On the nature of the plant ER exit sites

Identification and characterization of the COPII vesicle-forming GTPase Sar1 in Chlamydomonas

Eukaryotic cells are highly compartmentalized, requiring elaborate transport mechanisms to facilitate the movement of proteins between membrane-bound compartments. Most proteins synthesized in the endoplasmic reticulum (ER) are transported to the Golgi apparatus through COPII-mediated vesicular trafficking. Sar1, a small GTPase that facilitates the formation of COPII vesicles, plays a critical role in the early steps of this protein secretory pathway. Sar1 was characterized in yeast, animals and plants, but no Sar1 homolog has been identified and functionally analyzed in algae. Here we identified a putative Sar1 homolog (CrSar1) in the model green alga Chlamydomonas reinhardtii through amino acid sequence similarity. We employed site-directed mutagenesis to generate a dominant-negative mutant of CrSar1 (CrSar1DN). Using protein secretion assays, we demonstrate the inhibitory effect of CrSar1DN on protein secretion. However, different from previously studied organisms, ectopic expression of CrSar1DN did not result in collapse of the ER-Golgi interface in Chlamydomonas. Nonetheless, our data suggest a largely conserved role of CrSar1 in the ER-to-Golgi protein secretory pathway in green algae.

Suborganellar resolution imaging for the localisation of human glycosylation enzymes in tobacco Golgi bodies

Plant cells are a capable system for producing economically and therapeutically important proteins for a variety of applications, and are considered a safer production system than some existing hosts such as bacteria or yeasts. However, plants do not perform protein modifications in the same manner as mammalian cells do. This can impact on protein functionality for plant-produced human therapeutics. This obstacle can be overcome by creating a plant-based system capable of 'humanising' proteins of interest resulting in a glycosylation profile of synthetic plant-produced proteins as it would occur in mammalian systems. For this, the human glycosylation enzymes (HuGEs) involved in N-linked glycosylation N-acetylglucosaminyltransferase IV and V (GNTIV and GNTV), β-1,4-galactosyltransferase (B4GALT1), and α-2,6-sialyltransferase (ST6GAL) were expressed in plant cells. For these enzymes to carry out the stepwise glycosylation functions, they need to localise to late Golgi body cisternae. This was achieved by a protein targeting strategy of replacing the mammalian Golgi targeting domains (Cytoplasmic-Transmembrane-Stem (CTS) regions) with plant-specific ones. Using high-resolution and dynamic confocal microscopy, we show that GNTIV and GNTV were successfully targeted to the medial-Golgi cisternae while ST6GAL and B4GALT1 were targeted to trans-Golgi cisternae. Plant cells are a promising system to produce human therapeutics for example proteins used in enzyme replacement therapies. Plants can provide safer and cheaper alternatives to existing expression systems such as mammalian cell culture, bacteria or yeast. An important factor for the functionality of therapeutic proteins though are protein modifications specific to human cells. However, plants do not perform protein modifications in the same manner as human cells do. Therefore, plant cells need to be genetically modified to mimic human protein modifications patterns. The modification of importance here, is called N-linked glycosylation and adds specific sugar molecules onto the proteins. Here we show the expression of four human glycosylation enzymes, which are required for N-linked glycosylation, in plant cells. In addition, as these protein modifications are carried out in cells resembling a factory production line, it is important that the human glycosylation enzymes be placed in the correct cellular compartments and in the correct order. This is carried out in Golgi bodies. Golgi bodies are composed of several defined stacks termed cis-, medial and trans-Golgi body stacks. For correct protein function, two of these human glycosylation enzymes need to be placed in the medial-Golgi attacks and the other two in the trans-Golgi stacks. Using high-resolution laser microscopy in live plant cells, we show here that the human glycosylation enzymes are sent within the cells to the correct Golgi body stacks. These are first steps to modify plant cells in order to produce human therapeutics.

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

Retrograde transport in plants: Circular economy in the endomembrane system

The study of endomembrane trafficking is crucial for understanding how cells and whole organisms function. Moreover, there is a special interest in investigating endomembrane trafficking in plants, given its role in transport and accumulation of seed storage proteins and in secretion of cell wall material, arguably the two most essential commodities obtained from crops. The mechanisms of anterograde transport in the biosynthetic and endocytic pathways of plants have been thoroughly discussed in recent reviews, but, comparatively, retrograde trafficking pathways have received less attention. Retrograde trafficking is essential to recover membranes, retrieve proteins that have escaped from their intended localization, maintain homeostasis in maturing compartments, and recycle trafficking machinery for its reuse in anterograde transport reactions. Here, we review the current understanding on retrograde trafficking pathways in the endomembrane system of plants, discussing their integration with anterograde transport routes, describing conserved and plant-specific retrieval mechanisms at play, highlighting contentious issues and identifying open questions for future research.

Super resolution live imaging: The key for unveiling the true dynamics of membrane traffic around the Golgi apparatus in plant cells

In contrast to the relatively static image of the plants, the world inside each cell is surprisingly dynamic. Membrane-bounded organelles move actively on the cytoskeletons and exchange materials by vesicles, tubules, or direct contact between each other. In order to understand what is happening during those events, it is essential to visualize the working components in vivo. After the breakthrough made by the application of fluorescent proteins, the development of light microscopy enabled many discoveries in cell biology, including those about the membrane traffic in plant cells. Especially, super-resolution microscopy, which is becoming more and more accessible, is now one of the most powerful techniques. However, although the spatial resolution has improved a lot, there are still some difficulties in terms of the temporal resolution, which is also a crucial parameter for the visualization of the living nature of the intracellular structures. In this review, we will introduce the super resolution microscopy developed especially for live-cell imaging with high temporal resolution, and show some examples that were made by this tool in plant membrane research.