Neurofilaments are part of the high molecular weight complex containing neuronal cdc2-like kinase (nclk)

A cdc2-like kinase (nclk, comprised of cdk5 and its activator p25nck5a) that localizes primarily to neuronal cells has recently been identified. Although its precise physiological role remains unclear, a variety of nuclear and cytoplasmic proteins that may be targets for phosphorylation by this kinase have been suggested in various developmental and pathological states. Here we provide evidence for a functional association between nclk and neurofilament proteins: (i) brain neurofilament preparations include cdk5 and p25nck5a; (ii) nclk copurifies with neurofilament proteins using Mono-S and gel-filtration column chromatographic procedures; (iii) neurofilaments are coprecipitated with cdk5 kinase; and (iv) the addition of radiolabeled ATP to the immunoprecipitated complex results in phosphorylation of the cytoskeletal protein. These results are consistent with the formation of a functional macromolecular complex between nclk and neurofilaments in vivo and suggest a possible role for this kinase in regulating neuronal cytoskeletal networks.