TS-A2, TS-B, R-, and S-Caseins: Their Isolation, Composition, and Relationship to the β- and γ-Casein Polymorphs A2 and B

TS-A2, R-, S-, and TS-B caseins are minor components of micellar caseins typed β and γ-A2 and B. Based on amino acid analysis, TS-A2 and S-caseins form one pair of polymorphs and R- and TS-B another. The polymorphs in each pair differ only by a single amino acid substitution (Arg → Ser) also found in the β- and γ-variants. R- and TS-B are smaller than TS-A2 and S-casein by two amino acids in the N-terminal portion of the molecule. These proteins have a molecular weight of about 12,000 and contain no phosphorus or cystine but have one residue of tryptophan. These findings, together with results of partial sequencing, support the hypothesis that the proteins have the same structure as the C-terminal portions of the β- and γ-caseins.