Discovery of a protein required for photosynthetic membrane assembly

Characterizing the Self-Assembly Properties of Monoolein Lipid Isosteres

Living cells feature lipid compartments which exhibit a variety of shapes and structures that assist essential cellular processes. Many natural cell compartments frequently adopt convoluted nonlamellar lipid architectures that facilitate specific biological reactions. Improved methods for controlling the structural organization of artificial model membranes would facilitate investigations into how membrane morphology affects biological functions. Monoolein (MO) is a single-chain amphiphile which forms nonlamellar lipid phases in aqueous solution and has wide applications in nanomaterial development, the food industry, drug delivery, and protein crystallization. However, even if MO has been extensively studied, simple isosteres of MO, while readily accessible, have seen limited characterization. An improved understanding of how relatively minor changes in lipid chemical structure affect self-assembly and membrane topology could instruct the construction of artificial cells and organelles for modeling biological structures and facilitate nanomaterial-based applications. Here, we investigate the differences in self-assembly and large-scale organization between MO and two MO lipid isosteres. We show that replacing the ester linkage between the hydrophilic headgroup and hydrophobic hydrocarbon chain with a thioesther or amide functional group results in the assembly of lipid structures with different phases not resembling those formed by MO. Using light and cryo-electron microscopy, small-angle X-ray scattering, and infrared spectroscopy, we demonstrate differences in the molecular ordering and large-scale architectures of the self-assembled structures made from MO and its isosteric analogues. These results improve our understanding of the molecular underpinnings of lipid mesophase assembly and may facilitate the development of MO-based materials for biomedicine and as model lipid compartments.

The puzzle of chloroplast vesicle transport - involvement of GTPases

In the cytosol of plant cells vesicle transport occurs via secretory pathways among the endoplasmic reticulum network, Golgi bodies, secretory granules, endosome, and plasma membrane. Three systems transfer lipids, proteins and other important molecules through aqueous spaces to membrane-enclosed compartments, via vesicles that bud from donor membranes, being coated and uncoated before tethered and fused with acceptor membranes. In addition, molecular, biochemical and ultrastructural evidence indicates presence of a vesicle transport system in chloroplasts. Little is known about the protein components of this system. However, as chloroplasts harbor the photosynthetic apparatus that ultimately supports most organisms on the planet, close attention to their pathways is warranted. This may also reveal novel diversification and/or distinct solutions to the problems posed by the targeted intra-cellular trafficking of important molecules. To date two homologs to well-known yeast cytosolic vesicle transport proteins, CPSAR1 and CPRabA5e (CP, chloroplast localized), have been shown to have roles in chloroplast vesicle transport, both being GTPases. Bioinformatic data indicate that several homologs of cytosolic vesicle transport system components are putatively chloroplast-localized and in addition other proteins have been implicated to participate in chloroplast vesicle transport, including vesicle-inducing protein in plastids 1, thylakoid formation 1, snowy cotyledon 2/cotyledon chloroplast biogenesis factor, curvature thylakoid 1 proteins, and a dynamin like GTPase FZO-like protein. Several putative potential cargo proteins have also been identified, including building blocks of the photosynthetic apparatus. Here we discuss details of the largely unknown putative chloroplast vesicle transport system, focusing on GTPase-related components.

A mutable albino allele in rice reveals that formation of thylakoid membranes requires the SNOW-WHITE LEAF1 gene

Active DNA transposons are important tools for gene functional analysis. The endogenous non-autonomous transposon, nDart1-0, in rice (Oryza sativa L.) is expected to generate various transposon-insertion mutants because nDart1-0 elements tend to insert into genic regions under natural growth conditions. We have developed a specific method (nDart1-0-iPCR) for efficient detection of nDart1-0 insertions and successfully identified the SNOW-WHITE LEAF1 (SWL1) gene in a variegated albino (swl1-v) mutant obtained from the nDart1-promoted rice tagging line. The variegated albino phenotype was caused by insertion and excision of nDart1-0 in the 5'-untranslated region of the SWL1 gene predicted to encode an unknown protein with the N-terminal chloroplast transit peptide. SWL1 expression was detected in various rice tissues at different developmental stages. However, immunoblot analysis indicated that SWL1 protein accumulation was strictly regulated in a tissue-specific manner. In the swl1 mutant, formations of grana and stroma thylakoids and prolamellar bodies were inhibited. This study revealed that SWL1 is essential for the beginning of thylakoid membrane organization during chloroplast development. Furthermore, we provide a developmental perspective on the nDart1-promoted tagging line to characterize unidentified gene functions in rice.

Homologs of the yeast Tvp38 vesicle-associated protein are conserved in chloroplasts and cyanobacteria

Vesicle transfer processes in eukaryotes depend on specific proteins, which mediate the selective packing of cargo molecules for subsequent release out of the cells after vesicle fusion to the plasma membrane. The protein Tvp38 is conserved in yeasts and higher eukaryotes and potentially involved in vesicle transfer processes at the Golgi membrane. Members of the so-called "SNARE-associated proteins of the Tvp38-family" have also been identified in prokaryotes and those belong to the DedA protein family. Tvp38/DedA proteins are also conserved in cyanobacteria and chloroplasts. While only a single member of this family appears to be present in chloroplasts, cyanobacterial genomes typically encode multiple homologous proteins. Mainly based on our understanding of the DedA-homologous proteins of Escherichia coli, it appears likely that the function of these proteins in chloroplast and cyanobacteria involves stabilizing and organizing the structure of internal membrane systems.

