Golgins and GRASPs: holding the Golgi together

A structure-guided strategy to design Golgi apparatus-targeted type-I/II aggregation-induced emission photosensitizers for efficient photodynamic therapy

The Golgi apparatus (GA) is a vital target for anticancer therapy due to its sensitivity against reactive oxygen species (ROS)-induced oxidative stress that could lead to cell death. In this study, we designed a series of aggregation-induced emission (AIE)-based photosensitizers (TPAPyTZ, TPAPyTC, TPAPyTM, and TPAPyTI) carrying different ROS with selective GA-targeted ability. The in vitro study showed that TPAPyTZ and TPAPyTC displayed strong AIE characteristics, robust type-I/II ROS production capabilities, specific GA-targeted, high photostability, and high imaging quality. The cell-uptake of TPAPyTZ was found primarily through an energy-dependent caveolae/raft-mediated endocytosis pathway. Remarkably, TPAPyTZ induced GA-oxidative stress, leading to GA fragmentation, downregulation of GM130 expression, and activation of mitochondria caspase-related apoptosis during photodynamic therapy (PDT). In vivo experiments revealed that TPAPyTZ significantly inhibited tumor proliferation under lower-intensity white light irradiation with minimal side effects. Overall, our work presents a promising strategy for designing AIEgens for fluorescence imaging-guided PDT. Additionally, it enriched the collection of GA-targeted leads for the development of cancer theranostics capable of visualizing dynamic changes in the GA during cancer cell apoptosis, which could potentially enable early diagnosis applications in the future. STATEMENT OF SIGNIFICANCE: AIE luminogens (AIEgens) are potent phototheranostic agents that can exhibit strong fluorescence emission and enhance ROS production in the aggregate states. In this study, through the precise design of photosensitizers with four different electron-acceptors, we constructed a series of potent AIEgens (TPAPyTZ, TPAPyTC, TPAPyTM, and TPAPyTI) with strong fluorescence intensity and ROS generation capacity. Among them, TPAPyTZ with an extended π-conjugation displayed the strongest ROS generation ability and anti-tumor activity, resulting in an 88 % reduction in tumor weight. Our studies revealed that the enhanced activity of TPAPyTZ may be due to its unique Golgi apparatus (GA)-targeted ability, which causes GA oxidative stress followed by effective cancer cell apoptosis. This unique GA-targeted feature of TPAPyTZ remains rare in the reported AIEgens, which mainly target organelles such as lysosome, mitochondria, and cell membrane. The successful design of a GA-targeted and potent AIEgen could enrich the collection of GA-targeted luminogens, providing a lead theranostic for the further development of fluorescence imaging-guided PDT, and serving as a tool to explore the potential mechanism and discover new GA-specific drug targets.

Porcine reproductive and respiratory syndrome virus triggers Golgi apparatus fragmentation-mediated autophagy to facilitate viral self-replication

Macroautophagy/autophagy is a cellular degradation and recycling process that maintains the homeostasis of organisms. A growing number of studies have reported that autophagy participates in infection by a variety of viruses. Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe financial losses to the global swine industry. Although much research has shown that PRRSV triggers autophagy for its own benefits, the exact molecular mechanisms involved in PRRSV-triggered autophagy remain to be fully elucidated. In the current study, we demonstrated that PRRSV infection significantly induced Golgi apparatus (GA) fragmentation, which promoted autophagy to facilitate viral self-replication. Mechanistically, PRRSV nonstructural protein 2 was identified to interact with and degrade the Golgi reassembly and stacking protein 65 dependent on its papain-like cysteine protease 2 activity, resulting in GA fragmentation. Upon GA fragmentation, GA-resident Ras-like protein in brain 2 was disassociated from Golgi matrix protein 130 and subsequently bound to unc-51 like autophagy activating kinase 1 (ULK1), which enhanced phosphorylation of ULK1 and promoted autophagy. Taken together, all these results expand the knowledge of PRRSV-triggered autophagy as well as PRRSV pathogenesis to support novel potential avenues for prevention and control of the virus. More importantly, these results provide the detailed mechanism of GA fragmentation-mediated autophagy, deepening the understanding of autophagic processes.IMPORTANCEPorcine reproductive and respiratory syndrome virus (PRRSV) infection results in a serious swine disease affecting pig farming worldwide. Despite that numerous studies have shown that PRRSV triggers autophagy for its self-replication, how PRRSV induces autophagy is incompletely understood. Here, we identify that PRRSV Nsp2 degrades GRASP65 to induce GA fragmentation, which dissociates RAB2 from GM130 and activates RAB2-ULK1-mediated autophagy to enhance viral replication. This work expands our understanding of PRRSV-induced autophagy and PRRSV replication, which is beneficial for anti-viral drug development.

