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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.
AuthorsLars L E Sjögren, Noriaki Tanabe, Panagiotis Lymperopoulos, Nadir Z Khan, Steven R Rodermel, Henrik Aronsson, Adrian K Clarke
JournalThe Journal of biological chemistry (J Biol Chem) Vol. 289 Issue 16 Pg. 11318-11330 (Apr 18 2014) ISSN: 1083-351X [Electronic] United States
PMID24599948 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
Chemical References
  • Arabidopsis Proteins
  • Chloroplast Proteins
  • Heat-Shock Proteins
  • Protein Precursors
  • chloroplast 93-kD heat shock protein, Arabidopsis
  • Peptide Hydrolases
Topics
  • Arabidopsis (enzymology, genetics)
  • Arabidopsis Proteins (genetics, metabolism)
  • Chloroplast Proteins (genetics, metabolism)
  • Chloroplasts (enzymology, genetics)
  • Gene Knockout Techniques
  • Heat-Shock Proteins (genetics, metabolism)
  • Intracellular Membranes (enzymology)
  • Peptide Hydrolases (genetics, metabolism)
  • Protein Precursors (genetics, metabolism)
  • Proteolysis

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