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 Ser(502), Ser(528), and Ser(534) as target sites in chicken neurofilament middle molecular mass protein (NF-M) for casein kinase I (CKl) 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, Ser(464) and Ser(471), can also be phosphorylated by CKl 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 CKl 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 (VEEIIEET-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 alpha-internexin, and in mammalian NF-L are known to be associated with in vivo phosphorylation sites. We speculate that CKl phosphorylation in general, and these sites in particular, may be important in neurofilament function.