Characterization of Additional Casein Kinase I Sites in the C‐Terminal “Tail” Region of Chicken and Rat Neurofilament‐M

: In previous studies we have identified Ser502, Ser528, and Ser534 as target sites in chicken neurofilament middle molecular mass protein (NF‐M) for casein kinase I (CKI) in vitro and have shown that these sites are also phosphorylated in vivo. We now make use of a combination of molecular biological and protein chemical techniques to show that two additional in vivo phosphorylation sites in chicken NF‐M, Ser464 and Ser471, can also be phosphorylated by CKI in vitro. These two sites are conserved in higher vertebrate NF‐M molecules, and recombinant protein constructs containing the homologous rat NF‐M peptides can be phosphorylated by CKI in vitro, suggesting that phosphorylation of these sites is conserved at least in higher vertebrates. The two new sites are adjacent to a conserved peptide sequence (VEE‐IIEET‐V) found once in higher vertebrate NF‐M molecules and twice in lamprey NF‐180. Variants of this sequence are also found in neurofilament low and high molecular mass proteins (NF‐L and NF‐H) and α‐internexin, and in mammalian NF‐L are known to be associated with in vivo phosphorylation sites. We speculate that CKI phosphorylation in general, and these sites in particular, may be important in neurofilament function.