Monoclonal antibodies as probes of acetylcholine receptor structure. 2. Binding to native receptor

Binding of monoclonal antibodies top Torpedo californica acetylcholine receptor monomers solubilized in Triton X-100 was studied by centrifugation on sucrose gradients. Antibodies to alpha subunits were of two types. One type formed complexes of one antibody and one receptor monomer, independent of antibody/receptor ratio. We conclude that the binding sites for these antibodies are oriented on the two alpha subunits per monomer in such a way that each could be bound by one of the two binding sites of a single immunoglobulin molecule. Most antibodies were of this type. The other type of monoclonal antibody formed complexes of several sizes, including antibody cross-linked receptors, depending on the ratio of antibody to receptor. We conclude that the binding sites for these antibodies are oriented in such a way that the two alpha subunits per monomer could not be cross-linked by a single antibody molecule. A monoclonal antibody of this type raised against Electrophorus electricus receptors was used to show that this receptor also has two alpha subunits per monomer. This antibody cross-reacted with receptor from fetal calf muscle and was able to induce modulation of receptor in muscle cells in culture. This suggests that muscle receptor also has two alpha subunits and that the antibody can cross-link receptor in the plane of the membrane, as it does in solution, and thereby form complexes which enhance endocytosis and increase the rate of receptor destruction. The rate of antigenic modulation decreases at high antibody/receptor ratios, as expected if un-cross-linked complexes of two antibodies and one receptor were not destroyed at a faster rate. Antibodies which cross-link alpha subunits within a receptor monomer are frequent but would not be expected to be able to induce antigenic modulation. This provides one mechanism by which antisera of equivalent antireceptor titer might differ in their ability to induce antigenic modulation. An antibody which binds to denatured delta and gamma subunits forms complexes of only one antibody and one receptor monomer, independent of antibody ratio, as do antibodies thought to cross-link the two alpha subunits in a monomer. It apparently cross-links delta and gamma subunits within the monomer. Some of the monoclonal antibodies to alpha subunits can bind simultaneously to receptor, while the binding of others is mutually exclusive.

Monoclonal antibodies as probes of acetylcholine receptor structure. 1. Peptide mapping

The isolated subunits of the acetylocholine receptor from Torpedo californica were digested with proteolytic enzymes, and the resulting polypeptide fragments were analyzed by gel electrophoresis. We have identified those fragments which contain carbohydrate and those from the alpha subunit which are labelled with the acetylcholine binding site specific reagent [4-(N-maleimido)benzyl]tri[3H]methylammonium iodide. We have tested several monoclonal antibodies raised to the acetylcholine receptor from torpedo, some of which react with the denatured subunits [Tzartos, S.J., & Lindstrom, J.M. (1980) Proc. Natl. Acad. Sci. U.S.A.77, 755; Tzartos, S.J., & Lindstrom, J.M. (1981) in Monoclonal antibodies in Endocrine Research (Fellows, R., & Eisenbarth, G., Eds.) Raven Press (in press)]. The binding specificities of these antibodies to radioiodinated proteolytically generated fragments of the alpha subunit were determined by immunoprecipitation followed by gel electrophoresis. The antibodies tested fell into at least three main groups on the basis of their binding specificities. These antibodies were also tested for their capacity to bind to acetylcholine receptor solubilized in Triton X-100, sodium cholate, or sodium cholate supplemented with exogenous lipids. A monoclonal antibody raised to the denatured delta subunit, was tested for its ability to select radioiodinated proteolytic fragments of these subunits. These molecules provide probes for many sites on the acetylcholine receptor with affinities and specificities comparable to alpha-neurotoxins.

Acetylcholine receptors from Torpedo and Electrophorus have similar subunit structures

Previously, acetylcholine receptor purified from the electric organs of electric eels (Electrophorus electricus) and electric rays (Torpedo californica) (torpedo) had appeared to differ in subunit structure. Receptor from torpedo has the subunit structure alpha 2 beta gamma delta, but subunits corresponding only to alpha, beta, and gamma had been observed in receptor from eel. Here we report that if membrane fragments of eel electric organ are prepared and detergent extracted in the presence of iodoacetamide, then receptor purified from the extract contains a fourth subunit. Using monoclonal antibodies as well as conventional antisera, we show that the newly recognized subunit of receptor from eel corresponds to the delta subunit of torpedo. A monoclonal antibody to the delta subunit of torpedo cross-reacts with the gamma subunit and shows a similar cross-reaction between the delta' and gamma' subunits of receptor from eel, indicating the presence of an unexpected structural similarity. Although the function of the beta, gamma, and delta subunits remains unknown, these results support the concept that receptors from the electric organs of several species and probably also from muscle share a similarly complex subunit structure.