Plant .ALPHA.-Glucosidase: Molecular Analysis of Rice .ALPHA.-Glucosidase and Degradation Mechanism of Starch Granules in Germination Stage

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  • Nakai Hiroyuki
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Ito Tatsuya
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Tanizawa Shigeki
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Matsubara Kazuki
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Yamamoto Takeshi
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Okuyama Masayuki
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Mori Haruhide
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Chiba Seiya
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Sano Yoshio
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University
  • Kimura Atsuo
    Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University

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Other Title
  • 植物α-グルコシダーゼ，特にイネ酵素の分子解析と発芽時の澱粉粒分解に関する研究
  • Plant α-Glucosidase: Molecular Analysis of Rice α-Glucosidase and Degradation Mechanism of Starch Granules in Germination Stage
  • Plant アルファ Glucosidase Molecular Analysis of Rice アルファ Glucosidase and Degradation Mechanism of Starch Granules in Germination Stage

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Abstract

In germination of plant seeds, storage starch is principally degraded by the combination of amylolytic enzymes. As starch is an insoluble granule, a conventional view of the degradation pathway is that the initial attack is performed by α-amylase having the starch granule-binding ability. Plant α-glucosidase was also capable of adsorbing and hydrolyzing starch granules directly, indicating a possible second pathway: the direct liberation of glucose from starch granules by plant α-glucosidase rather than the α-amylase-mediated system. We found that the starch-binding site of plant α-glucosidase was situated in its C-terminal region, of which function was independent of the catalytic domain. Site-directed mutagenesis analysis on the aromatic amino acid residues conserved in this region revealed that Trp803 and Phe895 of rice α-glucosidase were responsible for binding to starch granules. Mold α-glucosidases were devoid of the ability to attack starch granules. In plant seeds, multiple α-glucosidases have been observed. Two types of α-glucosidases, insoluble and soluble enzymes, were found in the germinating stage of rice. Expression patterns of their activities classified 14 rice varieties into two groups (Groups 1 and 2). In Group 1 varieties, insoluble enzyme decreased immediately after germination. The soluble enzyme increased by de novo synthesis. Group 2 maintained a constant activity level of insoluble and soluble α-glucosidases in germination. From Groups 1 and 2, we selected varieties of Akamai and Nipponbare, respectively, of which analysis elucidated interesting molecular mechanisms of insoluble and soluble enzymes: i) isoform and isozyme formations by post-translational proteolysis as well as by chromosomal gene expression; ii) characterization of purified enzymes exhibiting different activities to starch granules.

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