Neurofilament protein phosphorylation. Species generality and reaction characteristics.

We have previously demonstrated that neurofilament protein (NFP) is endogenously phosphorylated in squid giant fiber axoplasm (1). Here the generality of this phenomenon is demonstrated, and optimal reaction conditions are described. Phosphorylation of Myxicola NFP is shown in vivo and in isolated axoplasm by 32P labeling from [32P]orthophosphoric acid and [gamma-32P]ATP, respectively. Phosphorylation of guinea pig NFP is shown in extracts of peripheral nerve and in enriched NFP preparations. Both Myxicola NFP subunits (150 and 160 kilodaltons) and all three guinea pig NFP subunits (200, 145, and 68 kilodaltons) are phosphorylated. Phosphorylation of 10-nm filament protein from cultured fibroblasts is also shown optimal reaction conditions with guinea pig enriched NFP are as follows. The reaction has high specificity for ATP as substrate. Divalent cations are required, Mn2+ being most effective. Monovalent cations decrease activity. The pH optimum is 8.0. Cyclic AMP and other low molecular weight factors have no significant modulatory effect.