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Caseins are highly phosphorylated milk proteins assembled in large colloidal structures termed micelles. In the milk of ruminants, as1-casein has been shown to be extensively phosphorylated. In this report we have determined the phosphorylation pattern of humanas1 -casein by a combi-nationofmatrix-assisted laserdesorptionmass spectrometry and amino acid sequence analysis. Three phosphorylation variants were identified. A nonphosphorylated form, a variant phosphorylated at Ser18 and a variant phosphory-lated at Ser18 and Ser26. Both phosphorylation sites are located in the amino acid recognition sequence of the mammary gland casein kinase | Eur. J. Biochem. 270 3651-3655 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03755.x The phosphorylation pattern of human as1-casein is markedly different from the ruminant species Esben S. Sorensen Lise Moller Maria Vinther Torben E. Petersen and Lone K. Rasmussen Protein Chemistry Laboratory Department of Molecular Biology University of Aarhus Denmark Caseins are highly phosphorylated milk proteins assembled in large colloidal structures termed micelles. In the milk of ruminants as1-casein has been shown to be extensively phosphorylated. In this report we have determined the phosphorylation pattern of human as1-casein by a combination of matrix-assisted laser desorption mass spectrometry and amino acid sequence analysis. Three phosphorylation variants were identified. A nonphosphorylated form a variant phosphorylated at Ser18 and a variant phosphory lated at Ser18 and Ser26. Both phosphorylation sites are located in the amino acid recognition sequence of the mammary gland casein kinase. Notably no phosphorylations were observed in the conserved region covering residues Ser70-Glu78 which is extensively phosphorylated in the ruminant as1-caseins. Keywords as1-casein human milk mammary gland casein kinase phosphorylation. Caseins are the predominant milk proteins of most mammalian species 1 . In ruminants about 75 of the milk protein content is constituted of caseins. The corresponding figure for human milk is only about 40 2 . In the milk of ruminants caseins interact with calcium phosphate forming large stable colloidal particles termed micelles. These micellar complexes make it possible to maintain a supersaturated calcium phosphate concentration in milk providing the newborn with sufficient calcium phosphate for the mineralization of the rapidly growing calcified tissues. In this context the phosphorylation of the individual caseins plays a significant role in the interaction with calcium phosphate and thereby the organization of the micelles. The ruminant caseins
