Characteristics of the protein kinase activity associated with rat neurofilament preparations

Abstract

Some properties of the protein kinase activity associated with neurofilaments isolated from the brain system and spinal cord of rats have been investigated. The activity had an apparent K m for ATP of 20 μM, a pH optimum of 8.0 and phosphorylated both serine and threonine residues in neurofilament proteins. Cyclic AMP had no effect on the vitro reaction and casein was a preferred exogenous substrate in comparison to histone. Phosphopeptide mapping of the 145 kDa subunit from neurofilaments phosphorylated in the presence and absence of microtubule proteins indicated that the neurofilament-associated activity was distinct from the microtubule-associated protein kinase. Limited proteolysis of neurofilaments with chymotrypsin indicated that the enzyme activity was not associated with a domain of the 200 kDa subunit which may form the side-arm projections on neurofilaments.