The dynamic roles of intracellular lipid droplets: from archaea to mammals

During the past decade, there has been a paradigm shift in our understanding of the roles of intracellular lipid droplets (LDs). New genetic, biochemical and imaging technologies have underpinned these advances, which are revealing much new information about these dynamic organelles. This review takes a comparative approach by examining recent work on LDs across the whole range of biological organisms from archaea and bacteria, through yeast and Drosophila to mammals, including humans. LDs probably evolved originally in microorganisms as temporary stores of excess dietary lipid that was surplus to the immediate requirements of membrane formation/turnover. LDs then acquired roles as long-term carbon stores that enabled organisms to survive episodic lack of nutrients. In multicellular organisms, LDs went on to acquire numerous additional roles including cell- and organism-level lipid homeostasis, protein sequestration, membrane trafficking and signalling. Many pathogens of plants and animals subvert their host LD metabolism as part of their infection process. Finally, malfunctions in LDs and associated proteins are implicated in several degenerative diseases of modern humans, among the most serious of which is the increasingly prevalent constellation of pathologies, such as obesity and insulin resistance, which is associated with metabolic syndrome.

Vipp1: a very important protein in plastids?!

As a key feature in oxygenic photosynthesis, thylakoid membranes play an essential role in the physiology of plants, algae, and cyanobacteria. Despite their importance in the process of oxygenic photosynthesis, their biogenesis has remained a mystery to the present day. A decade ago, vesicle-inducing protein in plastids 1 (Vipp1) was described to be involved in thylakoid membrane formation in chloroplasts and cyanobacteria. Most follow-up studies clearly linked Vipp1 to membranes and Vipp1 interactions as well as the defects observed after Vipp1 depletion in chloroplasts and cyanobacteria indicate that Vipp1 directly binds to membranes, locally stabilizes bilayer structures, and thereby retains membrane integrity. Here current knowledge about the structure and function of Vipp1 is summarized with a special focus on its relationship to the bacterial phage shock protein A (PspA), as both proteins share a common origin and appear to have retained many similarities in structure and function.

Plastids contain a second sec translocase system with essential functions

Proteins that are synthesized on cytoplasmic ribosomes but function within plastids must be imported and then targeted to one of six plastid locations. Although multiple systems that target proteins to the thylakoid membranes or thylakoid lumen have been identified, a system that can direct the integration of inner envelope membrane proteins from the stroma has not been previously described. Genetics and localization studies were used to show that plastids contain two different Sec systems with distinct functions. Loss-of-function mutations in components of the previously described thylakoid-localized Sec system, designated as SCY1 (At2g18710), SECA1 (At4g01800), and SECE1 (At4g14870) in Arabidopsis (Arabidopsis thaliana), result in albino seedlings and sucrose-dependent heterotrophic growth. Loss-of-function mutations in components of the second Sec system, designated as SCY2 (At2g31530) and SECA2 (At1g21650) in Arabidopsis, result in arrest at the globular stage and embryo lethality. Promoter-swap experiments provided evidence that SCY1 and SCY2 are functionally nonredundant and perform different roles in the cell. Finally, chloroplast import and fractionation assays and immunogold localization of SCY2-green fluorescent protein fusion proteins in root tissues indicated that SCY2 is part of an envelope-localized Sec system. Our data suggest that SCY2 and SECA2 function in Sec-mediated integration and translocation processes at the inner envelope membrane.

Etioplast and etio-chloroplast formation under natural conditions: the dark side of chlorophyll biosynthesis in angiosperms

Chloroplast development is usually regarded as proceeding from proplastids. However, direct or indirect conversion pathways have been described in the literature, the latter involving the etioplast or the etio-chloroplast stages. Etioplasts are characterized by the absence of chlorophylls (Chl-s) and the presence of a unique inner membrane network, the prolamellar body (PLB), whereas etio-chloroplasts contain Chl-s and small PLBs interconnected with chloroplast thylakoids. As etioplast development requires growth in darkness for several days, this stage is generally regarded as a nonnatural pathway of chloroplast development occurring only under laboratory conditions. In this article, we have reviewed the data in favor of the involvement of etioplasts and etio-chloroplasts as intermediary stage(s) in chloroplast formation under natural conditions, the molecular aspects of PLB formation and we propose a dynamic model for its regulation.

