STRUCTURAL DIVERSITY AMONG SUBTYPES OF SMALL-CONDUCTANCE CA2-ACTIVATED POTASSIUM CHANNELS()

I-125-Apamin and photolabile derivatives of the toxin have been used t o investigate the binding properties and subunit composition of small conductance Ca2+-activated potassium channels (SKCa, channels) express ed on plasma membranes from rat brain, rabbit liver, or rat pheochromo cytoma (PC12) cells, On all preparations, I-125-apamin recognized sing le classes of acceptor binding sites with similar high affinity (Kd si milar to 3-6 pM), Gallamine, however, was found to readily discriminat e between I-125-apamin accepters present in these preparations, showin g a maximal approx ninefold difference in affinity for accepters expre ssed by rabbit liver or PC 12 cells. Affinity-labeling patterns reveal ed the expression of different hetero-oligomeric combinations of high (86 or 59 kDa) and low (33 or 30 kDa) molecular mass I-125-apamin-bind ing polypeptides, consistent with pharmacological differences. Alterna tive expression of either 86- or 59-kDa polypeptides appeared to be th e most important factor influencing gallamine's affinity for SKCa chan nel subtypes, Both high- and low-molecular-mass polypeptides are integ ral membrane proteins, the latter being glycosylated in a tissue-speci fic manner. (C) 1997 Academic Press.