Visualizing Reversible Cisternal Stacking in Budding Yeast Pichia pastoris

Cisternal stacking is reversible, initiated at the "cis" side of the Golgi, and gets undone at the "trans" side in a continuous cycle in tune with the cisternal maturation. TGN peeling is a hallmark of such reversible cisternal stacking, but its visualization is challenging. In wild-type cells, TGN peeling of Golgi stack happens at a lower frequency, but the event itself occurs very rapidly, making it difficult to detect by microscopy. However, we have documented that TGN peeling becomes frequent in mutants of factors that play a role in reversible cisternal stacking, such as the GRIP domain Golgin PpImh1, Arl3, or Arl1 GTPase. In the present context, we describe the quantitative live microscopic methodology to visualize the TGN peeling effect in Pichia pastoris.

Retro-2 alters Golgi structure

Retro-2 directly interacts with an ER exit site protein, Sec16A, inhibiting ER exit of a Golgi tSNARE, Syntaxin5, which results in rapid re-distribution of Syntaxin5 to the ER. Recently, it was shown that SARS-CoV-2 infection disrupts the Golgi apparatus within 6–12 h, while its replication was effectively inhibited by Retro-2 in cultured human lung cells. Yet, exactly how Retro-2 may influence ultrastructure of the Golgi apparatus have not been thoroughly investigated. In this study, we characterized the effect of Retro-2 treatment on ultrastructure of the Golgi apparatus using electron microscopy and EM tomography. Our initial results on protein secretion showed that Retro-2 treatment does not significantly influence secretion of either small or large cargos. Ultra-structural study of the Golgi, however, revealed rapid accumulation of COPI-like vesicular profiles in the perinuclear area and a partial disassembly of the Golgi stack under electron microscope within 3–5 h, suggesting altered Golgi organization in these cells. Retro-2 treatment in cells depleted of GRASP65/55, the two well-known Golgi structural proteins, induced complete and rapid disassembly of the Golgi into individual cisterna. Taken together, these results suggest that Retro-2 profoundly alters Golgi structure to a much greater extent than previously anticipated.

An Update on the Key Factors Required for Plant Golgi Structure Maintenance

Plant Golgi apparatus serves as the central station of the secretory pathway and is the site where protein modification and cell wall matrix polysaccharides synthesis occur. The polarized and stacked cisternal structure is a prerequisite for Golgi function. Our understanding of Golgi structure maintenance and trafficking are largely obtained from mammals and yeast, yet, plant Golgi has many different aspects. In this review, we summarize the key players in Golgi maintenance demonstrated by genetic studies in plants, which function in ER-Golgi, intra-Golgi and post-Golgi transport pathways. Among these, we emphasize on players in intra-Golgi trafficking.

Golgi apparatus-targeted aggregation-induced emission luminogens for effective cancer photodynamic therapy

Golgi apparatus (GA) oxidative stress induced by in situ reactive oxygen species (ROS) could severely damage the morphology and function of GA, which may open up an avenue for effective photodynamic therapy (PDT). However, due to the lack of effective design strategy, photosensitizers (PSs) with specific GA targeting ability are in high demand and yet quite challenging. Herein, we report an aggregation-induced emission luminogen (AIEgen) based PS (TPE-PyT-CPS) that can effectively target the GA via caveolin/raft mediated endocytosis with a Pearson correlation coefficient up to 0.98. Additionally, the introduction of pyrene into TPE-PyT-CPS can reduce the energy gap between the lowest singlet state (S₁) and the lowest triplet state (T₁) (ΔE_ST) and exhibits enhanced singlet oxygen generation capability. GA fragmentation and cleavage of GA proteins (p115/GM130) are observed upon light irradiation. Meanwhile, the apoptotic pathway is activated through a crosstalk between GA oxidative stress and mitochondria in HeLa cells. More importantly, GA targeting TPE-T-CPS show better PDT effect than its non-GA-targeting counterpart TPE-PyT-PS, even though they possess very close ROS generation rate. This work provides a strategy for the development of PSs with specific GA targeting ability, which is of great importance for precise and effective PDT.