Mechanisms of lipid transport involved in organelle biogenesis in plant cells

Chloroplasts are the defining organelle of photoautotrophic plant cells. Photosynthetic light reactions and electron transport are the functions of an elaborate thylakoid membrane system inside chloroplasts. The lipid composition of photosynthetic membranes is characterized by a substantial fraction of nonphosphorous galactoglycerolipids reflecting the need of sessile plants to conserve phosphorus. Lipid transport and assembly of glycerolipids play an essential role in the biogenesis of the photosynthetic apparatus in developing chloroplasts. During chloroplast biogenesis, fatty acids are synthesized in the plastid and are exported to the endoplasmic reticulum, where they are incorporated into membrane lipids. Alternatively, lipids can also be assembled de novo at the inner envelope membrane of plastids in many plants. A rich repertoire of lipid exchange mechanisms involving the thylakoid membranes, the chloroplast inner and outer envelope membranes, and the endoplasmic reticulum is emerging. Studies of thylakoid biogenesis provide new insights into the general mechanisms of intermembrane lipid transfer.

The chloroplast HSP70B-CDJ2-CGE1 chaperones catalyse assembly and disassembly of VIPP1 oligomers in Chlamydomonas

The vesicle-inducing protein in plastids (VIPP1) is essential for the biogenesis of thylakoid membranes in cyanobacteria and plants. VIPP1 and its bacterial ancestor PspA form large homo-oligomeric rings of >1 MDa. We recently demonstrated that VIPP1 interacts with the chloroplast J-domain co-chaperone CDJ2 and its chaperone partner HSP70B, and hypothesized that the chaperones might be involved in the assembly and/or disassembly of VIPP1 oligomers. To test this hypothesis, we analysed the composition of VIPP1/chaperone complexes in Chlamydomonas reinhardtii cell extracts and monitored effects of the chaperones on VIPP1 assembly states in vitro. We found that CGE1, the chloroplast GrpE homologue, is also part of complexes with HSP70B, CDJ2 and VIPP1. We observed that CDJ2-VIPP1 accumulated as low- and high-molecular-weight complexes in ATP-depleted cell extracts, but as intermediate-size complexes in extracts supplemented with ATP. This was consistent with a role for the chaperones in VIPP1 assembly and disassembly. Using purified proteins, we could demonstrate that the chaperones indeed facilitated both the assembly and disassembly of VIPP1 oligomers. Electron microscopy studies revealed that, in contrast to PspA, VIPP1 rings assembled into rod-shaped supercomplexes that were morphologically similar to microtubule-like structures observed earlier in various plastid types. VIPP1 rods, too, were disassembled by the chaperones, and chaperone-mediated rod disassembly also occurred when VIPP1 lacked an approximately 30-aa C-terminal extension present in VIPP1 homologues but absent in PspA. By regulating the assembly state of VIPP1, the chloroplast HSP70 chaperone system may play an important role in the maintenance/biogenesis of thylakoid membranes.

Non-vesicular and vesicular lipid trafficking involving plastids

In plants, newly synthesized fatty acids are either directly incorporated into glycerolipids in the plastid or exported and assembled into lipids at the endoplasmic reticulum (ER). ER-derived glycerolipids serve as building blocks for extraplastidic membranes. Alternatively, they can return to the plastid where their diacylglycerol backbone is incorporated into the glycerolipids of the photosynthetic membranes, the thylakoids. Thylakoid lipids are assembled at the plastid envelope membranes and are transferred to the thylakoids. Under phosphate-limited growth conditions, galactolipids are exported from the outer plastid envelope membranes to extraplastidic membranes. Proteins, such as TRIGALACTOSYLDIACYLGLYCEROL1 (TGD1) or VESICLE-INDUCING PROTEIN IN PLASTIDS1 (VIPP1), which are involved in different aspects of plastid lipid trafficking phenomena have recently been identified and mechanistic models that are based on the analysis of these components have begun to emerge.

Biogenesis and origin of thylakoid membranes

Thylakoids are photosynthetically active membranes found in Cyanobacteria and chloroplasts. It is likely that they originated in photosynthetic bacteria, probably in close connection to the occurrence of photosystem II and oxygenic photosynthesis. In higher plants, chloroplasts develop from undifferentiated proplastids. These contain very few internal membranes and the whole thylakoid membrane system is built when chloroplast differentiation takes place. During cell and organelle division a constant synthesis of new thylakoid membrane material is required. Also, rapid adaptation to changes in light conditions and long term adaptation to a number of environmental factors are accomplished by changes in the lipid and protein content of the thylakoids. Thus regulation of synthesis and assembly of all these elements is required to ensure optimal function of these membranes.

A vesicle transport system inside chloroplasts

Intracellular transport via membrane vesicle traffic is a well known feature of eukaryotic cells. Yet, no vesicle transport system has been described for prokaryotes or organelles of prokaryotic origin, such as chloroplasts and mitochondria. Here we show that chloroplasts possess a vesicle transport system with features similar to vesicle traffic in homotypic membrane fusion. Vesicle formation and fusion is affected by specific inhibitors, e.g. nucleotide analogues, protein phosphatase inhibitors and Ca2+ antagonists. This vesicle transfer is an ongoing process in mature chloroplasts indicating that it represents an important new pathway in the formation and maintenance of the thylakoid membranes.