The Arabidopsis thaliana chloroplast proteome reveals pathway abundance and novel protein functions.
BACKGROUND
Chloroplasts are plant cell organelles of cyanobacterial origin. They perform essential metabolic and biosynthetic functions of global significance, including photosynthesis and amino acid biosynthesis. Most of the proteins that constitute the functional chloroplast are encoded in the nuclear genome and imported into the chloroplast after translation in the cytosol. Since protein targeting is difficult to predict, many nuclear-encoded plastid proteins are still to be discovered.


RESULTS
By tandem mass spectrometry, we identified 690 different proteins from purified Arabidopsis chloroplasts. Most proteins could be assigned to known protein complexes and metabolic pathways, but more than 30% of the proteins have unknown functions, and many are not predicted to localize to the chloroplast. Novel structure and function prediction methods provided more informative annotations for proteins of unknown functions. While near-complete protein coverage was accomplished for key chloroplast pathways such as carbon fixation and photosynthesis, fewer proteins were identified from pathways that are downregulated in the light. Parallel RNA profiling revealed a pathway-dependent correlation between transcript and relative protein abundance, suggesting gene regulation at different levels.


CONCLUSIONS
The chloroplast proteome contains many proteins that are of unknown function and not predicted to localize to the chloroplast. Expression of nuclear-encoded chloroplast genes is regulated at multiple levels in a pathway-dependent context. The combined shotgun proteomics and RNA profiling approach is of high potential value to predict metabolic pathway prevalence and to define regulatory levels of gene expression on a pathway scale.
Kleffmann
Torsten
T
Institute of Plant Sciences and Functional Genomics Center Zurich, Swiss Federal Institute of Technology, ETH Zentrum, LFW E51.1, Universitätstrasse 2, CH-8092 Zurich, Switzerland.
Russenberger
Doris
D
von Zychlinski
Anne
A
Christopher
Wayne
W
Sjölander
Kimmen
K
Gruissem
Wilhelm
W
Baginsky
Sacha
S
D017360:Arabidopsis / Q000737:chemistry* / Q000235:genetics; D002736:Chloroplasts / Q000737:chemistry*; D018506:Gene Expression Regulation, Plant; D013058:Mass Spectrometry; D020411:Oligonucleotide Array Sequence Analysis; D010940:Plant Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D020543:Proteome; D012333:RNA, Messenger / Q000032:analysis*
D010940:Plant Proteins; D020543:Proteome; D012333:RNA, Messenger