First published online October 29, 2004; 10.1104/pp.104.052928
Plant Physiology 136:3605-3615 (2004)
© 2004 American Society of Plant Biologists
CELL BIOLOGY AND SIGNAL TRANSDUCTION
A Stromal Hsp100 Protein Is Required for Normal Chloroplast Development and Function in Arabidopsis1,[w]
Diane Constan,
John E. Froehlich,
Sowkya Rangarajan and
Kenneth Keegstra*
Department of Energy Plant Research Laboratory (D.C., J.E.F., S.R., K.K.), Department of Plant Biology (K.K.), and Department of Biochemistry and Molecular Biology (K.K.), Michigan State University, East Lansing, Michigan 48824
Molecular chaperones are required for the translocation of many proteins across organellar membranes, presumably by providing energy in the form of ATP hydrolysis for protein movement. In the chloroplast protein import system, a heat shock protein 100 (Hsp100), known as Hsp93, is hypothesized to be the chaperone providing energy for precursor translocation, although there is little direct evidence for this hypothesis. To learn more about the possible function of Hsp93 during protein import into chloroplasts, we isolated knockout mutant lines that contain T-DNA disruptions in either atHSP93-V or atHSP93-III, which encode the two Arabidopsis (Arabidopsis thaliana) homologs of Hsp93. atHsp93-V mutant plants are much smaller and paler than wild-type plants. In addition, mutant chloroplasts contain less thylakoid membrane when compared to the wild type. Plastid protein composition, however, seems to be largely unaffected in atHsp93-V knockout plants. Chloroplasts isolated from the atHsp93-V knockout mutant line are still able to import a variety of precursor proteins, but the rate of import of some of these precursors is significantly reduced. These results indicate that atHsp93-V has an important, but not essential, role in the biogenesis of Arabidopsis chloroplasts. In contrast, knockout mutant plants for atHsp93-III, the second Arabidopsis Hsp93 homolog, had a visible phenotype identical to the wild type, suggesting that atHsp93-III may not play as important a role as atHsp93-V in chloroplast development and/or function.
1 This work was supported in part by the Energy Biosciences Program at the Department of Energy (to K.K.), by the Cell Biology Program at the National Science Foundation (to K.K. and J.F.), and by the Graduate Fellowship Program at the National Science Foundation (to D.C.).
[w] The online version of this article contains Web-only data.
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.052928.
* Corresponding author; e-mail keegstra{at}msu.edu; fax 517–353–9168.
Received September 3, 2004;
returned for revision September 22, 2004;
accepted September 22, 2004.
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