Characterization of a neurofilament-associated kinase that phosphorylates the middle molecular mass component of chicken neurofilaments.

We have examined the properties of a chicken neurofilament (NF) kinase partially purified from NF-enriched preparations. This kinase cosediments with NFs following extraction with Triton X-100 and can be separated in an active form from NFs by treatment with 0.8 M KCl. Sequential chromatography of the salt extract on DEAE-cellulose and phosphocellulose results in an approximately 500-fold increase in specific activity over endogenous NF preparations as measured by 32P-incorporation into the middle molecular mass component of NFs (NF-M). The kinase is Mg(2+)-dependent, second messenger-independent and inhibited by high concentrations of heparin. It shows selectivity for NF-M and evidence is presented that the kinase phosphorylates NF-M solely in the tail domain. The kinase can also phosphorylate the microtubule-associated proteins tau and MAP2 as well as mammalian NF-M, all of which share putative phosphorylation sequences with chicken NF-M.