Aggregation induced emission luminogen (AIEgen) based photosensitizers (PSs) have been developed for photodynamic cancer therapy. Here the authors report a series of AIEgen-based PSs that selectively target the Golgi apparatus, showing enhanced singlet oxygen generation and photodynamic therapy performance in cancer models.

Tankyrase-1-mediated degradation of Golgin45 regulates glycosyltransferase trafficking and protein glycosylation in Rab2-GTP-dependent manner

Altered glycosylation plays an important role during development and is also a hallmark of increased tumorigenicity and metastatic potentials of several cancers. We report here that Tankyrase-1 (TNKS1) controls protein glycosylation by Poly-ADP-ribosylation (PARylation) of a Golgi structural protein, Golgin45, at the Golgi. TNKS1 is a Golgi-localized peripheral membrane protein that plays various roles throughout the cell, ranging from telomere maintenance to Glut4 trafficking. Our study indicates that TNKS1 localization to the Golgi apparatus is mediated by Golgin45. TNKS1-dependent control of Golgin45 protein stability influences protein glycosylation, as shown by Glycomic analysis. Further, FRAP experiments indicated that Golgin45 protein level modulates Golgi glycosyltransferease trafficking in Rab2-GTP-dependent manner. Taken together, these results suggest that TNKS1-dependent regulation of Golgin45 may provide a molecular underpinning for altered glycosylation at the Golgi during development or oncogenic transformation.

The Role of GM130 in Nervous System Diseases

Golgi matrix protein 130 (GM130) is a Golgi-shaping protein located on the cis surface of the Golgi apparatus (GA). It is one of the most studied Golgin proteins so far. Its biological functions are involved in many aspects of life processes, including mitosis, autophagy, apoptosis, cell polarity, and directed migration at the cellular level, as well as intracellular lipid and protein transport, microtubule formation and assembly, lysosome function maintenance, and glycosylation modification. Mutation inactivation or loss of expression of GM130 has been detected in patients with different diseases. GM130 plays an important role in the development of the nervous system, but the studies on it are limited. This article reviewed the current research progress of GM130 in nervous system diseases. It summarized the physiological functions of GM130 in the occurrence and development of Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), microcephaly (MCPH), sepsis associated encephalopathy (SAE), and Ataxia, aiming to provide ideas for the further study of GM130 in nervous system disease detection and treatment.

Rapid degradation of GRASP55 and GRASP65 reveals their immediate impact on the Golgi structure

GRASP55 and GRASP65 have been implicated in stacking of Golgi cisternae and lateral linking of stacks within the Golgi ribbon. However, RNAi or gene knockout approaches to dissect their respective roles have often resulted in conflicting conclusions. Here, we gene-edited GRASP55 and/or GRASP65 with a degron tag in human fibroblasts, allowing for induced rapid degradation by the proteasome. We show that acute depletion of either GRASP55 or GRASP65 does not affect the Golgi ribbon, while chronic degradation of GRASP55 disrupts lateral connectivity of the ribbon. Acute double depletion of both GRASPs coincides with the loss of the vesicle tethering proteins GM130, p115, and Golgin-45 from the Golgi and compromises ribbon linking. Furthermore, GRASP55 and/or GRASP65 is not required for maintaining stacks or de novo assembly of stacked cisternae at the end of mitosis. These results demonstrate that both GRASPs are dispensable for Golgi stacking but are involved in maintaining the integrity of the Golgi ribbon together with GM130 and Golgin-45.

GM130 regulates pulmonary surfactant protein secretion in alveolar type II cells

Pulmonary surfactant is a lipid-protein complex secreted by alveolar type II epithelial cells and is essential for the maintenance of the delicate structure of mammalian alveoli to promote efficient gas exchange across the air-liquid barrier. The Golgi apparatus plays an important role in pulmonary surfactant modification and secretory trafficking. However, the physiological function of the Golgi apparatus in the transport of pulmonary surfactants is unclear. In the present study, deletion of GM130, which encodes for a matrix protein of the cis-Golgi cisternae, was shown to induce the disruption of the Golgi structure leading to impaired secretion of lung surfactant proteins and lipids. Specifically, the results of in vitro and in vivo analysis indicated that the loss of GM130 resulted in trapping of Sftpa in the endoplasmic reticulum, Sftpb and Sftpc accumulation in the Golgi apparatus, and an increase in the compensatory secretion of Sftpd. Moreover, global and epithelial-specific GM130 knockout in mice resulted in an enlargement of alveolar airspace and an increase in alveolar epithelial autophagy; however, surfactant repletion partially rescued the enlarged airspace defects in GM130-deficient mice. Therefore, our results demonstrate that GM130 and the mammalian Golgi apparatus play a critical role in the control of surfactant protein secretion in pulmonary epithelial cells.

