The trans-Golgi Network and the Golgi Stacks Behave Independently During Regeneration After Brefeldin A Treatment in Tobacco BY-2 Cells

On the nature of the plant ER exit sites

In plants, the endoplasmic reticulum (ER) and Golgi bodies are not only in close proximity, but are also physically linked. This unique organization raises questions about the nature of the transport vectors carrying cargo between the two organelles. Same as in metazoan and yeast cells, it was suggested that cargo is transported from the ER to Golgi cisternae via COPII-coated vesicles produced at ribosome-free ER exit sites (ERES). Recent developments in mammalian cell research suggest, though, that COPII helps to select secretory cargo, but does not coat the carriers leaving the ER. Furthermore, it was shown that mammalian ERES expand into a tubular network containing secretory cargo, but no COPII components. Because of the close association of the ER and Golgi bodies in plant cells, it was previously proposed that ERES and the Golgi comprise a secretory unit that travels over or with a motile ER membrane. In this study, we aimed to explore the nature of ERES in plant cells and took advantage of high-resolution confocal microscopy and imaged ERES labelled with canonical markers (Sar1a, Sec16, Sec24). We found that ERES are dynamically connected to Golgi bodies and most likely represent pre-cis-Golgi cisternae. Furthermore, we showed fine tubular connections from the ER to Golgi compartments (ERGo tubules) as well as fine protrusions from ERES/Golgi cisternae connecting with the ER. We suggest that these tubules observed between the ER and Golgi as well as between the ER and ERES are involved in stabilizing the physical connection between ER and ERES/Golgi cisternae, but may also be involved in cargo transport from the ER to Golgi bodies.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

Sec71 separates Golgi stacks in Drosophila S2 cells

Golgi stacks are the basic structural units of the Golgi. Golgi stacks are separated from each other and scattered in the cytoplasm of Drosophila cells. Here, we report that the ARF-GEF inhibitor Brefeldin A (BFA) induces the formation of BFA bodies, which are aggregates of Golgi stacks, trans-Golgi networks and recycling endosomes. Recycling endosomes are located in the centers of BFA bodies, while Golgi stacks surround them on their trans sides. Live imaging of S2 cells revealed that Golgi stacks repeatedly merged and separated on their trans sides, and BFA caused successive merger by inhibiting separation, forming BFA bodies. S2 cells carrying genome-edited BFA-resistant mutant Sec71M717L did not form BFA bodies at high concentrations of BFA; S2 cells carrying genome-edited BFA-hypersensitive mutant Sec71F713Y produced BFA bodies at low concentrations of BFA. These results indicate that Sec71 is the sole BFA target for BFA body formation and controls Golgi stack separation. Finally, we showed that impairment of Sec71 in fly photoreceptors induces BFA body formation, with accumulation of both apical and basolateral cargoes, resulting in inhibition of polarized transport.

Brefeldin A inhibits clathrin-dependent endocytosis and ion transport in Chara internodal cells

BACKGROUND

The Characeae are multicellular green algae, which are closely related to higher plants. Their internodal cells are a convenient model to study membrane transport and organelle interactions.

RESULTS

In this study, we report on the effect of brefeldin A (BFA), an inhibitor of vesicle trafficking, on internodal cells of Chara australis. BFA induced the commonly observed agglomeration of Golgi bodies and trans Golgi network into 'brefeldin compartments' at concentrations between 6 and 500 μM and within 30-120 min treatment. In contrast to most other cells, however, BFA inhibited endocytosis and significantly decreased the number of clathrin-coated pits and clathrin-coated vesicles at the plasma membrane. BFA did not inhibit secretion of organelles at wounds induced by puncturing or local light damage but prevented the formation of cellulosic wound walls probably because of insufficient membrane recycling. We also found that BFA inhibited the formation of alkaline and acid regions along the cell surface ('pH banding pattern') which facilitates carbon uptake required for photosynthesis; we hypothesise that this is due to insufficient recycling of ion transporters. During long-term treatments over several days, BFA delayed the formation of complex 3D plasma membranes (charasomes). Interestingly, BFA had no detectable effect on clathrin-dependent charasome degradation. Protein sequence analysis suggests that the peculiar effects of BFA in Chara internodal cells are due to a mutation in the guanine-nucleotide exchange factor GNOM required for recruitment of membrane coats via activation of ADP-ribosylation factor proteins.

CONCLUSIONS AND SIGNIFICANCE

This work provides an overview on the effects of BFA on different processes in C. australis. It revealed similarities but also distinct differences in vesicle trafficking between higher plant and algal cells. It shows that characean internodal cells are a promising model to study interactions between seemingly distant metabolic pathways.

