Structure of 8Sα globulin, the major seed storage protein of mung bean

The 8S globulins of mung bean [Vigna radiata (L.) Wilczek] are vicilin-type seed storage globulins which consist of three isoforms: 8S alpha, 8S alpha' and 8S beta. The three isoforms have high sequence identities with each other (around 90%). The structure of 8S alpha globulin has been determined for the first time by X-ray crystallographic analysis and refined at 2.65 angstrom resolution with a final R factor of 19.6% for 10-2.65 angstrom resolution data. The refined 8S alpha globulin structure consisted of 366 of the 423 amino-acid residues (one subunit of the biological trimer). With the exception of several disordered regions, the overall 8S alpha globulin structure closely resembled those of other seed storage 7S globulins. The 8S alpha globulin exhibited the highest degree of sequence identity (68%) and structural similarity (a root-mean-square deviation of 0.6 angstrom) with soybean beta-conglycinin beta (7S globulin). Their surface hydrophobicities are also similar to each other, although their solubilities differ under alkaline conditions at low ionic strength. This difference seems to be a consequence of charge-charge interactions and not hydrophobic interactions of the surfaces, based on a comparison of the electrostatic potentials of the molecular surfaces. The thermal stability of 8S alpha globulin is lower than that of soybean beta-conglycinin beta. This correlates with the cavity size derived from the crystal structure, although other structural features also have a small effect on the protein's thermal stability.