Sorting Signals, N-Terminal Modifications and Abundance of the Chloroplast Proteome

Characterization of the chloroplast proteome is needed to understand the essential contribution of the chloroplast to plant growth and development. Here we present a large scale analysis by nanoLC-Q-TOF and nanoLC-LTQ-Orbitrap mass spectrometry (MS) of ten independent chloroplast preparations from Arabidopsis thaliana which unambiguously identified 1325 proteins. Novel proteins include various kinases and putative nucleotide binding proteins. Based on repeated and independent MS based protein identifications requiring multiple matched peptide sequences, as well as literature, 916 nuclear-encoded proteins were assigned with high confidence to the plastid, of which 86% had a predicted chloroplast transit peptide (cTP). The protein abundance of soluble stromal proteins was calculated from normalized spectral counts from LTQ-Obitrap analysis and was found to cover four orders of magnitude. Comparison to gel-based quantification demonstrates that ‘spectral counting’ can provide large scale protein quantification for Arabidopsis. This quantitative information was used to determine possible biases for protein targeting prediction by TargetP and also to understand the significance of protein contaminants. The abundance data for 550 stromal proteins was used to understand abundance of metabolic pathways and chloroplast processes. We highlight the abundance of 48 stromal proteins involved in post-translational proteome homeostasis (including aminopeptidases, proteases, deformylases, chaperones, protein sorting components) and discuss the biological implications. N-terminal modifications were identified for a subset of nuclear- and chloroplast-encoded proteins and a novel N-terminal acetylation motif was discovered. Analysis of cTPs and their cleavage sites of Arabidopsis chloroplast proteins, as well as their predicted rice homologues, identified new species-dependent features, which will facilitate improved subcellular localization prediction. No evidence was found for suggested targeting via the secretory system. This study provides the most comprehensive chloroplast proteome analysis to date and an expanded Plant Proteome Database (PPDB) in which all MS data are projected on identified gene models

FUNCTIONAL CHARACTERIZATION OF THE CLP PROTEASE SYSTEM IN ARABIDOPSIS CHLOROPLASTS THROUGH REVERSE GENETICS AND PROTEOMICS

Proteases play an important role in regulating protein maturation, activity and life-time. The Clp protease system in Arabidopsis thaliana plastids accumulates at relatively high levels and consists of a proteolytic core and associated chaperones. The core is an assembly of five different catalytic ClpP subunits, four non-catalytic ClpR subunits, and two ClpS proteins with unknown function. ClpR,S are unique to photosynthetic organisms. Three ATP-dependent chaperones, ClpC1,C2,D, are expected to deliver substrates to the ClpPRS core. Control of Clp activity is not understood and Clp substrates are unknown. Arabidopsis T-DNA insertion Clp mutants were isolated and genotyped. Null mutants for ClpP4,P5 are embryo-lethal under both auto- and heterotrophic conditions. Mutants of ClpP3,R4 did not form seedlings under autotrophic conditions but developed albino seedlings under heterotrophic conditions, displaying limited greening under low light. Null mutants for the chaperones ClpC1 and ClpD have pale-green and wild-type phenotypes, respectively. ClpP,R core subunits are likely essential, while there are redundancies in the ClpC,D subfamily. Two mutants with partial loss of gene expression for ClpR1 and ClpR2 (clpr2-1) exhibited pale-green phenotypes, with clpr2-1 having a stronger phenotype. ClpR2 protein accumulation in clpr2-1 chloroplasts was 5-fold reduced, while the ClpPRS core was 3-fold downregulated, suggesting an induction of core composition heterogeneity. Stromal chaperones were upregulated several fold and ClpC was recruited to the thylakoid membrane. Thylakoid protein homeostasis was unbalanced as deduced from increased accumulation of thylakoid proteases, plastoglobules, protein precursors and degradation products. Clpr2-1 chloroplasts were smaller, with 30% less thylakoids than wild-type. Clearly, ClpR2 is not a redundant member of the Clp family and reduced CLPR2 gene expression has adverse effects on plastid and plant development. A comparative proteome analysis using differential stable isotope labeling of clpr2-1 and wild-type stroma identified 298 proteins, and 113 were quantified. The Calvin cycle was down-regulated, explaining the slower development of clpr2-1. The most striking response was the high accumulation of the chloroplast protein translation machinery and chaperones. This suggests that the ClpPRS core complex may be involved in regulation of plastid gene expression, providing a first understanding of the functional role of the Clp family in plastids.This work was supported by the grants from the National Sciece Foundation (NSF, #MCB 0343444) and the US Department of Energy (DOE, DE-FG02-04ER15560) to Klaas Jan van Wij

Chloroplasts: New Proteins, New Functions, and a Plastid Proteome Database[W]

