title: Structural features of a multisubstate cardiac mitoplast anion channel: inferences from single channel recording.
abstract: Ion channels from sheep cardiac mitoplast (inverted inner mitochondrial membrane vesicle) preparations were incorporated into voltage-clamped planar lipid bilayers. A low-conductance anion channel (approximately 40 or approximately 85 pS in symmetric 300 or 550 mM choline Cl, respectively), characterized by the presence of two well-defined substates, at approximately 25 and approximately 50% of the fully open level, was studied in detail. The substate behavior was consistent with a multibarelled channel containing four functionally coupled pores. At negative (cis-trans) membrane potentials, the putative protomers appeared to gate with substantial positive cooperativity, accounting for the apparent absence of a approximately 75% sublevel. At positive holding potentials, allosteric promoter interactions were more complicated, and the channel complex could be modeled as a dimer of dimers. The protochannels in one dimer ("dimer A") appeared to open independently of each other, and with a relatively high probability, while the monomers comprising the second dimer ("dimer B") were functionally coupled, could only open if both protomers in dimer A were open, and closed as soon as one of the monomers in dimer A shut. The channels also displayed Ca(2+)-(and Mg(2+)-) sensitive rectification related to bilayer lipid surface charge. By assuming that Ca2+ acted solely by screening surface charge, the membrane surface potential profile was used as a "microscopic ruler" to place one month of the channel within 10-11 A of the bilayer surface.
authors.affiliation: Department of Biochemistry, University of Edinburgh, Scotland, UK.
authors.forename: K A
authors.initials: KA
authors.lastname: Hayman
authors.forename: R H
authors.initials: RH
authors.lastname: Ashley
mesh_terms: D000818:Animals; D015220:Calcium Channels / Q000502:physiology; D018118:Chloride Channels / Q000502:physiology; D007473:Ion Channels / Q000502:physiology* / Q000648:ultrastructure; D008051:Lipid Bilayers; D008564:Membrane Potentials; D008929:Mitochondria, Heart / Q000502:physiology*; D012756:Sheep
chemical_list: D015220:Calcium Channels; D018118:Chloride Channels; D007473:Ion Channels; D008051:Lipid Bilayers