Biochemical Aspects of Neurotransmitter Receptors

A second pathway of activation of the Torpedo acetylcholine receptor channel

We have studied the interaction of the reversible acetylcholine esterase inhibitor (-)physostigmine (D-eserine) with the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue by means of ligand-induced ion flux into nAChR-rich membrane vesicles and of equilibrium binding. We find that (-) physostigmine induces cation flux (and also binds to the receptor) even in the presence of saturating concentrations of antagonists of acetylcholine, such as D-tubocurarine, alpha-bungarotoxin or antibody WF6. The direct action on the acetylcholine receptor is not affected by removal of the methylcarbamate function from the drug and thus is not due to carbamylation of the receptor. Antibodies FK1 and benzoquinonium antagonize channel activation (and binding) of eserine, suggesting that the eserine binding site(s) is separate from, but adjacent to, the acetylcholine binding site at the receptor. In addition to the channel activating site(s) with an affinity of binding in the 50 microM range, there exists a further class of low-affinity (Kd approximately mM) sites from which eserine acts as a direct blocker of the acetylcholine-activated channel. Our results suggest the existence of a second pathway of activation of the nAChR channel.

Organization of ligand binding sites at the acetylcholine receptor: a study with monoclonal antibodies

We have studied 20 monoclonal antibodies directed against both the solubilized and the membrane-bound receptor from Torpedo marmorata. We find the following: (i) Six of the antibodies compete with cholinergic ligands for receptor binding and, hence, are directed against the ligand binding regions. (ii) Of these six antibodies, two cross-react with receptor from Electrophorus electricus, rat myotubes, and chicken sympathetic ganglia. These two antibodies therefore define a preserved structure within the ligand binding regions. The other four antibodies bind to structures not common between the receptor preparations tested. (iii) From competition binding studies using internally 3H-labeled antibodies, nine nonoverlapping antigenic regions were defined at the surface of the receptor. Three of these regions overlap with the ligand binding regions. Since two of these three regions do not overlap with each other, two structurally distinct ligand binding regions must exist at the receptor. (iv) From competition binding studies with representative cholinergic ligands, the antibodies directed against the ligand binding regions can be subdivided into three groups: one group competes with all ligands tested; the second group competes with all ligands except the bismethonium compounds; the third group competes with all ligands except the bismethonium compounds and tubocurarine. The results are summarized in a model of the organization of ligand binding sites at the receptor: There are two ligand binding regions differing in their antigenic properties. Furthermore, either there exists separate sites for distinct groups of ligands within each of these binding regions or some ligands produce conformational changes of the receptor that reversibly abolish some antigenic sites. In any case, the cholinergic ligands must interact with the receptor by more and/or other structural determinants than are provided by the structure of acetylcholine.

Equilibrium binding of acetylcholine to the membrane-bound acetylcholine receptor

We have studied the binding of acetylcholine to membrane-bound acetylcholine receptor from Torpedo marmorata employing a highly accurate airfuge assay procedure. At equilibrium the receptor displays two classes of acetylcholine binding sites; these interact with only weak positive cooperativity. As a further difference to binding data deduced from electrophysiological dose/response curves, the equilibrium constants for the two classes of sites (Kd1 = 25 nM, Kd2 = 8 nM) are orders of magnitude lower than the concentration required for half-maximal response. Both the weaker-than-expected cooperativity of sites and the high binding affinities are likely to be due to desensitisation of the receptor during the period of incubation. The positively cooperative interaction of acetylcholine binding sites is only observed with membrane preparations obtained in the presence of appropriate chelating, sulfhydryl-blocking and active-serine-blocking agents. Aged membrane preparations loose the ability of site interactions while only small changes in the total number of binding sites are observed. In the absence of divalent ions, the affinity of binding of acetylcholine to the receptor is reduced. To assess the significance of the binding data obtained, several alternative reaction schemes for non-random binding to two sites at the receptor are considered. In addition, the effects of possible sources of experimental error on the shape of Scatchard plots are analysed.

Molecular forms and hydrodynamic properties of acetylcholine receptor from electric tissue

We have studied purified acetylcholine receptor proteins from Electrophorus electricus and Torpedo marmorata which function in both binding and reconstitution experiments. The molecular properties of these receptor-channel complexes were analyzed under non-denaturing conditions by polyacrylamide gradient gel electrophoresis, ultracentrifugation sedimentation studies and laser light scattering. The purified receptor proteins exist in different interconvertible forms depending on both the type and concentration of detergent present, and the presence or absence of an intersubunit disulfide bridge. Receptor purified in the absence of sulfhydryl-blocking agents exists in two monomeric and two dimeric forms at very low detergent concentrations (0.01-0.05% Tween 80). At intermediate detergent concentrations (0.4% Triton X-100) one monomeric and one dimeric form are present. Only the monomeric form remains at high levels of detergent (2% Triton X-100). This form has a sedimentation coefficient of 9.29 S, as measured by ultracentrifugation using Schlieren optics. If receptor is purified in the presence of sulfhydryl-blocking agents, conversion of dimers into monomers by high concentrations of detergent does not occur. Disulfide-reducing agents convert dimers into monomers independent of whether the receptor's free sulfhydryl groups are blocked or not. These findings imply that dimer formation is primarily due to hydrophobic interactions between monomers. When these interactions are reduced by high levels of detergent the intersubunit disulfide bridge is dissociated at the expense of newly formed interasubunit ones, as long as the sulfhydryl group(s) have remained unblocked. Monomers and dimers bind alpha-cobratoxin with the same affinity and kinetics. Dimers of disulfide-linked monomers are not required for the reconstitution of a functional ion-translocation system. The presence of EDTA during the purification of receptor proteins (as recommended by many groups to inhibit proteolysis) adversely influences the activity of the receptor in channel gating. These observations are discussed in terms of the requirements for a purification procedure yielding receptor preparations unaffected by proteolysis and functioning in both binding and ion translocation. The molecular weight of the receptor monomer was determined by several independent techniques and yielded values in the range of 250-300 x 10(3). With a translational diffusion constant D20, w = 2.95 x 10(-7) cm2 s-1 and a sedimentation coefficient s20, w = 9.29 S, the frictional coefficient ratio f/f0 = 1.51 was calculated for the receptor monomer from Torpedo marmorata. This indicates a considerable asymmetry in the shape of the detergent-solubilized receptor.