The two subfamilies of rice glutelin differ in both primary and higher-order structures

Abstract

Rice glutelin, which accounts for 70–80% of the total proteins of the seeds, consists of two nutritionally different subfamilies (A and B types). Although the similarity in primary sequences between the two subfamilies is as high as 60%, we established conditions to discriminate the two subfamilies when low amounts of antigen are analyzed by immunoblot methods. The glutelin α polypeptides can be resolved into six bands labeled α1 to α6 by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Gel filtration analysis showed that glutelin exists as a polymerized and a smaller molecular weight form. Immunoblot analysis of SDS-PAGE resolved polypeptides showed that α2, α3, and α4 are an A type and that these A types as well as α1, a B type, are polymerized. The polymerization tendency clearly differed between the two subfamilies except for α1, which may be derived from GluB-4 as suggested by analysis using Escherichia coli expression systems of glutelin cDNA regions corresponding to α polypeptides. GluB-4 and all the A type subunits have an extra Cys residue in the hypervariable regions, corresponding to the C-terminal region of α polypeptide. Accordingly, the extra Cys residue is hypothesized to be responsible for the polymerization of glutelin.