Arabidopsis FtsZ2-1 and FtsZ2-2 Are Functionally Redundant,But FtsZ-Based Plastid Division Is Not Essential for Chloroplast Partitioning or Plant Growth and Development

FtsZ1 and FtsZ2 are phylogenetically distinct families of FtsZ in plants that co-localize to mid-plastid rings and facilitate division of chloroplasts. In plants, altered levels of either FtsZ1 or FtsZ2 cause dose-dependent defects in chloroplast division; thus, studies on the functional relationship between FtsZgenes require careful manipulation of FtsZ levels in vivo. To define the functional relationship between the two FtsZ2 genes in Arabidopsis thaliana, FtsZ2-1 and FtsZ2-2, we expressed FtsZ2-1 in an ftsZ2-2 null mutant, and vice versa, and determined whether the chloroplast division defects were rescued in plants expressing different total levels of FtsZ2. Full rescue was observed when either the FtsZ2-1 or FtsZ2-2 level approximated total FtsZ2 levels in wild-type （WT）. Additionally, FtsZ2-2 interacts with ARC6, as shown previously for FtsZ2- 1. These data indicate that FtsZ2-1 and FtsZ2-2 are functionally redundant for chloroplast division in Arabidopsis. To rigorously validate the requirement of each FtsZ family for chloroplast division, we replaced FtsZ1 with FtsZ2 in vivo, and vice versa, while maintaining the FtsZ level in the transgenic plants equal to that of the total level in WT. Chloroplast division defects were not rescued, demonstrating conclusively that FtsZ1 and FtsZ2 are non-redundant for maintenance of WT chloroplast numbers. Finally, we generated ftsZtriple null mutants and show that plants completely devoid of FtsZ protein are viable and fertile. As plastids are presumably essential organelles, these findings suggest that an FtsZ-independent mode of plastid partitioning may occur in higher plants.     

Chloroplast division

Chloroplasts are descendants of cyanobacteria and divide by binary fission. Several components of the division apparatus have been identified in the past several years and we are beginning to appreciate the plastid division process at a mechanistic level. In this review, we attempt to summarize the most recent developments in the field and assemble these observations into a working model of plastid division in plants.     

Plastid division is mediated by combinatorial assembly of plastid division proteins

Plastids arise by division from pre-existing organelles, and with the recent characterization of several new components of plastid division our understanding of the division process in higher plants has improved dramatically. However, it is still not known how these different protein components act together during division. Here we analyse protein-protein interactions between all known stromal plastid division proteins. Using a combination of quantitative yeast two-hybrid assays, in planta co-localization studies, fluorescence resonance energy transfer and bimolecular fluorescence complementation assays we show that these proteins do not act in isolation but rather in protein complexes to govern appropriate plastid division. We have previously shown that AtMinD1 forms functional homodimers and we show here that in addition to homodimerization AtMinD1 also interacts with AtMinE1. Furthermore, AtMinE1 has the ability to homodimerize. We also demonstrate that proteins from both FtsZ families (AtFtsZ1-1 and AtFtsZ2-1) not only interact with themselves but also with each other, and we show that these interactions are not dependent on correct Z-ring formation. Further to this we demonstrate that ARC6 specifically interacts with the core domain of AtFtsZ2-1, but not with AtFtsZ1-1, providing in planta evidence for a functional difference between the two FtsZ protein families in plants. Our studies have enabled us to construct a meaningful intraplastidic protein-protein interaction map of all known stromal plastid division proteins in Arabidopsis.     

FtsHi1/ARC1 is an essential gene in Arabidopsis that links chloroplast biogenesis and division

