The Isolation and Characterization of the Major Polypeptides of the Seed Globulin of Cowpea (Vigna unguiculataL. Walp) and their Sequential Synthesis in Developing Seeds

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Abstract
Globulin protein formed the major protein fraction of mature seeds of cowpea; it was found to be heterogeneous when examined by using chromatography and zonal isoelectric precipitation. Both 7S and US globulin were present and the fraction was dissociated by SDS treatment into three major subunits with apparent molecular weights 56 000, 54 000, and 52 000 as determined in SDS-acrylamide gels. The individual major subunits were purified by using ion-exchange chromatography in 8 M urea and further characterized by chemical analysis. Two of the major subunits had low but different contents of S-containing amino acid residues and were probably subunits of 7S glycoproteins. The pattern of seed development was investigated and four phases were identified. The protein profile and the amino acid composition of the seeds changed during development; most of the seed globulin was synthesized in the third and fourth phases and the net synthesis of the three major sub-units of the globulin differed. The profile of the amino acid composition of the pod walls did not change during the period of synthesis of the seed globulin fraction. The essential amino acid content of the meal is determined by that of the globulin fraction except for cysteine/cystine which is mostly supplied by the albumin fraction, the latter probably containing some proteins rich in S-containing amino acids.