

  • Physiology

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