Consequences of impaired 1-MDa TIC complex assembly for the abundance and composition of chloroplast high-molecular mass protein complexes

Chloroplast import motor subunits FtsHi1 and FtsHi2 are located on opposite sides of the inner envelope membrane

Protein import into chloroplasts is powered by ATP hydrolysis in the stroma. Establishing the identity and functional mechanism of the stromal ATPase motor that drives import is critical for understanding chloroplast biogenesis. Recently, a complex consisting of Ycf2, FtsHi1, FtsHi2, FtsHi4, FtsHi5, FtsH12, and malate dehydrogenase was shown to be important for chloroplast protein import, and it has been proposed to act as the motor driving protein translocation across the chloroplast envelope into the stroma. To gain further mechanistic understanding of how the motor functions, we performed membrane association and topology analyses on two of its subunits, FtsHi1 and FtsHi2. We isolated cDNA clones encoding FtsHi1 and FtsHi2 preproteins to perform in vitro import experiments in order to determine the exact size of each mature protein. We also generated antibodies against the C-termini of the proteins, i.e., where their ATPase domains reside. Protease treatments and alkaline and high-salt extractions of chloroplasts with imported and endogenous proteins revealed that FtsHi1 is an integral membrane protein with its C-terminal portion located in the intermembrane space of the envelope, not the stroma, whereas FtsHi2 is a soluble protein in the stroma. We further complemented an FtsHi1-knockout mutant with a C-terminally tagged FtsHi1 and obtained identical results for topological analyses. Our data indicate that the model of a single membrane-anchored pulling motor at the stromal side of the inner membrane needs to be revised and suggest that the Ycf2-FtsHi complex may have additional functions.

Mutations in the chloroplast inner envelope protein TIC100 impair and repair chloroplast protein import and impact retrograde signaling

Chloroplast biogenesis requires synthesis of proteins in the nucleocytoplasm and the chloroplast itself. Nucleus-encoded chloroplast proteins are imported via multiprotein translocons in the organelle's envelope membranes. Controversy exists around whether a 1-MDa complex comprising TIC20, TIC100, and other proteins constitutes the inner membrane TIC translocon. The Arabidopsis thaliana cue8 virescent mutant is broadly defective in plastid development. We identify CUE8 as TIC100. The tic100cue8 mutant accumulates reduced levels of 1-MDa complex components and exhibits reduced import of two nucleus-encoded chloroplast proteins of different import profiles. A search for suppressors of tic100cue8 identified a second mutation within the same gene, tic100soh1, which rescues the visible, 1 MDa complex-subunit abundance, and chloroplast protein import phenotypes. tic100soh1 retains but rapidly exits virescence and rescues the synthetic lethality of tic100cue8 when retrograde signaling is impaired by a mutation in the GENOMES UNCOUPLED 1 gene. Alongside the strong virescence, changes in RNA editing and the presence of unimported precursor proteins show that a strong signaling response is triggered when TIC100 function is altered. Our results are consistent with a role for TIC100, and by extension the 1-MDa complex, in the chloroplast import of photosynthetic and nonphotosynthetic proteins, a process which initiates retrograde signaling.

The Plastid-Localized AtFtsHi3 Pseudo-Protease of Arabidopsis thaliana Has an Impact on Plant Growth and Drought Tolerance

While drought severely affects plant growth and crop production, the molecular mechanisms of the drought response of plants remain unclear. In this study, we demonstrated for the first time the effect of the pseudo-protease AtFtsHi3 of Arabidopsis thaliana on overall plant growth and in drought tolerance. An AtFTSHi3 knock-down mutant [ftshi3-1(kd)] displayed a pale-green phenotype with lower photosynthetic efficiency and Darwinian fitness compared to wild type (Wt). An observed delay in seed germination of ftshi3-1(kd) was attributed to overaccumulation of abscisic acid (ABA); ftshi3-1(kd) seedlings showed partial sensitivity to exogenous ABA. Being exposed to similar severity of soil drying, ftshi3-1(kd) was drought-tolerant up to 20 days after the last irrigation, while wild type plants wilted after 12 days. Leaves of ftshi3-1(kd) contained reduced stomata size, density, and a smaller stomatic aperture. During drought stress, ftshi3-1(kd) showed lowered stomatal conductance, increased intrinsic water-use efficiency (WUEi), and slower stress acclimation. Expression levels of ABA-responsive genes were higher in leaves of ftshi3-1(kd) than Wt; DREB1A, but not DREB2A, was significantly upregulated during drought. However, although ftshi3-1(kd) displayed a drought-tolerant phenotype in aboveground tissue, the root-associated bacterial community responded to drought.

The FtsHi Enzymes of Arabidopsis thaliana: Pseudo-Proteases with an Important Function

FtsH metalloproteases found in eubacteria, animals, and plants are well-known for their vital role in the maintenance and proteolysis of membrane proteins. Their location is restricted to organelles of endosymbiotic origin, the chloroplasts, and mitochondria. In the model organism Arabidopsis thaliana, there are 17 membrane-bound FtsH proteases containing an AAA+ (ATPase associated with various cellular activities) and a Zn2+ metalloprotease domain. However, in five of those, the zinc-binding motif HEXXH is either mutated (FtsHi1, 2, 4, 5) or completely missing (FtsHi3), rendering these enzymes presumably inactive in proteolysis. Still, homozygous null mutants of the pseudo-proteases FtsHi1, 2, 4, 5 are embryo-lethal. Homozygous ftshi3 or a weak point mutant in FTSHi1 are affected in overall plant growth and development. This review will focus on the findings concerning the FtsHi pseudo-proteases and their involvement in protein import, leading to consequences in embryogenesis, seed growth, chloroplast, and leaf development and oxidative stress management.

