Primary structure of bovine casein. Partial sequence

Abstract

This paper is the fifth of a series devoted to the primary structure of the bovine β‐casein A2. The first two communications dealt with the isolation, analysis and positioning in the peptide chain of the tryptic and “CNBr” peptides [1,2]. The last two described the sequence of 65 residues from the carboxyl‐terminus and of 32 residues from the amino‐terminus [3,4].We report here a partial sequence which includes 156 residues (out of the 209 residues of the β‐casein A2) exactly positioned in the peptide chain. Two segments including 53 residues (from position 36 to 68 and from 73 to 92) remain to be analyzed.In addition to the tryptic and “CNBr” peptides originating from β‐casein A2, two peptides issued from a tryptic digest of β‐casein B were used as starting material.The position of the substitution His→Gln which differenciates the β‐casein A2 from the β‐casein A3, already given in a preliminary communication [5], is indicated, as it is for the substitution Ser→Arg which represents one of the two differences occurring between the β‐casein A2 and the β‐casein B.Four of the five phosphorus atoms of the β‐casein A2 have been previously located in the NH2‐terminal tryptic peptide [4]. The fifth has been located at position 35. All the phosphorus atoms of the β‐casein A2 occur as phosphoserine.The complete structure of the β‐casein A2 will be given and discussed in the next and last paper.