Polypeptide Components of an Excitable Plasma Membrane

The excitable axonal membrane from garfish olfactory nerve has been purified by density gradient centrifugation using a modification of the Ficoll-sucrose method of Day et al. (Day, E. D., McMillan, P. N., Mickey, D. D., and Appel, S. H. (1971) Anal. Biochem. 39, 29). Three fractions are obtained with densities ranging from 1.051 to 1.064 g per ml. The membrane contains approximately 28% protein and the polypeptide chain composition has been analyzed on polyacrylamide gels in the presence of sodium dodecyl sulfate. The phosphorylated intermediate of the (Na+-K+)-ATPase has a molecular weight of 100,000 and the (Na+-K+)-ATPase activity is more than 90% of the total ATPase activity in the membrane.