Molecular weight in detergent solution of acetylcholine receptor from Torpedo californica

Acetylcholine receptor extracted in detergent solution from the electric tissue of Torpedo californica and purified by affinity chromatography contains predominantly two molecular weight species. These have been separated by sedimentation in a sucrose density gradient, and their molecular properties have been determined by sedimentation equilibrium and sedimentation velocity measurements in the analytical ultracentrifuge. The molecular weights of these species have been determined, without prior determination of the extent of detergent bound to them, by the adjustment of solvent density with D2O so as to blank out the contribution of bound detergent to the sedimentation potential. The molecular weights of the protein moieties are 250 000 and 500 000. Since these species are identical in specific activity and polypeptide composition they are related as monomer and dimer. The hydrodynamic properties of the detergent complexes of monomer and dimer were derived from combined measurements of sedimentation equilibrium and sedimentation velocity. The S20,w'S are 8.6 S and 12.8 S and the Stokes radii are 7.3 nm and 9.5 nm. For both monomer and dimer, the ratio of the Stokes radius to the minimum possible radius for the protein-detergent complex falls outside the range of values for globular proteins.