Immunochemical similarities between subunits of acetylcholine receptors from Torpedo, Electrophorus, and mammalian muscle

Abstract

Polypeptide chains composing acetylcholine receptors from the electric organs of Torpedo californica and Electrophorus electricus were purified and labeled with 125I. Immunochemical studies with these labeled chains showed that receptor from Electrophorus is composed of 3 chains corresponding to the .alpha., .beta. and .gamma. chains of receptor from Torpedo but lacks a chain corresponding to the .delta. chain of Torpedo. Receptor from mammalian muscle contains 4 groups of antigenic determinants corresponding to all 4 of the Torpedo chains. Binding of 125I-labeled chains was measured by quantitative immune precipitation and electrophoresis. Antisera to the following immunogens were used: denatured .alpha., .beta., .gamma. and .delta. chains of Torpedo receptor, native receptor from Torpedo and Electrophorus electric organs and from rat and fetal calf muscle, and human muscle receptor (from autoantisera of patients with myasthenia gravis). The 4 chains of Torpedo receptor were immunologically distinct from one another and from higher MW chains found in electric organ membranes. Antibodies to these chains reacted very efficiently with native Torpedo receptor, but the reverse was not true. Antibodies to native receptor from Torpedo and Electrophorus reacted slightly with each of the chains of the corresponding receptor. Cross-reaction between chains and antibodies to any native receptor was most obvious with the .alpha. chain of Torpedo or the corresponding .alpha. chain of Electrophorus. Antiserum to .alpha. chains exhibited higher titer against receptor from denervated rat muscle. Antibodies from myasthenia gravis patients did not cross-react detectably with 125I-labeled chains from electric organ receptors. Most interspecies cross-reaction occurred at conformationally dependent determinants whose subunit localization could not be determined by reaction with the denatured chains.