title: Chitinase III in pomegranate seeds (Punica granatum Linn.): a high-capacity calcium-binding protein in amyloplasts.
abstract: Chitinases are a class of ubiquitous proteins that are widely distributed in plants. Defense is the major natural role for chitinases, primarily against fungal pathogens. Little is known regarding their non-defensive roles in seeds. In this study, a new class III chitinase from pomegranate seeds (pomegranate seed chitinase, PSC) was isolated and purified to homogeneity. The native state of PSC is a monomer with a molecular weight of approximately 30 kDa. This chitinase naturally binds calcium ions with high capacity and low affinity, suggesting that PSC is a calcium storage protein. Consistent with this idea, its amino acid sequence (inferred from cDNA) is rich in acidic amino acid residues, especially Asp, similar to reported calcium storage proteins. The presence of calcium considerably improves the stability of the protein but has little effect on its enzymatic activity. Transmission electron microscopy analyses indicate that, similar to phytoferritin, this enzyme is widely distributed in the stroma of amyloplasts of the embryonic cells, suggesting that amyloplasts in seeds could serve as an alternative plastid for calcium storage. Indeed, the transmission electron microscopy results showed that, within the embryonic cells, calcium ions are mainly distributed in the stroma of the amyloplasts, consistent with a role for PSC in calcium storage. Thus, the plant appears to have evolved a new plastid for calcium storage in seeds. During seed germination, the content of this enzyme decreases with time, suggesting that it is involved in the germination process.
authors.affiliation: CAU & ACC Joint Laboratory of Space Food, College of Food Science & Nutritional Engineering, China Agricultural University, Key Laboratory of Functional Dairy, Ministry of Education, Beijing 100083, China.
authors.forename: Haixia
authors.initials: H
authors.lastname: Yang
authors.forename: Tuo
authors.initials: T
authors.lastname: Zhang
authors.forename: Taro
authors.initials: T
authors.lastname: Masuda
authors.forename: Chenyan
authors.initials: C
authors.lastname: Lv
authors.forename: Lei
authors.initials: L
authors.lastname: Sun
authors.forename: Guiqin
authors.initials: G
authors.lastname: Qu
authors.forename: Guanghua
authors.initials: G
authors.lastname: Zhao
mesh_terms: D000143:Acids / Q000378:metabolism; D000595:Amino Acid Sequence; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002688:Chitinases / Q000302:isolation & purification* / Q000378:metabolism; D004789:Enzyme Activation; D004795:Enzyme Stability; D018525:Germination; D029561:Lythraceae / Q000201:enzymology*; D046529:Microscopy, Electron, Transmission; D008969:Molecular Sequence Data; D059828:Plant Cells / Q000201:enzymology / Q000648:ultrastructure; D010940:Plant Proteins; D018087:Plastids / Q000201:enzymology* / Q000648:ultrastructure; D055314:Seed Storage Proteins / Q000378:metabolism; D012639:Seeds / Q000201:enzymology* / Q000648:ultrastructure; D016415:Sequence Alignment
chemical_list: D000143:Acids; D002135:Calcium-Binding Proteins; D010940:Plant Proteins; D055314:Seed Storage Proteins; D002688:Chitinases; C110892:chitinase, class III