title: Accumulation of small heat-shock protein homologs in the endoplasmic reticulum of cortical parenchyma cells in mulberry in association with seasonal cold acclimation.
abstract: Cortical parenchyma cells of mulberry (Morus bombycis Koidz.) trees acquire extremely high freezing tolerance in winter as a result of seasonal cold acclimation. The amount of total proteins in endoplasmic reticulum (ER)-enriched fractions isolated from these cells increased in parallel with the process of cold acclimation. Protein compositions in the ER-enriched fraction also changed seasonally, with a prominent accumulation of 20-kD (WAP20) and 27-kD (WAP27) proteins in winter. The N-terminal amino acid sequence of WAP20 exhibited homology to ER-localized small heat-shock proteins (smHSPs), whereas that of WAP27 did not exhibit homology to any known proteins. Like other smHSPs, WAP20 formed a complex of high molecular mass in native-polyacrylamide gel electrophoresis. Furthermore, not only WAP20 but also 21-kD proteins reacted with antibodies against WAP20. Fractionation of the crude microsomes by isopycnic sucrose-gradient centrifugation revealed that both WAP27 and WAP20 were distributed on a density corresponding to the fractions with higher activity of ER marker enzyme, suggesting localization of these proteins in the ER. When ER-enriched fractions were treated with trypsin in the absence of detergent, WAP20 and WAP27 were undigested, suggesting localization of these proteins inside the ER vesicle. The accumulation of a large quantity of smHSPs in the ER in winter as a result of seasonal cold acclimation indicates that these proteins may play a significant role in the acquisition of freezing tolerance in cortical parenchyma cells of mulberry trees.
authors.affiliation: Environmental Cryobiology Group, Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Ukaji
authors.forename: C
authors.initials: C
authors.lastname: Kuwabara
authors.forename: D
authors.initials: D
authors.lastname: Takezawa
authors.forename: K
authors.initials: K
authors.lastname: Arakawa
authors.forename: S
authors.initials: S
authors.lastname: Yoshida
authors.forename: S
authors.initials: S
authors.lastname: Fujikawa
mesh_terms: D000064:Acclimatization; D000595:Amino Acid Sequence; D003081:Cold Climate; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005615:Freezing; D006360:Heat-Shock Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012621:Seasons; D017386:Sequence Homology, Amino Acid; D014197:Trees / Q000166:cytology / Q000235:genetics / Q000378:metabolism*
chemical_list: D006360:Heat-Shock Proteins; D010940:Plant Proteins