Amino Terminal Sequence and Location of Phosphate Groups of the Major Human Casein

The amino acid sequence of the first 28 residues of the major human casein was determined. This protein in multiphosphorylated forms (0 to 5 phosphorous per molecule) was compared to cow beta-casein which is similar in composition but phosphorylated at a constant level. After sequencing the phosphate-free human casein, phosphorylated seryl and threonyl residues were located in three of the other phosphorylated forms by examining the aqueous layer of the phenylthiohydantoin conversion step during automatic liquid phase sequencing. Phosphate groups on specific seryl/threonyl residues suggest a biosynthetic mechanism involving stepwise phosphorylation or dephosphorylation.