The Arabidopsis arc1 (accumulation and replication of chloroplasts 1) mutant has pale seedlings and smaller, more numerous chloroplasts than the wild type. Previous work has suggested that arc1 affects the timing of chloroplast division but does not function directly in the division process. We isolated ARC1 by map‐based cloning and discovered it encodes FtsHi1 (At4g23940), one of several FtsHi proteins in Arabidopsis. These poorly studied proteins resemble FtsH metalloproteases important for organelle biogenesis and protein quality control but are presumed to be proteolytically inactive. FtsHi1 bears a predicted chloroplast transit peptide and localizes to the chloroplast envelope membrane. Phenotypic studies showed that arc1 (hereafter ftsHi1‐1), which bears a missense mutation, is a weak allele of FtsHi1 that disrupts thylakoid development and reduces de‐etiolation efficiency in seedlings, suggesting that FtsHi1 is important for chloroplast biogenesis. Consistent with this finding, transgenic plants suppressed for accumulation of an FtsHi1 fusion protein were often variegated. A strong T‐DNA insertion allele, ftsHi1‐2, caused embryo‐lethality, indicating that FtsHi1 is an essential gene product. A wild‐type FtsHi1 transgene rescued both the chloroplast division and pale phenotypes of ftsHi1‐1 and the embryo‐lethal phenotype of ftsHi1‐2. FtsHi1 overexpression produced a subtle increase in chloroplast size and decrease in chloroplast number in wild‐type plants while suppression led to increased numbers of small chloroplasts, providing new evidence that FtsHi1 negatively influences chloroplast division. Taken together, our analyses reveal that FtsHi1 functions in an essential, envelope‐associated process that may couple plastid development with division.     

Direct interaction of FtsZ and MreB is required for septum synthesis and cell division in Escherichia coli

How bacteria coordinate cell growth with division is not well understood. Bacterial cell elongation is controlled by actin–MreB while cell division is governed by tubulin–FtsZ. A ring‐like structure containing FtsZ (the Z ring) at mid‐cell attracts other cell division proteins to form the divisome, an essential protein assembly required for septum synthesis and cell separation. The Z ring exists at mid‐cell during a major part of the cell cycle without contracting. Here, we show that MreB and FtsZ of Escherichia coli interact directly and that this interaction is required for Z ring contraction. We further show that the MreB–FtsZ interaction is required for transfer of cell‐wall biosynthetic enzymes from the lateral to the mature divisome, allowing cells to synthesise the septum. Our observations show that bacterial cell division is coupled to cell elongation via a direct and essential interaction between FtsZ and MreB.     

Crystal structure of a conserved domain in the intermembrane space region of the plastid division protein ARC6

The chloroplast division machinery is composed of numerous proteins that assemble as a large complex to divide double‐membraned chloroplasts through binary fission. A key mediator of division‐complex formation is ARC6, a chloroplast inner envelope protein and evolutionary descendant of the cyanobacterial cell division protein Ftn2. ARC6 connects stromal and cytosolic contractile rings across the two membranes through interaction with an outer envelope protein within the intermembrane space (IMS). The ARC6 IMS region bears a structurally uncharacterized domain of unknown function, DUF4101, that is highly conserved among ARC6 and Ftn2 proteins. Here we report the crystal structure of this domain from Arabidopsis thaliana ARC6. The domain forms an α/β barrel open towards the outer envelope membrane but closed towards the inner envelope membrane. These findings provide new clues into how ARC6 and its homologs contribute to chloroplast and cyanobacterial cell division.     

叶绿体分裂蛋白PDV1胞质侧结构域可溶性表达条件的研究及纯化

目的:探索叶绿体分裂蛋白PLASTID DIVISION1(PDV1)胞质侧结构域的高效可溶性表达条件,并得到高纯度目的蛋白。方法:通过改变表达载体种类、基因片段大小、诱导剂浓度、诱导温度的方法,以及运用分子伴侣的协助,实现目的蛋白高效可溶性表达。通过镍柱亲和层析和分子筛层析纯化目的蛋白。结果:(1)带His标签的目的蛋白大部分以包涵体形式存在于沉淀中;(2)截掉疏水区域并与增溶标签GST或NusA融合表达,再通过改变诱导表达条件,可以实现PDV1胞质侧结构域的可溶性表达;(3)比较目的蛋白可溶性表达量,选择高效可溶性表达体系,并在该条件下纯化得到高纯度目的蛋白。结论:PDV1胞质侧结构域的高效可溶性表达及纯化,为进一步研究该蛋白的结构及其在叶绿体分裂过程中的作用奠定了一定基础。     

The topological specificity factor AtMinE1 is essential for correct plastid division site placement in Arabidopsis

