Disulphide Linkages Occur in Many Polypeptides of Rice Protein Fractions: a Two-dimensional Gel Electrophoretic Study

Most of the work on rice storage proteins focused on the major fraction - glutelin. In contrast to previous reports of the occurrence of single legumin-like polypeptide pair in glutelin, two-dimensional gel electrophoresis revealed many glutelin bands, with some having and some lacking disulphide-linkages. Five legumin-like polypeptide pairs exhibited wide heterogeneity over a range of molecular weight (M-r) 25 to 60 kDa, each having a large subunit (M-r ranged from 18 to 40.5 kDa) disulphide-bonded to a small subunit (M-r ranged from 16.5 to 18.0 kDa). A band of 49 kDa was homodimeric with two subunits of 29 kDa each; a polypeptide of 51 kDa which altered position to ones corresponding to 53 kDa and 57 kDa on 2-D gels contained intrapolypeptide linkages. Polypeptides of 65 kDa and 60 kDa occurred as aggregates of 110 kDa. The number of polypeptides in other seed protein fractions albumins, globulins and prolamins, varied from four to ten. One of the albumin bands had intrapolypeptide disulphide linkages (20 kDa) and the globulins contained two such bands (13.5 and 20 kDa). Thus, the present study provides a description of the polypeptide composition of different rice protein fractions that is finely resolved with respect to the occurrence of disulphide linkages.