Photoaffinity labeling and quaternary structure of the acetylcholine receptor from Torpedo californica.

Abstract

Membrane fragments from electric tissue of Torpedo californica containing nicotinic acetylcholine receptor were composed of 4 different polypeptide chains with MW of 40,000 (.alpha.), 48,000 (.beta.), 62,000 (.gamma.) and 66,000 (.delta.). The .alpha. and .beta. chains were still present in all and .gamma. and .delta. in some of the receptor preparations after Triton X-100 extraction and purification by affinity chromatography. All components of the receptor reacted covalently with the photoaffinity label 4-azido-2-nitrobenzyltrimethylammonium fluoroborate, the .delta. chain incorporating less of the reagent compared to the .alpha. and .beta. chains. Agonists and antagonists containing a quaternary ammonium group protected all chains against the label; the principal neurotoxin from Naja naja siamensis protected the .alpha. chain only. The .alpha. chain apparently binds the neurotoxin from Naja naja, and the .alpha. and .beta. chains are involved in the binding of ligands with quaternary ammonium groups. The function of the .gamma. and .delta. chains remains to be determined.