Legumin and vicilin, storage proteins of legume seeds

If information about the identity, or even genetic history, of a grain sample is sought, it should be possible to read this information from the grain-protein composition. As direct products of gene transcription and translation, proteins contain a wealth of genetic information, ready to be read off, given the appropriate techniques. Zuckerkandl and Pauling (1965) have described the relationship between chemical composition and genotype (or even progenitors) as occurring at several different levels. Semantides (sense-carrying molecules, such as DNA, RNA and proteins) provide reliable information about genotype; this is not so for episemantic molecules (the products of enzymes) or for asemantic molecules (not specifically produced by the organism), since in these latter cases, environmental or extraneous influences might be expected to predominate.

Identification of cereal varieties by gel electrophoresis of the grain proteins

Studies on Brassica seed proteins. I. The low molecular weight proteins in rapeseed. Isolation and characterization.

The 12 S globulin of rapeseed represents an oligomeric protein with a molecular weight of 300,000. It is composed of 6 subunits, which are arranged in a trigonal antiprism with the point group symmetry 32 (D3). Each subunit contains smaller units (polypeptide chains) with molecular weights in the range of 18,500 to 31,000. The protein contains the following 4 polypeptide chains differing by their molecular weights in the SDS-electrophoresis; 18,500 ± 800, 21,100 ± 500, 26,800 ± 900, 31,200 ± 1,600. According to its quaternary structure the protein dissociates under milieu conditions.



The secondary structure of the protein is characterized by a low (11 %) content of α-helix and a relatively high (31%) content of β-conformation.



Owing to its structure and physico-chemical properties the rapeseed protein is closely related to other 11/12 S globulins in the seeds of different plant species and botanical families.

The structure of the 12 S globulin from rapeseed (Brassica napus L.)

Supplemental methionine in a complete culture medium increased the methionine content of the protein fraction of cultured soybean (Glycine max L. Merrill) cotyledons (Thompson, Madison, Muenster 1981 Phytochemistry 20: 941-945). To explain the observed increase in protein methionine, we have measured the amounts and subunit compositions of 7S and 11S storage proteins and determined the amino acid compositions of the three major protein fractions (2-5S, 7S, 11S) of seeds developed on plants and of cultured cotyledons grown in the presence or absence of supplemental l-methionine. Development of cultured cotyledons was representative of development of seeds on plants. The ratios of 11S to 7S proteins, the subunit contents, and amino acid compositions of their storage protein fractions were similar, but not identical. Supplemental methionine increased the mole percent methionine in each of the three protein fractions of cultured cotyledons and changed the amounts of several other amino acids. Supplemental methionine inhibited expression of the 7S beta-subunit gene. Concomitant with the absence of the beta-subunit, which contains no methionine, was an increase in the ratio of 11S to 7S proteins, and an increase in the methionine content of the subunits composing these fractions. Inhibition of beta-subunit gene expression by methionine in cultured cotyledons provides a reproducible, easily controlled system for the study of eucaryotic gene expression.

Effects of exogenous methionine on storage protein composition of soybean cotyledons cultured in vitro.

Two proteins, previously described by the authors as BI (S020, buffer 12 S) and AIVS (S020, w 1.7 S) have been isolated from eight varieties of rapeseed belonging to Brassica campestris L. and Brassica napus L. species. These proteins have similar chromatographic and electrophoretic characteristics but differ in amino acid composition, particularly with regard to the sulfur-containing amino acids. One of the rapeseed samples was obtained from plants grown on sulfur-deficient soil. The sulfur deficiency produced, in the mature seed, a much reduced protein content and appeared to affect the structure of the protein BI.

C. M. Christ

Papers

Species and varietal differences in the proteins of rapeseed