Knockout of RSN1, TVP18 or CSC1-2 causes perturbation of Golgi cisternae in Pichia pastoris

The structural organization of the Golgi stacks in mammalian cells is intrinsically linked to function, including glycosylation, but the role of morphology is less clear in lower eukaryotes. Here we investigated the link between the structural organization of the Golgi and secretory pathway function using Pichia pastoris as a model system. To unstack the Golgi cisternae, we disrupted 18 genes encoding proteins in the secretory pathway without loss of viability. Using biosensors, confocal microscopy and transmission electron microscopy we identified three strains with irreversible perturbations in the stacking of the Golgi cisternae, all of which had disruption in genes that encode proteins with annotated function as or homology to calcium/calcium permeable ion channels. Despite this, no variation in the secretory pathway for ER size, whole cell glycomics or recombinant protein glycans was observed. Our investigations showed the robust nature of the secretory pathway in P. pastoris and suggest that Ca²⁺ concentration, homeostasis or signalling may play a significant role for Golgi stacking in this organism and should be investigated in other organisms.

Nonredundant Roles of GRASP55 and GRASP65 in the Golgi Apparatus and Beyond

It has been demonstrated that two Golgi stacking proteins, GRASP55 and GRASP65, self-interact to form trans-oligomers that tether adjacent Golgi membranes into stacks and ribbons in mammalian cells. This ensures proper functioning of the Golgi apparatus in protein trafficking and processing. More recently, GRASP proteins have drawn extensive attention from researchers due to their diverse and essential roles in and out of the Golgi in different organisms. In this review, we summarize their established roles in Golgi structure formation and function under physiological conditions. We then highlight the emerging and divergent roles for individual GRASP proteins, focusing on GRASP65 in cell migration and apoptosis and GRASP55 in unconventional protein secretion and autophagy under stress or pathological conditions.

Living on the edge: the role of Atgolgin-84A at the plant ER-Golgi interface

The plant Golgi apparatus is responsible for the processing of proteins received from the endoplasmic reticulum (ER) and their distribution to multiple destinations within the cell. Golgi matrix components, such as golgins, have been identified and suggested to function as putative tethering factors to mediate the physical connections between Golgi bodies and the ER network. Golgins are proteins anchored to the Golgi membrane by the C-terminus either through transmembrane domains or interaction with small regulatory GTPases. The golgin N-terminus contains long coiled-coil domains, which consist of a number of α-helices wrapped around each other to form a structure similar to a rope being made from several strands, reaching into the cytoplasm. In animal cells, golgins are also implicated in specific recognition of cargo at the Golgi.Here, we investigate the plant golgin Atgolgin-84A for its subcellular localization and potential role as a tethering factor at the ER-Golgi interface. For this, fluorescent fusions of Atgolgin-84A and an Atgolgin-84A truncation lacking the coiled-coil domains (Atgolgin-84AΔ1-557) were transiently expressed in tobacco leaf epidermal cells and imaged using high-resolution confocal microscopy. We show that Atgolgin-84A localizes to a pre-cis-Golgi compartment that is also labelled by one of the COPII proteins as well as by the tether protein AtCASP. Upon overexpression of Atgolgin-84A or its deletion mutant, transport between the ER and Golgi bodies is impaired and cargo proteins are redirected to the vacuole. LAY DESCRIPTION: The Golgi apparatus is a specialised compartment found in mammalian and plant cells. It is the post office of the cell and packages proteins into small membrane boxes for transport to their destination in the cell. The plant Golgi apparatus consist of many separate Golgi bodies and is responsible for the processing of proteins received from the endoplasmic reticulum (ER) and their distribution to multiple destinations within the cell. Specialised proteins called golgins have been suggested to tether Golgi bodies and the ER. Here we investigate the plant golgin Atgolgin-84A for its exact within the Golgi body and its potential role as a tethering factor at the ER-Golgi interface. For this, we have fused Atgolgin-84A with a fluorescent protein from jellyfish and we are producing this combination in tobacco leaf cells. This allows us to see the protein using laser microscopy. We show that Atgolgin-84A localises to a compartment between the ER and Golgi that is also labelled by the tether protein AtCASP. When Atgolgin-84A is produced in high amounts in the cell, transport between the ER and Golgi bodies is inhibited and proteins are redirected to the vacuole.

