title: Structural diversity among subtypes of small-conductance Ca2+-activated potassium channels.
abstract: 125I-Apamin and photolabile derivatives of the toxin have been used to investigate the binding properties and subunit composition of small conductance Ca2+-activated potassium channels (SK(Ca) channels) expressed on plasma membranes from rat brain, rabbit liver, or rat pheochromocytoma (PC12) cells. On all preparations, 125I-apamin recognized single classes of acceptor binding sites with similar high affinity (Kd approximately 3-6 pM). Gallamine, however, was found to readily discriminate between 125I-apamin acceptors present in these preparations, showing a maximal approx nine-fold difference in affinity for acceptors expressed by rabbit liver or PC12 cells. Affinity-labeling patterns revealed the expression of different hetero-oligomeric combinations of high (86 or 59 kDa) and low (33 or 30 kDa) molecular mass 125I-apamin-binding polypeptides, consistent with pharmacological differences. Alternative expression of either 86- or 59-kDa polypeptides appeared to be the most important factor influencing gallamine's affinity for SK(Ca) channel subtypes. Both high- and low-molecular-mass polypeptides are integral membrane proteins, the latter being glycosylated in a tissue-specific manner.
authors.affiliation: Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, London, United Kingdom.
authors.forename: J D
authors.initials: JD
authors.lastname: Wadsworth
authors.forename: S
authors.initials: S
authors.lastname: Torelli
authors.forename: K B
authors.initials: KB
authors.lastname: Doorty
authors.forename: P N
authors.initials: PN
authors.lastname: Strong
mesh_terms: D000345:Affinity Labels; D000818:Animals; D001030:Apamin / Q000378:metabolism; D001923:Brain Chemistry; D002118:Calcium / Q000494:pharmacology; D005703:Gallamine Triethiodide / Q000378:metabolism; D015640:Ion Channel Gating; D008099:Liver / Q000737:chemistry; D016716:PC12 Cells; D015221:Potassium Channels / Q000737:chemistry* / Q000145:classification / Q000378:metabolism; D024681:Potassium Channels, Calcium-Activated; D011817:Rabbits; D051381:Rats; D051640:Small-Conductance Calcium-Activated Potassium Channels
chemical_list: D000345:Affinity Labels; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D051640:Small-Conductance Calcium-Activated Potassium Channels; D001030:Apamin; D005703:Gallamine Triethiodide; D002118:Calcium