Highly purified preparations of bovine brain and rabbit nerve root neurofilaments were found to be lacking in protein kinase activity when either histone FIIA or the neurofilaments themselves were used as acceptors. There was no augmentation of activity in the presence of cyclic AMP. Addition of microtubule proteins prepared by cycles of assembly and disassembly resulted in phosphorylation of histone, phosphorylation of tubulin and the microtubule-associated proteins, and phosphorylation of neurofilament subunits. The phosphorylation of neurofilaments was predominantly in the 150,000-dalton species and was completely cyclic AMP dependent.

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