In plant cells, plastids divide by binary fission involving a complex pathway of events. Although there are clear similarities between bacterial and plastid division, limited information exists regarding the mechanism of plastid division in higher plants. Here we demonstrate that AtMinE1, an Arabidopsis homologue of the bacterial MinE topological specificity factor, is an essential integral component of the plastid division machinery. In prokaryotes MinE imparts topological specificity during cell division by blocking division apparatus assembly at sites other than midcell. We demonstrate that overexpression of AtMinE1 in E. coli results in loss of topological specificity and minicell formation suggesting evolutionary conservation of MinE mode of action. We further show that AtMinE1 can indeed act as a topological specificity factor during plastid division revealing that AtMinE1 overexpression in Arabidopsis seedlings results in division site misplacement giving rise to multiple constrictions along the length of plastids. In agreement with cell division studies in bacteria, AtMinE1 and AtMinD1 show distinct intraplastidic localisation patterns suggestive of dynamic localisation behaviour. Taken together our findings demonstrate that AtMinE1 is an evolutionary conserved topological specificity factor, most probably acting in concert with AtMinD1, required for correct plastid division in Arabidopsis.     

Plant FtsZ1 and FtsZ2 expressed in a eukaryotic host: GTPase activity and self-assembly

Plants and algae contain the FtsZ1 and FtsZ2 protein families that perform specific, non-redundant functions in plastid division. In vitro studies of chloroplast division have been hampered by the lack of a suitable expression system. Here we report the expression and purification of FtsZ1-1 and FtsZ2-1 from Arabidopsis thaliana using a eukaryotic host. Specific GTPase activities were determined and found to be different for FtsZ1-1 vs. FtsZ2-1. The purified proteins readily assembled into previously unreported assembly products named type-I and -II filaments. In contrast to bacterial FtsZ, the Arabidopsis proteins do not form bundled sheets in the presence of Ca²⁺.     

Effects of mutations in Arabidopsis FtsZ1 on plastid division, FtsZ ring formation and positioning, and FtsZ filament morphology in vivo

In plants, chloroplast division FtsZ proteins have diverged into two families, FtsZ1 and FtsZ2. FtsZ1 is more divergent from its bacterial counterparts and lacks a C-terminal motif conserved in most other FtsZs. To begin investigating FtsZ1 structure-function relationships, we first identified a T-DNA insertion mutation in the single FtsZ1 gene in Arabidopsis thaliana, AtFtsZ1-1. Homozygotes null for FtsZ1, though impaired in chloroplast division, could be isolated and set seed normally, indicating that FtsZ1 is not essential for viability. We then mapped five additional atftsZ1-1 alleles onto an FtsZ1 structural model and characterized chloroplast morphologies, FtsZ protein levels and FtsZ filament morphologies in young and mature leaves of the corresponding mutants. atftsZ1-1(G267R), atftsZ1-1(R298Q) and atftsZ1-1(Delta404-433) exhibit reduced FtsZ1 accumulation but wild-type FtsZ2 levels. The semi-dominant atftsZ1-1(G267R) mutation caused the most severe phenotype, altering a conserved residue in the predicted T7 loop. atftsZ1-1(G267R) protein accumulates normally in young leaves but is not detected in rings or filaments. atftsZ1-1(R298Q) has midplastid FtsZ1-containing rings in young leaves, indicating that R298 is not critical for ring formation or positioning despite its conservation. atftsZ1-1(D159N) and atftsZ1-1(G366A) both have overly long, sometimes spiral-like FtsZ filaments, suggesting that FtsZ dynamics are altered in these mutants. However, atftsZ1-1(D159N) exhibits loss of proper midplastid FtsZ positioning while atftsZ1-1(G366A) does not. Finally, truncation of the FtsZ1 C-terminus in atftsZ1-1(Delta404-433) impairs chloroplast division somewhat but does not prevent midplastid Z ring formation. These alleles will facilitate understanding of how the in vitro biochemical properties of FtsZ1 are related to its in vivo function.     

A 4-aminofurazan derivative-A189-inhibits assembly of bacterial cell division protein FtsZ in vitro and in vivo

Out of 95,000 commercially available chemical compounds screened by the anucleate cell blue assay, 138 selected hit compounds were further screened. As a result, A189, a 4-aminofurazan derivative was found to inhibit FtsZ GTPase with an IC(50) of 80 mug/ml and to exhibit antibacterial activity against Staphylococcus aureus and Escherichia coli. Light scattering demonstrated that A189 inhibited FtsZ assembly in vitro, and microscopic observation of A189-treated E. coli indicated that A189 perturbed FtsZ ring formation and made bacterial cells filamentous. However, nucleoids staining with DAPI revealed that A189 did not affect DNA replication and chromosome segregation in bacterial filamentous cells. Furthermore, A189 made sulA-deleted E. coli cells filamentous. Taken together, these findings suggest that A189 inhibits FtsZ GTPase activity, resulting in perturbation of FtsZ ring formation, which leads to bacterial cell death.     

