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Molecular Plant Advance Access originally published online on September 30, 2009
Molecular Plant 2009 2(6):1325-1335; doi:10.1093/mp/ssp082
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© The Author 2009. Published by the Molecular Plant Shanghai Editorial Office in association with Oxford University Press on behalf of CSPP and IPPE, SIBS, CAS.

Chloroplast Proteins without Cleavable Transit Peptides: Rare Exceptions or a Major Constituent of the Chloroplast Proteome?

Ute Armbruster2, Alexander Hertle2, Elina Makarenko, Jessica Zühlke, Mathias Pribil, Angela Dietzmann, Ivo Schliebner, Elena Aseeva, Elena Fenino, Michael Scharfenberg, Christian Voigt and Dario Leister1

Lehrstuhl für Botanik, Department Biologie I, Ludwig-Maximilians-Universität München, Menzinger Str. 67, D-80638 München, Germany

1 To whom correspondence should be addressed. E-mail leister{at}lrz.uni-muenchen.de, fax +49-89-2180-74599, tel. +49-89-2180-74550.

Most chloroplast proteins (cp proteins) are nucleus-encoded, synthesized on cytosolic ribosomes as precursor proteins containing a presequence (cTP), and post-translationally imported via the Tic/Toc complex into the organelle, where the cTP is removed. Only a few unambiguous instances of cp proteins that do not require cTPs (non-canonical cp proteins) have been reported so far. However, the survey of data from large-scale proteomic studies presented here suggests that the fraction of such proteins in the total cp proteome might be as large as ~30%. To explore this discrepancy, we chose a representative set of 28 putative non-canonical cp proteins, and used in vitro import and Red Fluorescent Protein (RFP)-fusion assays to determine their sub-cellular destinations. Four proteins, including embryo defective 1211, glycolate oxidase 2, protein disulfide isomerase-like protein (PDII), and a putative glutathione S-transferase, could be unambiguously assigned to the chloroplast. Several others (‘potential cp proteins’) were found to be imported into chloroplasts in vitro, but failed to localize to the organelle when RFP was fused to their C-terminal ends. Extrapolations suggest that the fraction of cp proteins that enter the inner compartments of the organelle, although they lack a cTP, might be as large as 11.4% of the total cp proteome. Our data also support the idea that cytosolic proteins that associate with the cp outer membrane might account for false positive cp proteins obtained in earlier studies.

Key Words: Chloroplast biology • mitochondria • organelle biogenesis/function • protein targeting

2 These authors contributed equally to this work.


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