research papers 824 doi:10.1107/S090744490601804X Acta Cryst. (2006). D62, 824–832 Acta Crystallographica Section D Biological Crystallography ISSN 0907-4449 Structure of 8Sa globulin, the major seed storage protein of mung bean Takafumi Itoh, a Roberta N. Garcia, b Motoyasu Adachi, a ‡ Yukie Maruyama, a Evelyn Mae Tecson-Mendoza, b Bunzo Mikami a and Shigeru Utsumi a * a Food Quality Design and Development, Graduate School of Agriculture, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan, and b Institute of Plant Breeding, College of Agriculture, University of the Philippines Los Ban ˜os, College, Laguna 4031, Philippines ‡ Present address: Center for Neutron Science, Tokai Research Establishment, Japan Atomic Energy Research Institute, 2-4 Shirakata, Tokai-mura, Ibaraki 319-1195, Japan. Correspondence e-mail: sutsumi@kais.kyoto-u.ac.jp # 2006 International Union of Crystallography Printed in Denmark – all rights reserved The 8S globulins of mung bean [Vigna radiata (L.) Wilczek] are vicilin-type seed storage globulins which consist of three isoforms: 8S, 8S 0 and 8S. The three isoforms have high sequence identities with each other (around 90%). The structure of 8S globulin has been determined for the first time by X-ray crystallographic analysis and refined at 2.65 A ˚ resolution with a final R factor of 19.6% for 10–2.65 A ˚ resolution data. The refined 8S 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 globulin structure closely resembled those of other seed storage 7S globulins. The 8S globulin exhibited the highest degree of sequence identity (68%) and structural similarity (a root-mean-square deviation of 0.6 A ˚ ) with soybean -conglycinin  (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 globulin is lower than that of soybean -conglycinin . 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. Received 10 March 2006 Accepted 16 May 2006 PDB Reference: 8S globulin, 2cv6, r2cv6sf. 1. Introduction Mung beans [Vigna radiata (L.) Wilczek] are a popular food crop in Asia, South America, Australia and the USA and are similar to other legumes such as soybeans, jack beans and kidney beans. Mung bean seeds contain about 20–25% protein and are a major source of protein, especially in developing countries. The major seed proteins of mung bean are storage globulins of the basic 7S type (3%), vicilin type (8S; 90%) and legumin type (11S; 8%) (Tecson-Mendoza et al., 2001). The 8S globulins, which are the major storage globulins of mung bean, have molecular weights of about 150 kDa and consist of three homologous isoforms, 8S, 8S 0 and 8S, which exhibit high homology (about 90% identity) to each other and have a molecular weight of about 49 kDa, indicating that the native 8S globulin consists of heterotrimers (Tecson-Mendoza et al., 2001; Bernardo et al. , 2004). Like other 7S globulins, they have no disulfide linkages (Bernardo et al. , 2004). Their amino-acid sequences exhibit similarity (about 68%) to soybean -con- glycinin  (a 7S globulin). It has been reported that soybean