title: Quantitative analysis of the chloroplast molecular chaperone ClpC/Hsp93 in Arabidopsis reveals new insights into its localization, interaction with the Clp proteolytic core, and functional importance.
abstract: The molecular chaperone ClpC/Hsp93 is essential for chloroplast function in vascular plants. ClpC has long been held to act both independently and as the regulatory partner for the ATP-dependent Clp protease, and yet this and many other important characteristics remain unclear. In this study, we reveal that of the two near-identical ClpC paralogs (ClpC1 and ClpC2) in Arabidopsis chloroplasts, along with the closely related ClpD, it is ClpC1 that is the most abundant throughout leaf maturation. An unexpectedly large proportion of both chloroplast ClpC proteins (30% of total ClpC content) associates to envelope membranes in addition to their stromal localization. The Clp proteolytic core is also bound to envelope membranes, the amount of which is sufficient to bind to all the similarly localized ClpC. The role of such an envelope membrane Clp protease remains unclear although it appears uninvolved in preprotein processing or Tic subunit protein turnover. Within the stroma, the amount of oligomeric ClpC protein is less than that of the Clp proteolytic core, suggesting most if not all stromal ClpC functions as part of the Clp protease; a proposal supported by the near abolition of Clp degradation activity in the clpC1 knock-out mutant. Overall, ClpC appears to function primarily within the Clp protease, as the principle stromal protease responsible for maintaining homeostasis, and also on the envelope membrane where it possibly confers a novel protein quality control mechanism for chloroplast preprotein import.
authors.affiliation: From the Department of Biological and Environmental Sciences, Gothenburg University, Box 461, 405 30 Gothenburg, Sweden and.
authors.forename: Lars L E
authors.initials: LLE
authors.lastname: Sjögren
authors.affiliation: From the Department of Biological and Environmental Sciences, Gothenburg University, Box 461, 405 30 Gothenburg, Sweden and.
authors.forename: Noriaki
authors.initials: N
authors.lastname: Tanabe
authors.affiliation: From the Department of Biological and Environmental Sciences, Gothenburg University, Box 461, 405 30 Gothenburg, Sweden and.
authors.forename: Panagiotis
authors.initials: P
authors.lastname: Lymperopoulos
authors.affiliation: From the Department of Biological and Environmental Sciences, Gothenburg University, Box 461, 405 30 Gothenburg, Sweden and.
authors.forename: Nadir Z
authors.initials: NZ
authors.lastname: Khan
authors.affiliation: the Department of Genetics, Development and Cell Biology, Iowa State University, Ames, Iowa 50011.
authors.forename: Steven R
authors.initials: SR
authors.lastname: Rodermel
authors.affiliation: From the Department of Biological and Environmental Sciences, Gothenburg University, Box 461, 405 30 Gothenburg, Sweden and.
authors.forename: Henrik
authors.initials: H
authors.lastname: Aronsson
authors.affiliation: From the Department of Biological and Environmental Sciences, Gothenburg University, Box 461, 405 30 Gothenburg, Sweden and. Electronic address: adrian.clarke@bioenv.gu.se.
authors.forename: Adrian K
authors.initials: AK
authors.lastname: Clarke
mesh_terms: D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D060365:Chloroplast Proteins / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts / Q000201:enzymology* / Q000235:genetics; D055786:Gene Knockout Techniques; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D007425:Intracellular Membranes / Q000201:enzymology*; D010447:Peptide Hydrolases / Q000235:genetics / Q000378:metabolism*; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D059748:Proteolysis
chemical_list: D029681:Arabidopsis Proteins; D060365:Chloroplast Proteins; D006360:Heat-Shock Proteins; D010447:Peptide Hydrolases; D011498:Protein Precursors; C574007:AT5G50920 protein, Arabidopsis
keywords: Chloroplast; Molecular Chaperone; Protease; Protein Degradation; Protein Translocation