Protein phosphorylation and sodium conductance in nerve membrane.

High molecular weight proteins extracted from the walking nerves of the shore crab (Carcinus maenas) exhibit a cycle of phosphorylation-dephosphorylation that is influenced by neurotropic compounds and inorganic ions. The net phosphorylation state of the proteins is increased in the presence of K+ ions and decreased with Na+ ions. In the absence of Mg2+ there is no phosphorylation. Ca2+ ions at low concentrations are necessary for optimal phosphorylation. At high concentration (above 0.1 mM), Ca2+ ions are inhibitory. Neurotropic compounds generally inhibit the phosphorylation process. More specifically, tetrodotoxin and veratridine, depending on the ionic composition of the medium, have opposite effects on the phosphorylation process, a result in agreement with their known physiological action. It is suggested that the high molecular weight components thus identified are part of the sodium permeation sites and that the conductance state of those sites is controlled by a phosphorylation process.