拟南芥叶绿体分裂突变体pdl37的基因鉴定与分析

pd137是经甲基磺酸乙脂（ethyl methane sulphonate, EMS）诱变并通过筛选得到的一个拟南芥叶绿体分裂突变体。该突变体的叶绿体表型与野生型相比有很大差异： 叶绿体面积显著增大, 细胞中叶绿体数量明显减少。遗传分析显示pd137的突变表型受隐性单基因控制。本研究通过遗传作图将该突变基因粗定位于拟南芥2号染色体的分子标记CH2-13.70和CH2-16.0区间内。该区间内已知的与叶绿体分裂相关的基因只有FtsZ2-1。对FtsZ2-1基因的测序结果显示pd137突变体的FtsZ2-1基因第505位碱基发生了无义突变, 使蛋白质翻译提前终止。该突变还严重影响了FtsZ2-1基因的mRNA水平。转基因互补实验进一步验证了该突变体表型是由于FtsZ2-1基因突变引起。本项工作为研究叶绿体分裂的机制提供了新材料和一些有用的线索。     

CGL160-mediated recruitment of the coupling factor CF1 is required for efficient thylakoid ATP synthase assembly,photosynthesis, and chloroplast development in Arabidopsis

Chloroplast ATP synthases consist of a membrane-spanning coupling factor (CF_O) and a soluble coupling factor (CF₁). It was previously demonstrated that CONSERVED ONLY IN THE GREEN LINEAGE160 (CGL160) promotes the formation of plant CF_O and performs a similar function in the assembly of its c-ring to that of the distantly related bacterial Atp1/UncI protein. Here, we show that in Arabidopsis (Arabidopsis thaliana) the N-terminal portion of CGL160 (AtCGL160N) is required for late steps in CF₁-CF_O assembly. In plants that lacked AtCGL160N, CF₁-CF_O content, photosynthesis, and chloroplast development were impaired. Loss of AtCGL160N did not perturb c-ring formation, but led to a 10-fold increase in the numbers of stromal CF₁ subcomplexes relative to that in the wild type. Co-immunoprecipitation and protein crosslinking assays revealed an association of AtCGL160 with CF₁ subunits. Yeast two-hybrid assays localized the interaction to a stretch of AtCGL160N that binds to the DELSEED-containing CF₁-β subdomain. Since Atp1 of Synechocystis (Synechocystis sp. PCC 6803) could functionally replace the membrane domain of AtCGL160 in Arabidopsis, we propose that CGL160 evolved from a cyanobacterial ancestor and acquired an additional function in the recruitment of a soluble CF₁ subcomplex, which is critical for the modulation of CF₁-CF_O activity and photosynthesis.

The green-lineage specific N-terminal domain of CGL160 recruits coupling factor 1 and its lack affects chloroplast development, photosynthesis and thylakoid ATP synthase assembly in Arabidopsis.

IN A NUTSHELL Background: Thylakoid ATP synthases are impressive molecular engines that harness the light-driven proton gradient to generate ATP during photosynthesis. Their molecular mode of operation and atomic structure have been elucidated, but their assembly process is still under investigation. Specific auxiliary factors assist in ATP synthase assembly and prevent the accumulation of dead-end products or deleterious intermediates. CGL160 is one such factor and consists of a membrane and an N-terminal domain. The membrane domain of CGL160 is distantly related to bacterial Atp1 proteins, which are also present in cyanobacteria. Previous studies demonstrated that CGL160 promotes efficient formation of the membranous c-ring of thylakoid ATP synthases in Arabidopsis thaliana. Question: What is the function of the green lineage-specific N-terminal domain of CGL160 in thylakoid ATP synthase assembly, and what is the evolutionary relationship between CGL160 and Atp1? Findings: Here, we showed that the N-terminal domain of CGL160 is required for the late steps in thylakoid ATP synthase assembly and recruits the stromal ATP synthase intermediate coupling factor CF₁. The assembly step is critical for chloroplast development in the dark, ATP synthase activity, and photosynthesis in A. thaliana. We also revealed that Atp1 from the cyanobacterium Synechocystis spec PCC 6803 could functionally replace the membrane domain of CGL160 in A. thaliana. These results indicated that Atp1 operates in c-ring assembly in cyanobacteria and that CGL160 evolved from its cyanobacterial ancestor Atp1. However, CGL160 acquired an additional function in linking a soluble ATP synthase intermediate to a membranous subcomplex. Next steps: The next steps are to identify all auxiliary factors required for the assembly of thylakoid ATP synthases and to understand their precise function in ATP synthase formation. Detailed knowledge of the factors and the assembly process could provide elegant strategies for adjusting proton circuits and altering the ATP budget in crops or other photosynthetic organisms.     