Endomembrane architecture and dynamics during secretion of the extracellular matrix of the unicellular charophyte, Penium margaritaceum

The extracellular matrix (ECM) of many charophytes, the assemblage of green algae that are the sister group to land plants, is complex, produced in large amounts, and has multiple essential functions. An extensive secretory apparatus and endomembrane system are presumably needed to synthesize and secrete the ECM, but structural details of such a system have not been fully characterized. Penium margaritaceum is a valuable unicellular model charophyte for studying secretion dynamics. We report that Penium has a highly organized endomembrane system, consisting of 150-200 non-mobile Golgi bodies that process and package ECM components into different sets of vesicles that traffic to the cortical cytoplasm, where they are transported around the cell by cytoplasmic streaming. At either fixed or transient areas, specific cytoplasmic vesicles fuse with the plasma membrane and secrete their constituents. Extracellular polysaccharide (EPS) production was observed to occur in one location of the Golgi body and sometimes in unique Golgi hybrids. Treatment of cells with brefeldin A caused disruption of the Golgi body, and inhibition of EPS secretion and cell wall expansion. The structure of the endomembrane system in Penium provides mechanistic insights into how extant charophytes generate large quantities of ECM, which in their ancestors facilitated the colonization of land.

Differentiation of Trafficking Pathways at Golgi Entry Core Compartments and Post-Golgi Subdomains

Eukaryotic cells have developed specialized membrane structures called organelles, which compartmentalize cellular functions and chemical reactions. Recent improvements in microscopy and membrane compartment isolation techniques are now sophisticating our view. Emerging evidences support that there are distinct sub-populations or subdomains, which are spatially and/or temporally segregated within one type of organelle, contributing to specify differential sorting of various cargos to distinct destinations of the cell. In plant cells, the Golgi apparatus represents a main trafficking hub in which entry occurs through a Golgi Entry Core Compartment (GECCO), that remains to be further characterized, and sorting of cargos is mediated through multiple transport pathways with different sets of regulator proteins at the post-Golgi compartment trans-Golgi network (TGN). Both GECCO and TGN are differentiated sub-populations as compared to the rest of Golgi, and moreover, further subdomain formation within TGN is suggested to play a key role for cargo sorting. In this review, we will summarize recent findings obtained on organelle subdomains, and their relationship with cargo entry at and exit from the Golgi apparatus.

Spatiotemporal dissection of the trans-Golgi network in budding yeast

The trans-Golgi network (TGN) acts as a sorting hub for membrane traffic. It receives newly synthesized and recycled proteins, and sorts and delivers them to specific targets such as the plasma membrane, endosomes and lysosomes/vacuoles. Accumulating evidence suggests that the TGN is generated from the trans-most cisterna of the Golgi by maturation, but the detailed transition processes remain obscure. Here, we examine spatiotemporal assembly dynamics of various Golgi/TGN-resident proteins in budding yeast by high-speed and high-resolution spinning-disk confocal microscopy. The Golgi–TGN transition gradually proceeds via at least three successive stages: the ‘Golgi stage’ where glycosylation occurs; the ‘early TGN stage’, which receives retrograde traffic; and the ‘late TGN stage’, where transport carriers are produced. During the stage transition periods, earlier and later markers are often compartmentalized within a cisterna. Furthermore, for the late TGN stage, various types of coat/adaptor proteins exhibit distinct assembly patterns. Taken together, our findings characterize the identity of the TGN as a membrane compartment that is structurally and functionally distinguishable from the Golgi.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The TGN displays two sub-stages of maturation: ‘early TGN’, when retrograde traffic is received, and ‘late TGN’, when transport carriers are produced. At the late TGN, various coat/adaptor proteins exhibit distinct assembly dynamics.

Meeting report - Cellular gateways: expanding the role of endocytosis in plant development

The occasion of The Company of Biologists' workshop 'Cellular gateways: expanding the role of endocytosis in plant development' on 22-25 April 2018, at Wiston House, an Elizabethan mansion in West Sussex, England, witnessed stimulating and lively discussions on the mechanism and functions of endocytosis in plant cells. The workshop was organized by Jenny Russinova, Daniël Van Damme (both VIB/University of Ghent, Belgium) and Takashi Ueda (National Institute for Basic Biology, Okazaki, Japan), and aimed to bridge the gap in knowledge about the endocytic machinery and its cargos in the plant field.

Use of Brefeldin A and Wortmannin to Dissect Post-Golgi Organelles Related to Vacuolar Transport in Arabidopsis thaliana

Eukaryotic cells comprise various organelles surrounded by the membrane. Each organelle is characterized by unique proteins and lipids and has its own specific functions. Single membrane-bounded organelles, including the Golgi apparatus, endosomes, and vacuoles are connected by membrane trafficking. Identifying the organelle localization of a protein of interest is essential for determining the proteins physiological functions. Here, we describe methods for determining protein subcellular localization using the inhibitors brefeldin A and wortmannin in Arabidopsis thaliana.

Merging roads: chemical tools and cell biology to study unconventional protein secretion

The endomembrane trafficking network is highly complex and dynamic, with both conventional and so-called unconventional routes which are in essence recently discovered pathways that are poorly understood in plants. One approach to dissecting endomembrane pathways that we have pioneered is the use of chemical biology. Classical genetic manipulations often deal with indirect pleiotropic phenotypes resulting from the perturbation of key players of the trafficking routes. Many of these difficulties can be circumvented using small molecules to modify or disrupt the function or localization of key proteins regulating these pathways. In this review, we summarize how small molecules have been used as probes to define these pathways, and how they could be used to increase current knowledge of unconventional protein secretion pathways.