Disruption of the Golgi apparatus mediates zinc deficiency-induced impairment of cognitive function in mice

Zinc deficiency is reported to be a global problem that affects cognitive function. The mechanism underlying zinc deficiency-induced impairment of cognitive function is still obscure. In this study, we treated KM mice (Kun Ming mice) with zinc chelator TPEN (N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine) by i.p. injection. NOR (New Object Recognition) tests demonstrated that TPEN can impair the cognitive function of KM mice. Disruption of the GRASP55/Golgin45 complex, and even the Golgi apparatus, was also observed in hippocampus cells by TPEN treatment. Further investigation by IHF showed that enrichment of Aβ peptides occurs in neurons of the cerebral tissue of mice, suggesting that amyloidosis may mediate TPEN-induced impairment of cognitive function. This research not only clarifies that zinc plays an important role in Golgi organization in vivo, but also gives us a possible novel pathway underlying AD development.

Molecular Characterization and Immuno-Reactivity Patterns of a Novel Plasmodium falciparum Armadillo-Type Repeat Protein, PfATRP

Nearly half of the genes in the Plasmodium falciparum genome have not yet been functionally investigated. We used homology-based structural modeling to identify multiple copies of Armadillo repeats within one uncharacterized gene expressed during the intraerythrocytic stages, PF3D7_0410600, subsequently referred to as P. falciparum Armadillo-Type Repeat Protein (PfATRP). Soluble recombinant PfATRP was expressed in a bacterial expression system, purified to apparent homogeneity and the identity of the recombinant PfATRP was confirmed by mass spectrometry. Affinity-purified α-PfATRP rabbit antibodies specifically recognized the recombinant protein. Immunofluorescence assays revealed that α-PfATRP rabbit antibodies reacted with P. falciparum schizonts. Anti-PfATRP antibody exhibited peripheral staining patterns around the merozoites. Given the localization of PfATRP in merozoites, we tested for an egress phenotype during schizont arrest assays and demonstrated that native PfATRP is inaccessible on the surface of merozoites in intact schizonts. Dual immunofluorescence assays with markers for the inner membrane complex (IMC) and microtubules suggest partial colocalization in both asexual and sexual stage parasites. Using the soluble recombinant PfATRP in a screen of plasma samples revealed that malaria-infected children have naturally acquired PfATRP-specific antibodies.

Golgi organization is regulated by proteasomal degradation

The Golgi is a dynamic organelle whose correct assembly is crucial for cellular homeostasis. Perturbations in Golgi structure are associated with numerous disorders from neurodegeneration to cancer. However, whether and how dispersal of the Golgi apparatus is actively regulated under stress, and the consequences of Golgi dispersal, remain unknown. Here we demonstrate that 26S proteasomes are associated with the cytosolic surface of Golgi membranes to facilitate Golgi Apparatus-Related Degradation (GARD) and degradation of GM130 in response to Golgi stress. The degradation of GM130 is dependent on p97/VCP and 26S proteasomes, and required for Golgi dispersal. Finally, we show that perturbation of Golgi homeostasis induces cell death of multiple myeloma in vitro and in vivo, offering a therapeutic strategy for this malignancy. Taken together, this work reveals a mechanism of Golgi-localized proteasomal degradation, providing a functional link between proteostasis control and Golgi architecture, which may be critical in various secretion-related pathologies.

Correct Golgi assembly is important to cellular homeostasis but regulation of its structure under stress remains unclear. Here, the authors identify stress-induced degradation of GM130 by Golgi-localized 26S proteasomes, leading to Golgi dispersal.

