Proteolytic nicking of the acetylcholine receptor

Abstract

Low concentrations of papain rapidly cleave solubilized or membrane-bound acetylcholine receptor (AcChR) from Torpedo californica into a wide range of small fragments. The .alpha. subunits of the receptor are most resistant to cleavage. After solubilization in sodium dodecyl sulfate [SDS] solutions the fragments are dissociated, and on electrophoresis the apparent subunit composition is reduced from 4 types (.alpha., .beta., .gamma. and .delta.) to only .alpha. and finally, with large amounts of papain, to fragments even smaller than .alpha.. Prior to dissociation in SDS, the proteolytic fragments remain physically and functionally associated. The receptor which was degraded so as to apparently contain only .alpha. subunits, or even no obvious subunits, still retains antigenic determinants corresponding to each subunit, still retains its characteristic size and doughnut shape when examined by EM, and still sediments as dimers on sucrose gradients. Proteolytically nicked receptor remains fully functional in carbamylcholine-induced 22Na+ flux. Inadequate inhibition of proteases during purification of receptor could account for reports from some laboratories that they have purified receptors containing only .alpha. subunits or fragments of .alpha. subunits. The strong noncovalent association between AcChR subunits which has precluded their separation except under denaturing conditions in SDS was demonstrated.