Molecular weight in detergent solution of acetylcholine receptor from Torpedo californica

Abstract

Acetylcholine receptor extracted in detergent solution from the electric tissue of T. californica and purified by affinity chromatography contains predominantly 2 MW species. These were separated by sedimentation in a sucrose density gradient, and their molecular properties determined by sedimentation equilibrium and sedimentation velocity measurements in the analytical ultracentrifuge. The MW of these species were 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 MW of the protein moieties were 250,000 and 500,000. Since these species were identical in specific activity and polypeptide composition, they were 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 were 8.6 S and 12.8 S and the Stokes radii were 7.3 and 9.5 nm. For monomer and dimer the ratio of the Stokes radius to the minimum possible radius for the protein-detergent complex fell outside the range of values for globular proteins.