Conformational flexibility of GRASPs and their constituent PDZ subdomains reveals structural basis of their promiscuous interactome

The Golgi complex is a central component of the secretory pathway, responsible for several critical cellular functions in eukaryotes. The complex is organized by the Golgi matrix that includes the Golgi reassembly and stacking protein (GRASP), which was shown to be involved in cisternae stacking and lateral linkage in metazoan. GRASPs also have critical roles in other processes, with an unusual ability to interact with several different binding partners. The conserved N terminus of the GRASP family includes two PSD-95, DLG, and ZO-1 (PDZ) domains. Previous crystallographic studies of orthologues suggest that PDZ1 and PDZ2 have similar conformations and secondary structure content. However, PDZ1 alone mediates nearly all interactions between GRASPs and their partners. In this work, NMR, synchrotron radiation CD, and molecular dynamics (MD) were used to examine the structure, flexibility, and stability of the two constituent PDZ domains. GRASP PDZs are structured in an unusual β₃ α₁ β₄ β₅ α₂ β₆ β₁ β₂ secondary structural arrangement and NMR data indicate that the PDZ1 binding pocket is formed by a stable β₂ -strand and a more flexible and unstable α₂ -helix, suggesting an explanation for the higher PDZ1 promiscuity. The conformational free energy profiles of the two PDZ domains were calculated using MD simulations. The data suggest that, after binding, the protein partner significantly reduces the conformational space that GRASPs can access by stabilizing one particular conformation, in a partner-dependent fashion. The structural flexibility of PDZ1, modulated by PDZ2, and the coupled, coordinated movement between the two PDZs enable GRASPs to interact with multiple partners, allowing them to function as promiscuous, multitasking proteins.

The Golgi apparatus and cell polarity: Roles of the cytoskeleton, the Golgi matrix, and Golgi membranes

Membrane trafficking plays a crucial role in cell polarity by directing lipids and proteins to specific subcellular locations in the cell and sustaining a polarized state. The Golgi apparatus, the master organizer of membrane trafficking, can be subdivided into three layers that play different mechanical roles: a cytoskeletal layer, the so-called Golgi matrix, and the Golgi membranes. First, the outer regions of the Golgi apparatus interact with cytoskeletal elements, mainly actin and microtubules, which shape, position, and orient the organelle. Closer to the Golgi membranes, a matrix of long coiled-coiled proteins not only selectively captures transport intermediates but also participates in signaling events during polarization of membrane trafficking. Finally, the Golgi membranes themselves serve as active signaling platforms during cell polarity events. We review here the recent findings that link the Golgi apparatus to cell polarity, focusing on the roles of the cytoskeleton, the Golgi matrix, and the Golgi membranes.

The golgin PpImh1 mediates reversible cisternal stacking in the Golgi of the budding yeast Pichia pastoris

The adhesive force for cisternal stacking of Golgi needs to be reversible - to be initiated and undone in a continuous cycle to keep up with the cisternal maturation. Microscopic evidence in support of such a reversible nature of stacking, in the form of 'TGN peeling,' has been reported in various species, suggesting a potential evolutionarily conserved mechanism. However, knowledge of such mechanism has remained sketchy. Here, we have explored this issue in the budding yeast Pichia pastoris which harbors stacked Golgi. We observed that deletion of GRIP domain golgin P. pastoris (Pp)IMH1 increases the peeling of late cisterna, causing unstacking of the Golgi stack. Our results suggest that the PpImh1 dimer mediates reversible stacking through a continuous association-dissociation cycle of its GRIP domain to the middle and late Golgi cisterna under the GTP hydrolysis-based regulation of Arl3-Arl1 GTPase cascade switch. The reversible cisternal stacking function of PpImh1 is independent of its vesicle-capturing function. Since GRIP domain proteins are conserved in plants, animals and fungi, it is plausible that this reversible mechanism of Golgi stacking is evolutionarily conserved.This article has an associated First Person interview with the first author of the paper.

Form and function of the Golgi apparatus: scaffolds, cytoskeleton and signalling

In addition to the classical functions of the Golgi in membrane transport and glycosylation, the Golgi apparatus of mammalian cells is now recognised to contribute to the regulation of a range of cellular processes, including mitosis, DNA repair, stress responses, autophagy, apoptosis and inflammation. These processes are often mediated, either directly or indirectly, by membrane scaffold molecules, such as golgins and GRASPs which are located on Golgi membranes. In many cases, these scaffold molecules also link the actin and microtubule cytoskeleton and influence Golgi morphology. An emerging theme is a strong relationship between the morphology of the Golgi and regulation of a variety of signalling pathways. Here, we review the molecular regulation of the morphology of the Golgi, especially the role of the golgins and other scaffolds in the interaction with the microtubule and actin networks. In addition, we discuss the impact of the modulation of the Golgi ribbon in various diseases, such as neurodegeneration and cancer, to the pathology of disease.

