Optical manipulation reveals strong attracting forces at membrane contact sites between endoplasmic reticulum and chloroplasts

Reticulon-like proteins in Arabidopsis thaliana: structural organization and ER localization

Reticulons are proteins that have been found predominantly associated with the endoplasmic reticulum in yeast and mammalian cells. While their functions are still poorly understood, recent findings suggest that they participate in the shaping of the tubular endoplamic reticulum (ER). Although reticulon-like proteins have been identified in plants, very little is known about their cellular localization and functions. Here, we characterized the reticulon-like protein family of Arabidopsis thaliana. Three subfamilies can be distinguished on the basis of structural organization and sequence homology. We investigated the subcellular localization of two members of the largest subfamily, i.e. AtRTNLB2 and AtRTNLB4, using fluorescent protein tags. The results demonstrate for the first time that plant reticulon-like proteins are associated with the ER. Both AtRTNLB proteins are located in the tubular ER but AtRTNLB4 is also found in the lamellar ER cisternae, and in ER tubules in close association with the chloroplasts. Similarity in protein structure and subcellular localization between AtRTNLB2 and mammalian reticulons suggests that they could assume similar basic functions inside the cell.

Chloroplast envelope membranes: a dynamic interface between plastids and the cytosol

Chloroplasts are bounded by a pair of outer membranes, the envelope, that is the only permanent membrane structure of the different types of plastids. Chloroplasts have had a long and complex evolutionary past and integration of the envelope membranes in cellular functions is the result of this evolution. Plastid envelope membranes contain a wide diversity of lipids and terpenoid compounds serving numerous biochemical functions and the flexibility of their biosynthetic pathways allow plants to adapt to fluctuating environmental conditions (for instance phosphate deprivation). A large body of knowledge has been generated by proteomic studies targeted to envelope membranes, thus revealing an unexpected complexity of this membrane system. For instance, new transport systems for metabolites and ions have been identified in envelope membranes and new routes for the import of chloroplast-specific proteins have been identified. The picture emerging from our present understanding of plastid envelope membranes is that of a key player in plastid biogenesis and the co-ordinated gene expression of plastid-specific protein (owing to chlorophyll precursors), of a major hub for integration of metabolic and ionic networks in cell metabolism, of a flexible system that can divide, produce dynamic extensions and interact with other cell constituents. Envelope membranes are indeed one of the most complex and dynamic system within a plant cell. In this review, we present an overview of envelope constituents together with recent insights into the major functions fulfilled by envelope membranes and their dynamics within plant cells.

Membrane contact sites: physical attachment between chloroplasts and endoplasmic reticulum revealed by optical manipulation

Chloroplasts and their surrounding cell are highly interdependent. One example is lipid metabolism, where the cell depends on its chloroplasts to provide fatty acids for lipid synthesis in the endoplasmic reticulum (ER) and in turn, chloroplasts rely on import of lipid precursors from the ER. Despite its fundamental importance, the route for lipid trafficking into and out of chloroplasts remains unknown. Biochemical studies of plant membrane lipid metabolism have suggested the possibility of lipid transport at membrane contact sites (MCSs) between the ER and chloroplasts. With the aid of optical manipulation, we recently could present physical evidence for this association. Leaf protoplasts isolated from Arabidopsis thaliana expressing green fluorescent protein (GFP) in the ER lumen were observed by confocal microscopy. A laser scalpel was used to rupture the protoplasts. ER fragments associated with the released chloroplasts could be stretched out by optical tweezers but remained attached to the chloroplast surface, even when a stretching force of 400 pN was applied. We thus provided the first physical evidence for MCSs between two membranes and we propose for the ER-chloroplast pair, that such tight associations are involved in bidirectional lipid trafficking between the two compartments.

Channeling of eukaryotic diacylglycerol into the biosynthesis of plastidial phosphatidylglycerol

Plastidial glycolipids contain diacylglycerol (DAG) moieties, which are either synthesized in the plastids (prokaryotic lipids) or originate in the extraplastidial compartment (eukaryotic lipids) necessitating their transfer into plastids. In contrast, the only phospholipid in plastids, phosphatidylglycerol (PG), contains exclusively prokaryotic DAG backbones. PG contributes in several ways to the functions of chloroplasts, but it is not known to what extent its prokaryotic nature is required to fulfill these tasks. As a first step toward answering this question, we produced transgenic tobacco plants that contain eukaryotic PG in thylakoids. This was achieved by targeting a bacterial DAG kinase into chloroplasts in which the heterologous enzyme was also incorporated into the envelope fraction. From lipid analysis we conclude that the DAG kinase phosphorylated eukaryotic DAG forming phosphatidic acid, which was converted into PG. This resulted in PG with 2-3 times more eukaryotic than prokaryotic DAG backbones. In the newly formed PG the unique Delta3-trans-double bond, normally confined to 3-trans-hexadecenoic acid, was also found in sn-2-bound cis-unsaturated C18 fatty acids. In addition, a lipidomics technique allowed the characterization of phosphatidic acid, which is assumed to be derived from eukaryotic DAG precursors in the chloroplasts of the transgenic plants. The differences in lipid composition had only minor effects on measured functions of the photosynthetic apparatus, whereas the most obvious phenotype was a significant reduction in growth.

Laser Surgery and Optical Trapping in a Laser Scanning Microscope

Optical manipulation opens up many new possibilities for experiments in the field of microbiology and is a very powerful tool for investigating cellular structure. In this emerging field imaging retains an important role, and systems that combine advanced imaging techniques with optical manipulation tools, such as laser scalpels or optical tweezers, are an important starting point for researchers. We present a flexible experimental platform that contains both a laser scalpel and optical tweezers, in combination with confocal and multiphoton microscopy. A simple manipulation of the external optics is used to retain the three-dimensional imaging capabilities of the microscopes. Two applications of the system are presented. In the first, the laser scalpel is used to initiate diffusion of a fluorescent dye through Escherichia coli mutants, which exhibit abnormal cell division, forming filaments, or chains of bacteria. The diffusion assay is used to assess the potential for the exchange of cytoplasmic material between neighboring cells. The second application investigates the binding of endoplasmic reticulum (ER) to chloroplasts in Pisum sativum (garden pea). Individual plant protoplasts are ruptured using the laser scalpel, allowing individual chloroplasts to be trapped and manipulated. Strands of the ER which are attached to the chloroplast are identified. The magnitude and nature of the binding between the chloroplast and the ER are investigated.