The decrement in functional capacity of the nicotinic receptor on intact BC3H-1 cells has been simultaneously compared with the fractional occupation of the receptor by cobra α-toxin. A parabolic, concave inward relationship between the fractional occupation of receptors by α-toxin and the decrement in permeability response is observed when the latter is tested over a range of agonist concentrations. Since α-toxin binding appears equivalent at each site on the receptor, the observed relationship is accommodated by a model where activation of a permeability response requires agonist occupation of two toxin-binding sites per functional receptor. Furthermore, the binding of α-toxin and agonist appears to be mutually exclusive, but occupation of either of the two sites by α-toxin is sufficient to block the functional capacity of the receptor. Consistent with this model, when a major fraction of sites is occupied by α-toxin, the concentration dependence for either carbamylcholine-mediated activation or desensitization of the remaining functional receptors is not detectably altered and retains positive cooperativity. In contrast, progressive occupation of the available sites by α-toxin leads to a decrease in apparent affinity and a corresponding loss of positive cooperativity for agonist occupation functions generated upon instantaneous or following equilibrium exposure to the agonist. At high degrees of fractional occupancy by α-toxin, where the dominant species capable of binding agonist would contain a single bound toxin molecule, the Hill coefficient for the equilibrium occupation function for full agonists falls from a value of 1.4 to 0.7 By contrast, the binding isotherms for antagonists which typically exhibit values less than 1.0 are not altered following fractional irreversible occupation by α-toxin. Thus, the two binding sites on the receptor oligomer are not intrinsically equivalent for the binding of agonists and reversible antagonists. A scheme for desensitization of the receptor is presented which incorporates both nonequivalence in the two agonist binding-sites and the maintenance of symmetry in the receptor states undergoing transitions.
|Original language||English (US)|
|Number of pages||13|
|Journal||Journal of Biological Chemistry|
|State||Published - 1980|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology