Identification and Characterization of the Seed Storage Proteins from Alfalfa (Medicago sativa)

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Abstract
The major seed storage proteins in alfalfa are medicagin (a legumin-like globulin), alfin (a vicilin-like globulin) and a family of Lower Molecular Weight albumins (LMW13). These comprise 30%, 10% and 20%, respectively, of the total extractable protein from cotyledons of mature seeds. Alfin is a heterogeneous oligomeric protein (Mr∼ 150 kD) composed of polypeptides ranging in size from Mr 50 to 14 kD (α1,-α6; 50, 38, 32, 20, 16 and 14 kD, respectively). Medicagin is also a high molecular weight oligomeric protein, but requires high concentrations of salt for solubilization. It is comprised of a family of individually distinct subunits, each composed of an acidic polypeptide (A1–A9; Mr 49 to 39 kD) linked via disulphide bond(s) to a basic polypeptide (B1, B2, B3; Mr 24, 23 and 20 kD, respectively). This pairing is highly specific and two families are recognizable on the basis of the B polypeptide (B3 or B1/B2). Subunits (Mr∼ 50–65 kD) are assembled as trimers (8S) or larger oligomers (12S–15S) in mature seeds. The lower molecular weight albumins (LMW1–3) are acidic (pl<6), and consist of sets of disulphide-bonded polypeptides (Mr 15 and 11 kD).