The Physiological Functions of the Golgin Vesicle Tethering Proteins

The golgins comprise a family of vesicle tethering proteins that act in a selective manner to tether transport vesicles at the Golgi apparatus. Tethering is followed by membrane fusion to complete the delivery of vesicle-bound cargo to the Golgi. Different golgins are localized to different regions of the Golgi, and their ability to selectively tether transport vesicles is important for the specificity of vesicle traffic in the secretory pathway. In recent years, our mechanistic understanding of golgin-mediated tethering has greatly improved. We are also beginning to appreciate how the loss of golgin function can impact upon physiological processes through the use of animal models and the study of human disease. These approaches have revealed that loss of a golgin causes tissue-restricted phenotypes, which can vary in severity and the cell types affected. In many cases, it is possible to attribute these phenotypes to a defect in vesicular traffic, although why certain tissues are sensitive to loss of a particular golgin is still, in most cases, unclear. Here, I will summarize recent progress in our understanding of golgins, focusing on the physiological roles of these proteins, as determined from animal models and the study of disease in humans. I will describe what these in vivo analyses have taught us, as well as highlight less understood aspects, and areas for future investigations.

Golgipathies in Neurodevelopment: A New View of Old Defects

The Golgi apparatus (GA) is involved in a whole spectrum of activities, from lipid biosynthesis and membrane secretion to the posttranslational processing and trafficking of most proteins, the control of mitosis, cell polarity, migration and morphogenesis, and diverse processes such as apoptosis, autophagy, and the stress response. In keeping with its versatility, mutations in GA proteins lead to a number of different disorders, including syndromes with multisystem involvement. Intriguingly, however, > 40% of the GA-related genes known to be associated with disease affect the central or peripheral nervous system, highlighting the critical importance of the GA for neural function. We have previously proposed the term "Golgipathies" in relation to a group of disorders in which mutations in GA proteins or their molecular partners lead to consequences for brain development, in particular postnatal-onset microcephaly (POM), white-matter defects, and intellectual disability (ID). Here, taking into account the broader role of the GA in the nervous system, we refine and enlarge this emerging concept to include other disorders whose symptoms may be indicative of altered neurodevelopmental processes, from neurogenesis to neuronal migration and the secretory function critical for the maturation of postmitotic neurons and myelination.

A SEPT1-based scaffold is required for Golgi integrity and function

Compartmentalization of membrane transport and signaling processes is of pivotal importance to eukaryotic cell function. While plasma membrane compartmentalization and dynamics are well known to depend on the scaffolding function of septin GTPases, the roles of septins at intracellular membranes have remained largely elusive. Here, we show that the structural and functional integrity of the Golgi depends on its association with a septin 1 (SEPT1)-based scaffold, which promotes local microtubule nucleation and positioning of the Golgi. SEPT1 function depends on the Golgi matrix protein GM130 (also known as GOLGA2) and on centrosomal proteins, including CEP170 and components of γ-tubulin ring complex (γ-Turc), to facilitate the perinuclear concentration of Golgi membranes. Accordingly, SEPT1 depletion triggers a massive fragmentation of the Golgi ribbon, thereby compromising anterograde membrane traffic at the level of the Golgi.

Golgi Structure and Function in Health, Stress, and Diseases

The Golgi apparatus is a central intracellular membrane-bound organelle with key functions in trafficking, processing, and sorting of newly synthesized membrane and secretory proteins and lipids. To best perform these functions, Golgi membranes form a unique stacked structure. The Golgi structure is dynamic but tightly regulated; it undergoes rapid disassembly and reassembly during the cell cycle of mammalian cells and is disrupted under certain stress and pathological conditions. In the past decade, significant amount of effort has been made to reveal the molecular mechanisms that regulate the Golgi membrane architecture and function. Here we review the major discoveries in the mechanisms of Golgi structure formation, regulation, and alteration in relation to its functions in physiological and pathological conditions to further our understanding of Golgi structure and function in health and diseases.

