Functional stability of Torpedo acetylcholine receptor. Effects of protease treatment

The effect of tryptic degradation on structural and functional properties of the membrane-bound acetylcholine receptor from Torpedo californica has been investigated. Under conditions of proteolysis which resulted in extensive degradation of receptor subunits, the membrane preparations retained their full capability of mediating agonist-induced cation flux as measured in rapid kinetic experiments. Low concentrations on trypsin also cleaved receptor dimers to monomers, and this effect was paralleled by degradation of the Mr 65 000 subunits which are known to contain sulfhydryl group(s) involved in receptor dimerization through an interchain disulfide bond(s). This conversion to monomers occurred at lower trypsin concentrations when the enzyme was added to the outside of the vesicles compared with the effects observed when the enzyme was present inside the vesicles. Similarly Mr 43 000 protein consistently found in preparations of the membrane-bound acetylcholine receptor, which can readily be removed without apparent effect on receptor function, displayed greater susceptibility to proteolysis when trypsin was added to the exterior medium rather than inside the vesicles. The results emphasize the full functionality of the monomeric form of the acetylcholine receptor comprised of four polypeptides.