Abundance of metalloprotease FtsH12 modulates chloroplast development in Arabidopsis thaliana

The ATP-dependent metalloprotease FtsH12 (filamentation temperature sensitive protein H 12) has been suggested to participate in a heteromeric motor complex, driving protein translocation into the chloroplast. FtsH12 was immuno-detected in proplastids, seedlings, leaves, and roots. Expression of Myc-tagged FtsH12 under its native promotor allowed identification of FtsHi1, 2, 4, and 5, and plastidic NAD-malate dehydrogenase, five of the six interaction partners in the suggested import motor complex. Arabidopsis thaliana mutant seedlings with reduced FTSH12 abundance exhibited pale cotyledons and small, deformed chloroplasts with altered thylakoid structure. Mature plants retained these chloroplast defects, resulting in slightly variegated leaves and lower chlorophyll content. Label-free proteomics revealed strong changes in the proteome composition of FTSH12 knock-down seedlings, reflecting impaired plastid development. The composition of the translocon on the inner chloroplast membrane (TIC) protein import complex was altered, with coordinated reduction of the FtsH12-FtsHi complex subunits and accumulation of the 1 MDa TIC complex subunits TIC56, TIC214 and TIC22-III. FTSH12 overexpressor lines showed no obvious phenotype, but still displayed distinct differences in their proteome. N-terminome analyses further demonstrated normal proteolytic maturation of plastid-imported proteins irrespective of FTSH12 abundance. Together, our data suggest that FtsH12 has highest impact during seedling development; its abundance alters the plastid import machinery and impairs chloroplast development.

Chromoplast differentiation in bell pepper (Capsicum annuum) fruits

We report here a detailed analysis of the proteome adjustments that accompany chromoplast differentiation from chloroplasts during bell pepper (Capsicum annuum) fruit ripening. While the two photosystems are disassembled and their constituents degraded, the cytochrome b₆ f complex, the ATPase complex, and Calvin cycle enzymes are maintained at high levels up to fully mature chromoplasts. This is also true for ferredoxin (Fd) and Fd-dependent NADP reductase, suggesting that ferredoxin retains a central role in the chromoplasts' redox metabolism. There is a significant increase in the amount of enzymes of the typical metabolism of heterotrophic plastids, such as the oxidative pentose phosphate pathway (OPPP) and amino acid and fatty acid biosynthesis. Enzymes of chlorophyll catabolism and carotenoid biosynthesis increase in abundance, supporting the pigment reorganization that goes together with chromoplast differentiation. The majority of plastid encoded proteins decline but constituents of the plastid ribosome and AccD increase in abundance. Furthermore, the amount of plastid terminal oxidase (PTOX) remains unchanged despite a significant increase in phytoene desaturase (PDS) levels, suggesting that the electrons from phytoene desaturation are consumed by another oxidase. This may be a particularity of non-climacteric fruits such as bell pepper that lack a respiratory burst at the onset of fruit ripening.

Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system

Chloroplasts are photosynthetic plant organelles descended from a bacterial ancestor. The vast majority of chloroplast proteins are synthesized in the cytosol and then imported into the chloroplast post-translationally. Translocation complexes exist in the organelle's outer and inner envelope membranes (termed TOC and TIC, respectively) to facilitate protein import. These systems recognize chloroplast precursor proteins and mediate their import in an energy-dependent manner. However, many unanswered questions remain regarding mechanistic details of the import process and the participation and functions of individual components; for example, the cytosolic events that mediate protein delivery to chloroplasts, the composition of the TIC apparatus, and the nature of the protein import motor all require resolution. The flux of proteins through TOC and TIC varies greatly throughout development and in response to specific environmental cues. The import process is, therefore, tightly regulated, and it has emerged that the ubiquitin-proteasome system (UPS) plays a key role in this regard, acting at several different steps in the process. The UPS is involved in: the selective degradation of transcription factors that co-ordinate the expression of chloroplast precursor proteins; the removal of unimported chloroplast precursor proteins in the cytosol; the inhibition of chloroplast biogenesis pre-germination; and the reconfiguration of the TOC apparatus in response to developmental and environmental signals in a process termed chloroplast-associated protein degradation. In this review, we highlight recent advances in our understanding of protein import into chloroplasts and how this process is regulated by the UPS.

Proteomic dissection of the chloroplast: Moving beyond photosynthesis

Chloroplast, the photosynthetic machinery, converts photoenergy to ATP and NADPH, which powers the production of carbohydrates from atmospheric CO₂ and H₂O. It also serves as a major production site of multivariate pro-defense molecules, and coordinate with other organelles for cell defense. Chloroplast harbors 30-50% of total cellular proteins, out of which 80% are membrane residents and are difficult to solubilize. While proteome profiling has illuminated vast areas of biological protein space, a great deal of effort must be invested to understand the proteomic landscape of the chloroplast, which plays central role in photosynthesis, energy metabolism and stress-adaptation. Therefore, characterization of chloroplast proteome would not only provide the foundation for future investigation of expression and function of chloroplast proteins, but would open up new avenues for modulation of plant productivity through synchronizing chloroplastic key components. In this review, we summarize the progress that has been made to build new understanding of the chloroplast proteome and implications of chloroplast dynamicsing generate metabolic energy and modulating stress adaptation.