Neurofilament protein is phosphorylated in the squid giant axon.

Abstract

Phosphorylation of neurofilament protein from squid axoplasm was studied. Phosphorylation was demonstrated by 32P labeling of protein during incubation of axoplasm with [.gamma.-32P]ATP. When the labeled proteins are separated by SDS[sodium dodecyl sulfate]-polyacrylamide gel electrophoresis (SDS-PAGE), 2 bands, at 2.0 .times. 105 daltons and > 4 .times. 105 daltons, contain the bulk of the 32P. The 2.0 .times. 105-dalton phosphorylated polypeptide comigrates on SDS-PAGE with 1 of the subunits of squid neurofilament protein. Both major phosphorylated polypeptides cofractionated with neurofilaments in discontinuous sucrose gradient centrifugation and on gel filtration chromatography on Sepharose 4B. The protein-phosphate bond behaved like a phospho-ester and labeled phospho-serine was identified in an acid hydrolysate of the protein. The generality of this phenomenon in various species and its possible physiological significance were discussed.