The Structure of Legumin, a Storage Protein of Broad Bean (Vicia faba) Seed

Abstract

Legumin from broad bean seed has been carboxymethylated and analysed. The protein contained three N‐terminal residues, leucine, glycine and threonine. Three polypeptide chains were found by polyacrylamide gel electrophoresis of the dissociated protein; their molecular weights were 56000, 42000 and 23000. 140 peptides were found on fingerprints of tryptic digests of S‐carboxymethyl‐legumin. Of these 14 were radioactive in digests of [14C]carboxymethylated legumin, and 16 were radioactive in digests of legumin prepared from material labelled in vivo with [35S]sulphate. Three new end groups were found after cyanogen bromide cleavage corresponding to Met‐Val, Met‐Phe and Met‐Pro sequences. Taken in conjunction with the amino acid analyses these peptide mapping experiments suggest that the protein contains a total sequence weight of 115000, which is in agreement with the finding of three polypeptide chains whose weights total 121000. On the basis of dye binding studies and of the incorporation in vivo of uniformly 14C‐labelled amino acid mixture, it is suggested that the polypeptide chains of molecular weights 56000, 42000 and 23000 are present in molar ratios of 1:4:6 or 1:3:6, respectively. From consideration of experimental accuracy the 1:3:6 structure is preferred. The proposed 10 chains model for legumin has a molecular weight of 320000. Copyright © 1970, Wiley Blackwell. All rights reserved