Divalent cation dependent phosphorylation of proteins in squid giant axon

In vitro and in situ (after intracellular infusion) incubation of axoplasm from the squid giant axon with [gamma-32P]ATP produces a phosphorylation of primarily two proteins (of mol.wt. 200,000 and greater than 400,000). The phosphorylation of these proteins is stimulated by Mg2+, inhibited by Ca2+, and unaffected by 10(-7) to 10(-5) M cyclic nucleotides. The 200 kdalton and greater than 400 kdalton phosphorylated peaks appear to be neurofilament proteins, and phosphorylation of these peaks in situ is decreased by electrical stimulation of the axon.