An extensive analysis of the Arabidopsis thaliana peripheral and integral thylakoid membrane proteome was performed by sequential extractions with salt, detergent, and organic solvents, followed by multidimensional protein separation steps (reverse-phase HPLC and one- and two-dimensional electrophoresis gels), different enzymatic and nonenzymatic protein cleavage techniques, mass spectrometry, and bioinformatics. Altogether, 154 proteins were identified, of which 76 (49%) were α-helical integral membrane proteins. Twenty-seven new proteins without known function but with predicted chloroplast transit peptides were identified, of which 17 (63%) are integral membrane proteins. These new proteins, likely important in thylakoid biogenesis, include two rubredoxins, a potential metallochaperone, and a new DnaJ-like protein. The data were integrated with our analysis of the lumenal-enriched proteome. We identified 83 out of 100 known proteins of the thylakoid localized photosynthetic apparatus, including several new paralogues and some 20 proteins involved in protein insertion, assembly, folding, or proteolysis. An additional 16 proteins are involved in translation, demonstrating that the thylakoid membrane surface is an important site for protein synthesis. The high coverage of the photosynthetic apparatus and the identification of known hydrophobic proteins with low expression levels, such as cpSecE, Ohp1, and Ohp2, indicate an excellent dynamic resolution of the analysis. The sequential extraction process proved very helpful to validate transmembrane prediction. Our data also were cross-correlated to chloroplast subproteome analyses by other laboratories. All data are deposited in a new curated plastid proteome database (PPDB) with multiple search functions (http://cbsusrv01.tc.cornell.edu/users/ppdb/). This PPDB will serve as an expandable resource for the plant community

Subunits of the Plastid ClpPR Protease Complex Have Differential Contributions to Embryogenesis, Plastid Biogenesis, and Plant Development in Arabidopsis[C][W]

The plastid ClpPR protease complex in Arabidopsis thaliana consists of five catalytic ClpP and four noncatalytic ClpR subunits. An extensive analysis of the CLPR family and CLPP5 is presented to address this complexity. Null alleles for CLPR2 and CLPR4 showed delayed embryogenesis and albino embryos, with seedling development blocked in the cotyledon stage; this developmental block was overcome under heterotrophic conditions, and seedlings developed into small albino to virescent seedlings. By contrast, null alleles for CLPP5 were embryo lethal. Thus, the ClpPR proteins make different functional contributions. To further test for redundancies and functional differences between the ClpR proteins, we overexpressed full-length cDNAs for ClpR1, R2, R3, R4 in clpr1, clpr2 and clpr4 mutants. This showed that overexpression of ClpR3 can complement for the loss of ClpR1, but not for the loss of ClpR2 or ClpR4, indicating that ClpR3 can functionally substitute ClpR1. By contrast, ClpR1, R2 and R4 could not substitute each other. Double mutants of weak CLPR1 and 2 alleles were seedling lethal, showing that a minimum concentration of different ClpR proteins is essential for Clp function. Microscopy and large-scale comparative leaf proteome analyses of a CLPR4 null allele demonstrate a central role of Clp protease in chloroplast biogenesis and protein homeostasis; substrates are discussed. Lack of transcriptional and translational feedback regulation within the CLPPR gene family indicates that regulation of Clp activity occurs through Clp complex assembly and substrate delivery

Central functions of the lumenal and peripheral thylakoid proteome of Arabidopsis determined by experimentation and genome-wide prediction

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Central Functions of the Lumenal and Peripheral Thylakoid Proteome of Arabidopsis Determined by Experimentation and Genome-Wide Prediction

Experimental proteome analysis was combined with a genome-wide prediction screen to characterize the protein content of the thylakoid lumen of Arabidopsis chloroplasts. Soluble thylakoid proteins were separated by two-dimensional electrophoresis and identified by mass spectrometry. The identities of 81 proteins were established, and N termini were sequenced to validate localization prediction. Gene annotation of the identified proteins was corrected by experimental data, and an interesting case of alternative splicing was discovered. Expression of a surprising number of paralogs was detected. Expression of five isomerases of different classes suggests strong (un)folding activity in the thylakoid lumen. These isomerases possibly are connected to a network of peripheral and lumenal proteins involved in antioxidative response, including peroxiredoxins, m-type thioredoxins, and a lumenal ascorbate peroxidase. Characteristics of the experimentally identified lumenal proteins and their orthologs were used for a genome-wide prediction of the lumenal proteome. Lumenal proteins with a typical twin-arginine translocation motif were predicted with good accuracy and sensitivity and included additional isomerases and proteases. Thus, prime functions of the lumenal proteome include assistance in the folding and proteolysis of thylakoid proteins as well as protection against oxidative stress. Many of the predicted lumenal proteins must be present at concentrations at least 10,000-fold lower than proteins of the photosynthetic apparatus