The Plant Cell, Vol. 13, 2823-2839,
December 2001, Copyright © 2001,
American Society of Plant Biologists
The Chloroplastic GrpE Homolog of Chlamydomonas
Two Isoforms Generated by Differential Splicing
Michael Schroda1,a,
Olivier Vallona,
Julian P. Whiteleggeb,
Christoph F. Beckc and
Francis-André Wollmana
a Institut de Biologie Physico-Chimique, Unité Propre de Recherche 1261, 13 rue Pierre et Marie Curie, 75005 Paris, France
b Department of Chemistry and Biochemistry, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095
c Institut für Biologie III, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany
1 To whom correspondence should be addressed. E-mail ms{at}bop5.biologie.uni-freiburg.de; fax 49-761-203-2601
In eubacteria and mitochondria, Hsp70 chaperone activity is controlled by the nucleotide exchange factor GrpE. We have identified the chloroplastic GrpE homolog of Chlamydomonas, CGE1, as an 26-kD protein coimmunoprecipitating with the stromal HSP70B protein. When expressed in Escherichia coli, CGE1 can functionally replace GrpE and interacts physically with DnaK. CGE1 is encoded by a single-copy gene that is induced strongly by heat shock and slightly by light. Alternative splicing generates two isoforms that differ only by two residues in the N-terminal part. The larger form is synthesized preferentially during heat shock, whereas the smaller one dominates at lower temperatures. Fractions of both HSP70B and CGE1 associate with chloroplast membranes in an ATP-sensitive manner. By colorless native PAGE and pulse labeling, CGE1 monomers were found to assemble rapidly into dimers and tetramers. In addition, CGE1 was found to form ATP-sensitive complexes with HSP70B of 230 and 120 kD, the latter increasing dramatically after heat shock.
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