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PLANT PHYSIOLOGY , Vol 114, Issue 1 29-37, Copyright © 1997 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Abundant Accumulation of the Calcium-Binding Molecular Chaperone Calreticulin in Specific Floral Tissues of Arabidopsis thaliana
D. E. Nelson, B. Glaunsinger and H. J. Bohnert
Department of Biochemistry (D.E.N., B.G., H.J.B.), and Departments of Plant Sciences and Molecular and Cellular Biology (H.J.B.), University of Arizona, Biosciences West, Tucson, Arizona 85721
Calreticulin (CRT) is a calcium-binding protein in the endoplasmic
reticulum (ER) with an established role as a molecular chaperone. An
additional function in signal transduction, specifically in calcium
distribution, is suggested but not proven. We have analyzed the expression
pattern of Arabidopsis thaliana CRTs for a comparison with these proposed
roles. Three CRT genes were expressed, with identities of the encoded
proteins ranging from 54 to 86%. Protein motifs with established functions
found in CRTs of other species were conserved. CRT was found in all of the
cells in low amounts, whereas three distinct floral tissues showed abundant
expression: secreting nectaries, ovules early in development, and a set of
subepidermal cells near the abaxial surface of the anther. Localization in
the developing endosperm, which is characterized by high protein synthesis
rates, can be reconciled with a specific chaperone function. Equally,
nectar production and secretion, a developmental stage marked by abundant
ER, may require abundant CRT to accommodate the traffic of secretory
proteins through the ER. Localization of CRT in the anthers, which are
degenerating at the time of maximum expression of CRT, cannot easily be
reconciled with a chaperone function but may indicate a role for CRT in
anther maturation or dehiscence.
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