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

• Published in: Molecular plant 2009-11, Vol.2 (6), p.1325-1335
• Main Authors: Armbruster, Ute, Hertle, Alexander, Makarenko, Elina, Zühlke, Jessica, Pribil, Mathias, Dietzmann, Angela, Schliebner, Ivo, Aseeva, Elena, Fenino, Elena, Scharfenberg, Michael, Voigt, Christian, Leister, Dario
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Chloroplast biology; Chloroplasts - metabolism; Databases, Protein; DNA, Complementary - genetics; Luminescent Proteins - metabolism; mitochondria; organelle biogenesis/function; Organelles - metabolism; Pisum sativum - metabolism; Plant Leaves - metabolism; Plant Proteins - genetics; Plant Proteins - metabolism; protein targeting; Proteome - metabolism; Red Fluorescent Protein; RNA, Messenger - genetics; Seedlings - metabolism; Transcription, Genetic; 乙醇酸氧化酶; 叶绿体; 成分; 红色荧光蛋白; 蛋白质组; 裂解; 谷胱甘肽S; 转运
• ISSN: 1674-2052; 1752-9867
• DOI: 10.1093/mp/ssp082

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.