Multiple phosphorylation sites in mammalian neurofilament polypeptides.

The 68,000, 145,000, and 200,000 dalton subunits of neurofilaments from rat spinal cord and brain stem were shown by chemical phosphate analysis to contain about 3, 9, and 22 mol of phosphate/mol of polypeptide, respectively. All three subunits contained phosphoserine while the 145,000 dalton polypeptide also contained phosphothreonine. The high phosphate content of the 200,000 dalton polypeptide was reflected by the extensive shifts both in its isoelectric point and apparent molecular weight after treatment of neurofilaments with alkaline phosphatase.