Multiple forms of α-glucosidase in rice seeds (Oryza sativa L., var Nipponbare)

Two isoforms of α-glucosidases (ONG2-I and ONG2-II) were purified from dry rice seeds (Oryza sativa L., var Nipponbare). Both ONG2-I and ONG2-II were the gene products of ONG2 mRNA expressed in ripening seeds. Each enzyme consisted of two components of 6 kDa-peptide and 88 kDa-peptide encoded by this order in ONG2 cDNA (ong2), and generated by post-translational proteolysis. The 88 kDa-peptide of ONG2-II had 10 additional N-terminal amino acids compared with the 88 kDa-peptide of ONG2-I. The peptides between 6 kDa and 88 kDa components (26 amino acids for ONG2-I and 16 for ONG2-II) were removed by post-translational proteolysis. Proteolysis induced changes in adsorption and degradation of insoluble starch granules. We also obtained three α-glucosidase cDNAs (ong1, ong3, and ong4) from ripening seeds. The ONG1, ONG2, and ONG4 genes were situated in distinct locus of rice genome. The transcripts encoding ONG2 and ONG3 were generated by alternative splicing. Members of α-glucosidase multigene family are differentially expressed during ripening and germinating stages in rice.