EGY1 encodes a membrane-associated and ATP-independent metalloprotease that is required for chloroplast development

Chloroplast development requires coordinated expression of both nuclear- and chloroplast-encoded genes. To better understand the roles played by nuclear-encoded chloroplast proteins in chloroplast biogenesis, we isolated an Arabidopsis mutant, egy1-1, which has a dual phenotype, reduced chlorophyll accumulation and abnormal hypocotyl gravicurvature. Subsequent map-based cloning and DNA sequencing of the mutant gene revealed a 10-bp deletion in an EGY1 gene, which encodes a 59-kDa metalloprotease that contains eight trans-membrane domains at its C-terminus, and carries out beta-casein degradation in an ATP-independent manner. EGY1 protein accumulation varies between tissue types, being most prominent in leaf and stem tissues, and is responsive to light and ethylene. EGY1-GFP hybrid proteins are localized in the chloroplast. egy1 mutant chloroplasts had reduced granal thylakoids and poorly developed lamellae networks. Furthermore, the accumulation of chlorophyll a/b binding proteins of the light-harvesting complexes I and II (Lhca and Lhcb) are significantly decreased in three separate loss-of-function egy1 mutants. Taken together, these results suggest that EGY1 metalloprotease is required for chloroplast development and, hence, a defective EGY1 gene has pleiotropic effects both on chloroplast development and on ethylene-dependent gravitropism of light-grown hypocotyls.     

RAF2 is a RuBisCO assembly factor in Arabidopsis thaliana

Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the reaction between gaseous carbon dioxide (CO₂) and ribulose‐1,5‐bisphosphate. Although it is one of the most studied enzymes, the assembly mechanisms of the large hexadecameric RuBisCO is still emerging. In bacteria and in the C4 plant Zea mays, a protein with distant homology to p terin‐4α‐c arbinolamine d ehydratase (PCD) has recently been shown to be involved in RuBisCO assembly. However, studies of the homologous PCD‐like protein (RAF2, RuBisCO assembly factor 2) in the C3 plant Arabidopsis thaliana (A. thaliana) have so far focused on its role in hormone and stress signaling. We investigated whether A. thalianaRAF2 is also involved in RuBisCO assembly. We localized RAF2 to the soluble chloroplast stroma and demonstrated that raf2 A. thaliana mutant plants display a severe pale green phenotype with reduced levels of stromal RuBisCO. We concluded that the RAF2 protein is probably involved in RuBisCO assembly in the C3 plant A. thaliana.     

Chlororespiratory reduction 6 is a novel factor required for accumulation of the chloroplast NAD(P)H dehydrogenase complex in Arabidopsis

The chloroplast NAD(P)H dehydrogenase (NDH) complex is involved in photosystem I cyclic electron transport and chlororespiration in higher plants. An Arabidopsis (Arabidopsis thaliana) chlororespiratory reduction 6 (crr6) mutant lacking NDH activity was identified by means of chlorophyll fluorescence imaging. Accumulation of the NDH complex was impaired in crr6. Physiological characterization of photosynthetic electron transport indicated the specific defect of the NDH complex in crr6. In contrast to the CRR7 protein that was recently identified as a potential novel subunit of the NDH complex by means of the same screening, the CRR6 protein was stable under the crr2 mutant background in which the NDH complex does not accumulate. The CRR6 gene (At2g47910) encodes a novel protein without any known motif. Although CRR6 does not have any transmembrane domains, it is localized in the thylakoid membrane fraction of the chloroplast. CRR6 is conserved in phototrophs, including cyanobacteria, from which the chloroplast NDH complex has evolutionally originated, but not in Chlamydomonas reinhardtii, in which the NDH complex is absent. We believe that CRR6 is a novel specific factor for the assembly or stabilization of the NDH complex.