Depolarization induced change in the enzymatic radio iodination of a protein on the internal surface of the squid giant axon membrane

An enzymatic iodination procedure was used to probe for accessible proteins in the outer and inner membrane surfaces of the squid giant axon. The iodinated proteins obtained from the external surface, reflecting the Schwann cell surfaces and basal laminae, had a wide distribution of molecular weights, with a dominance at > 200,000 daltons. Internal membrane iodination by intracellular perfusion of the axon with the enzymatic mixture yielded two major peaks of activity: at 68,000 and 12,000 daltons. The dominant 12,000 dalton peak was selectively reduced in iodination by potassium depolarization of the axon.