Highlight | Molecular mechanisms
After having identified the protein ceQORH as a novel protein of the chloroplast envelope, researchers at the Cell & Plant Physiology laboratory, in collaboration with a team of UJF Grenoble show that this protein is imported into plastids by an independent machinery the classical pathway OCD.
Like mitochondria, chloroplasts only code for a tiny fraction of their proteins (less than a hundred). Most of these proteins, which number several thousand, are actually coded for by the nuclear genome, and then transferred into the cytosol in the form of precursors before being imported into the organelle. The chloroplast's limiting double membrane (or envelope) contains a system (TOC and TIC) that catalyzes the import of these precursors from the cell's cytosol to the chloroplast's various sub-compartments. Except for proteins located at the surface of the chloroplasts, these precursors all display an N-terminal transit sequence that is cleaved off once the protein has been imported into the organelle.
This transit sequence plays two separate roles:
1) it ensures the precursor is recognised by the import receptors (TOC) located on the surface of the organelles, and
2) it guarantees that the protein is maintained in a destructured, and therefore non-functional, state outside the organelle.
The Exploring the Dynamics of Proteomes team and the Plant Cell Physiology laboratory have been jointly running a proteomics programme focused on the study of the chloroplast envelope, and during which researchers identified several, previously unknown, envelope components [1, 2]. Among these components, the ceQORH protein has the unique feature of containing a central transit sequence that is not cleaved off once the protein has been imported into the chloroplasts [3, 4].
Working in tandem with the iRTSV Imaging Platform and the Plastids and Cell Differentiation Laboratory at the Joseph Fourier University, researchers at the Plant Cell Physiology laboratory have now shown  that the ceQORH protein is imported into plastids via a system that is fully independent of the usual TOC pathway. This research, which uses in vitro and in vivo data, also shows that the protein is not systematically located in chloroplasts in planta.
These findings suggest that evolution may have driven the appearance of proteins capable of being targeted to organelles while lacking a cleavable targeting sequence in order to enable protein folding and function despite an alternative subcellular location.
 Ferro M, Salvi D, Riviere-Rolland H, Vermat T, Seigneurin-Berny D, Grunwald D, Garin J, Joyard J and Rolland N
Integral membrane proteins of the chloroplast envelope: Identification and subcellular localization of new transporters.
Proceedings of the National Academy of Sciences USA, 2002
 Ferro M, Salvi D, Brugière S, Miras S, Kowalski S, Louwagie M, Garin J, Joyard J and Rolland N
Proteomics of the chloroplast envelope membranes from Arabidopsis thaliana.
Molecular and Cellular Proteomics, 2003
 Miras S, Salvi D, Ferro M, Grunwald D, Garin J, Joyard J and Rolland N
Non-canonical transit peptide for import into the chloroplast.
Journal of Biological Chemistry, 2002
 Miras S, Salvi D, Rolland N, Joyard J, Ferro M, Garin J and Grunwald D
Peptide d’adressage plastidial
Brevet FR0207729. Numéro de publication internationale WO2004/001050 A1
 Miras S, Salvi D, Piette L, Seigneurin-Berny D, Grunwald D, Reinbothe C, Joyard J, Reinbothe S and Rolland N
Toc159- and Toc75-independent import of a transit sequence-less precursor into the inner envelope of chloroplasts.
Journal of Biological Chemistry, 2007
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