Identification and isolation of methionine-cysteine rich proteins in soybean seed

Introgression of leginsulin, a cysteine-rich protein, and high-protein trait from an Asian soybean plant introduction genotype into a North American experimental soybean line

Soybean is an important protein source for both humans and animals. However, soybean proteins are relatively poor in the sulfur-containing amino acids, cysteine and methionine. Improving the content of endogenous proteins rich in sulfur-containing amino acids could enhance the nutritive value of soybean meal. Leginsulin, a cysteine-rich peptide, predominantly accumulates in Asian soybean accessions but not in most North American cultivars. By screening diverse soybean accessions from the USDA Soybean Germplasm Collection, we were able to identify one plant introduction, PI 427138, as a high-protein line with relatively high amounts of both elemental sulfur and leginsulin. We introgressed these desirable traits from PI 427138 into an experimental line with the aim of improving the overall protein content and quality of seed proteins. Biochemical characterization of inbred progenies from the cross of LD00-3309 with PI 427138 grown at six locations revealed stable ingression of high protein, high elemental sulfur, and high leginsulin accumulation. Comparison of soybean seed proteins resolved by high-resolution 2-D gel electrophoresis in combination with Delta2D image analysis software revealed preferential accumulation of a few glycinin subunits contributed to the increased protein content in the introgressed lines. Amino acid analysis revealed that even though the leginsulin introgressed lines had higher protein, leginsulin, and elemental sulfur, the overall concentration of sulfur-containing amino acids was not significantly altered when compared with the parental lines. The experimental soybean lines developed during this study (Leg-3, Leg-7, and Leg-8) lack A5, A4, and B3 glycinin subunits and could be utilized in breeding programs to develop high-quality tofu cultivars.

Molecular strategies to improve the nutritional quality of legume proteins

Legume proteins are relatively deficient in the sulfur amino acids. Among the different strategies to increase the sulfur amino acid contents of legumes, we have chosen to increase the biosynthesis of endogenous, non-abundant sulfur-rich proteins we have identified in soybean seed. We identified and isolated an 8 kDa sulfur-rich protein from the albumin fraction of soybean seed. Subsequently, a cDNA designated Gm2S-1, has been isolated and characterized. The Gm2S-1 cDNA encodes a pre-proprotein that contains a signal peptide and a precursor protein that undergoes post-translational processing to yield the mature 8 kDa protein containing 7.8% methionine and 7.8% cysteine. The Gm2S-1 gene is an ideal candidate for overexpression to improve the nutritional quality of soybean and other legumes.