Accumulation of Small Heat-Shock Protein Homologs in the Endoplasmic Reticulum of Cortical Parenchyma Cells in Mulberry in Association with Seasonal Cold Acclimation

• Published in: Plant physiology (Bethesda) Vol. 120; no. 2; pp. 481 - 489
• Main Authors: UKAJI, N, KUWABARA, C, TAKEZAWA, D, ARAKAWA, K, YOSHIDA, S, FUJIKAWA, S
• Format: Journal Article
• Language: English
• Published: Rockville, MD American Society of Plant Physiologists 01-06-1999; American Society of Plant Biologists
• Subjects: Acclimatization; Accumulation; Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; Amino Acid Sequence; Amino acids; Antibodies; Biological and medical sciences; Cell biochemistry; Cell physiology; Centrifugation; Climatic adaptation. Acclimatization; Cold Climate; Endoplasmic Reticulum - metabolism; Environmental and Stress Physiology; Fractionation; Freezing; Fundamental and applied biological sciences. Psychology; Gels; General agronomy. Plant production; Heat-Shock Proteins - chemistry; Heat-Shock Proteins - genetics; Heat-Shock Proteins - metabolism; Molecular Sequence Data; Molecular Weight; Parenchyma; Plant cells; Plant physiology and development; Plant Proteins - chemistry; Plant Proteins - genetics; Plant Proteins - metabolism; Proteins; Seasons; Sequence Homology, Amino Acid; Small heat shock proteins; Trees - cytology; Trees - genetics; Trees - metabolism; Winter; Temperature; Cold; Seasonal variation; Dicotyledones; Angiospermae; Heat shock protein; Environmental factor; Spermatophyta; Moraceae; Biological accumulation; Endoplasmic reticulum; Adaptation
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.120.2.481

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.