Molecular Cloning of Mouse Mammary Gland ϰ-Casein: Comparison With Rat ϰ-Casein and Rat and Human γ-Fibrinogen

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Abstract
A cDNA clone for .kappa.-casein mRNA from the lactating mouse mammary gland was isolated and its nucleotide sequence determined. Analysis of the deduced amino acid sequence revealed a precursor protein with a 21-amino-acid signal sequence and a mature protein of 160 amino acids, the mature mouse protein being 3 amino acids longer than the rat .kappa.-casein. Northern blot analysis of the lactating rat and mouse mammary gland showed a specific mRNA for rat .kappa.-casein and two distinct mRNAs for mouse .kappa.-casein. This result is explained by the presence of two putative polyadenylation sites in mouse .kappa.-casein cDNA, whereas rat .kappa.-casein cDNA has only one polyadenylation site. Comparison of the nucleotide sequence and of the deduced amino acid sequence of .kappa.-casein from mouse with that of the rat showed 85% homology between the two sequences. However, when amino acid sequences of .kappa.-casein from rat and mouse were compared with ovine .kappa.-casein, only a 45% homology was observd. Amino acid sequences of .kappa.-casein from rat, mouse, and sheep were 36.53% homologous with rat and human .gamma.-fibrinogen. The extent of homology was similar (32%) when nucleotide sequences of corresponding cDNAs were compared. The stretches of homology existing at different regions between the two proteins were more confined toward the amino-terminal half of .gamma.-fibrinogen. However, when nucleotide sequences were compared, mouse .kappa.-casein cDNA showed homology only with the second half of the rat .gamma.-fibrinogen cDNA, i.e., between nucleotides 661-1135. The homology with the human .gamma.-fibrinogen cDNA spanned over two regions, one between nucleotides 1-328 and the second between nucleotides 591-726. Thus, it is hypothesized that .kappa.-casein, a milk-clotting protein, and .gamma.-fibrinogen, a blood-clotting protein, may have evolved from a common ancestor.