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An alternative pathway for protein import into chloroplasts

​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.

Published on 3 December 2007

​​​​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 [5] 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.

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