Modifications at the A-domain of the chloroplast import receptor Toc159

Understanding protein import in diverse non-green plastids

The spectacular diversity of plastids in non-green organs such as flowers, fruits, roots, tubers, and senescing leaves represents a Universe of metabolic processes in higher plants that remain to be completely characterized. The endosymbiosis of the plastid and the subsequent export of the ancestral cyanobacterial genome to the nuclear genome, and adaptation of the plants to all types of environments has resulted in the emergence of diverse and a highly orchestrated metabolism across the plant kingdom that is entirely reliant on a complex protein import and translocation system. The TOC and TIC translocons, critical for importing nuclear-encoded proteins into the plastid stroma, remain poorly resolved, especially in the case of TIC. From the stroma, three core pathways (cpTat, cpSec, and cpSRP) may localize imported proteins to the thylakoid. Non-canonical routes only utilizing TOC also exist for the insertion of many inner and outer membrane proteins, or in the case of some modified proteins, a vesicular import route. Understanding this complex protein import system is further compounded by the highly heterogeneous nature of transit peptides, and the varying transit peptide specificity of plastids depending on species and the developmental and trophic stage of the plant organs. Computational tools provide an increasingly sophisticated means of predicting protein import into highly diverse non-green plastids across higher plants, which need to be validated using proteomics and metabolic approaches. The myriad plastid functions enable higher plants to interact and respond to all kinds of environments. Unraveling the diversity of non-green plastid functions across the higher plants has the potential to provide knowledge that will help in developing climate resilient crops.

A multifaceted analysis reveals two distinct phases of chloroplast biogenesis during de-etiolation in Arabidopsis

Light triggers chloroplast differentiation whereby the etioplast transforms into a photosynthesizing chloroplast and the thylakoid rapidly emerges. However, the sequence of events during chloroplast differentiation remains poorly understood. Using Serial Block Face Scanning Electron Microscopy (SBF-SEM), we generated a series of chloroplast 3D reconstructions during differentiation, revealing chloroplast number and volume and the extent of envelope and thylakoid membrane surfaces. Furthermore, we used quantitative lipid and whole proteome data to complement the (ultra)structural data, providing a time-resolved, multi-dimensional description of chloroplast differentiation. This showed two distinct phases of chloroplast biogenesis: an initial photosynthesis-enabling 'Structure Establishment Phase' followed by a 'Chloroplast Proliferation Phase' during cell expansion. Moreover, these data detail thylakoid membrane expansion during de-etiolation at the seedling level and the relative contribution and differential regulation of proteins and lipids at each developmental stage. Altogether, we establish a roadmap for chloroplast differentiation, a critical process for plant photoautotrophic growth and survival.

Phosphoproteomic analysis of chromoplasts from sweet orange during fruit ripening

Like other types of plastids, chromoplasts have essential biosynthetic and metabolic activities which may be regulated via post-translational modifications, such as phosphorylation, of their resident proteins. We here report a proteome-wide mapping of in vivo phosphorylation sites in chromoplast-enriched samples prepared from sweet orange [Citrus sinensis (L.) Osbeck] at different ripening stages by titanium dioxide-based affinity chromatography for phosphoprotein enrichment with LC-MS/MS. A total of 109 plastid-localized phosphoprotein candidates were identified that correspond to 179 unique phosphorylation sites in 135 phosphopeptides. On the basis of Motif-X analysis, two distinct types of phosphorylation sites, one as proline-directed phosphorylation motif and the other as casein kinase II motif, can be generalized from these identified phosphopeptides. While most identified phosphoproteins show high homology to those already identified in plastids, approximately 22% of them are novel based on BLAST search using the public databases PhosPhAt and P(3) DB. A close comparative analysis showed that approximately 50% of the phosphoproteins identified in citrus chromoplasts find obvious counterparts in the chloroplast phosphoproteome, suggesting a rather high-level of conservation in basic metabolic activities in these two types of plastids. Not surprisingly, the phosphoproteome of citrus chromoplasts is also characterized by the lack of phosphoproteins involved in photosynthesis and by the presence of more phosphoproteins implicated in stress/redox responses. This study presents the first comprehensive phosphoproteomic analysis of chromoplasts and may help to understand how phosphorylation regulates differentiation of citrus chromoplasts during fruit ripening.

Targeting and assembly of components of the TOC protein import complex at the chloroplast outer envelope membrane

The translocon at the outer envelope membrane of chloroplasts (TOC) initiates the import of thousands of nuclear encoded preproteins required for chloroplast biogenesis and function. The multimeric TOC complex contains two GTP-regulated receptors, Toc34 and Toc159, which recognize the transit peptides of preproteins and initiate protein import through a β-barrel membrane channel, Toc75. Different isoforms of Toc34 and Toc159 assemble with Toc75 to form structurally and functionally diverse translocons, and the composition and levels of TOC translocons is required for the import of specific subsets of coordinately expressed proteins during plant growth and development. Consequently, the proper assembly of the TOC complexes is key to ensuring organelle homeostasis. This review will focus on our current knowledge of the targeting and assembly of TOC components to form functional translocons at the outer membrane. Our analyses reveal that the targeting of TOC components involves elements common to the targeting of other outer membrane proteins, but also include unique features that appear to have evolved to specifically facilitate assembly of the import apparatus.

A transit peptide-like sorting signal at the C terminus directs the Bienertia sinuspersici preprotein receptor Toc159 to the chloroplast outer membrane

Although Toc159 is known to be one of the key GTPase receptors for selective recognition of chloroplast preproteins, the mechanism for its targeting to the chloroplast surface remains unclear. To compare the targeting of these GTPase receptors, we identified two Toc159 isoforms and a Toc34 from Bienertia sinuspersici, a single-cell C₄ species with dimorphic chloroplasts in individual chlorenchyma cells. Fluorescent protein tagging and immunogold studies revealed that the localization patterns of Toc159 were distinctive from those of Toc34, suggesting different targeting pathways. Bioinformatics analyses indicated that the C-terminal tails (CTs) of Toc159 possess physicochemical and structural properties of chloroplast transit peptides (cTPs). These results were further confirmed by fluorescent protein tagging, which showed the targeting of CT fusion proteins to the chloroplast surface. The CT of Bs Toc159 in reverse orientation functioned as a cleavable cTP that guided the fluorescent protein to the stroma. Moreover, a Bs Toc34 mutant protein was retargeted to the chloroplast envelope using the CTs of Toc159 or reverse sequences of other cTPs, suggesting their conserved functions. Together, our data show that the C terminus and the central GTPase domain represent a novel dual domain-mediated sorting mechanism that might account for the partitioning of Toc159 between the cytosol and the chloroplast envelope for preprotein recognition.