DjA1 maintains Golgi integrity via interaction with GRASP65

In mammalian cells, the Golgi reassembly stacking protein of 65 kDa (GRASP65) has been implicated in both Golgi stacking and ribbon linking by forming trans-oligomers. To better understand its function and regulation, we used biochemical methods to identify the DnaJ homolog subfamily A member 1 (DjA1) as a novel GRASP65-binding protein. In cells, depletion of DjA1 resulted in Golgi fragmentation, short and improperly aligned cisternae, and delayed Golgi reassembly after nocodazole washout. In vitro, immunodepletion of DjA1 from interphase cytosol reduced its activity to enhance GRASP65 oligomerization and Golgi membrane fusion, while adding purified DjA1 enhanced GRASP65 oligomerization. DjA1 is a cochaperone of Heat shock cognate 71-kDa protein (Hsc70), but the activity of DjA1 in Golgi structure formation is independent of its cochaperone activity or Hsc70, rather, through DjA1-GRASP65 interaction to promote GRASP65 oligomerization. Thus, DjA1 interacts with GRASP65 to enhance Golgi structure formation through the promotion of GRASP65 trans-oligomerization.

Intersectin-1 interacts with the golgin GCC88 to couple the actin network and Golgi architecture

The maintenance of the Golgi ribbon relies on a dynamic balance between the actin and microtubule networks; however, the pathways controlling actin networks remain poorly defined. Previously, we showed that the trans-Golgi network (TGN) membrane tether/golgin, GCC88, modulates the Golgi ribbon architecture. Here, we show that dispersal of the Golgi ribbon by GCC88 is dependent on actin and the involvement of nonmuscle myosin IIA. We have identified the long isoform of intersectin-1 (ITSN-1), a guanine nucleotide exchange factor for Cdc42, as a novel Golgi component and an interaction partner of GCC88 responsible for mediating the actin-dependent dispersal of the Golgi ribbon. We show that perturbation of Golgi morphology by changes in membrane flux, mediated by silencing the retromer subunit Vps26, or in a model of neurodegeneration, induced by Tau overexpression, are also dependent on the ITSN-1-GCC88 interaction. Overall, our study reveals a role for a TGN golgin and ITSN-1 in linking to the actin cytoskeleton and regulating the balance between a compact Golgi ribbon and a dispersed Golgi, a pathway with relevance to pathophysiological conditions.

The Golgi architecture and cell sensing

An array of signalling molecules are located at the Golgi apparatus, including phosphoinositides, small GTPases, kinases, and phosphatases, which are linked to multiple signalling pathways. Initially considered to be associated predominantly with membrane trafficking, signalling pathways at the Golgi are now recognised to regulate a diverse range of higher-order functions. Many of these signalling pathways are influenced by the architecture of the Golgi. In vertebrate cells, the Golgi consists of individual stacks fused together into a compact ribbon structure and the function of this ribbon structure has been enigmatic. Notably, recent advances have identified a role for the Golgi ribbon in regulation of cellular processes. Fragmentation of the Golgi ribbon results in modulation of many signalling pathways. Various diseases and disorders, including cancer and neurodegeneration, are associated with the loss of the Golgi ribbon and the appearance of a dispersed fragmented Golgi. Here, we review the emerging theme of the Golgi as a cell sensor and highlight the relationship between the morphological status of the Golgi in vertebrate cells and the modulation of signalling networks.

Targeted protein unfolding uncovers a Golgi-specific transcriptional stress response

In eukaryotic cells, organelle-specific stress-response mechanisms are vital for maintaining cellular homeostasis. The Golgi apparatus, an essential organelle of the secretory system, is the major site of protein modification and sorting within a cell and functions as a platform for spatially regulated signaling. Golgi homeostasis mechanisms that regulate organelle structure and ensure precise processing and localization of protein substrates remain poorly understood. Using a chemical biology strategy to induce protein unfolding, we uncover a Golgi-specific transcriptional response. An RNA-sequencing profile of this stress response compared with the current state-of-the-art Golgi stressors, nigericin and xyloside, demonstrates the enhanced precision of Golgi targeting achieved with our system. The data set further reveals previously uncharacterized genes that we find to be essential for Golgi structural integrity. These findings highlight the Golgi's ability to sense misfolded proteins and establish new aspects of Golgi autoregulation.

α‑synuclein induces apoptosis of astrocytes by causing dysfunction of the endoplasmic reticulum‑Golgi compartment

Although previous work has demonstrated that the overexpression of wild-type or mutant α-synuclein (α-syn) can induce cell death via a number of different mechanisms, including oxidative stress, dysfunction of the ubiquitin-proteasome degradation system, mitochondrial damage and endoplasmic reticulum (ER) stress, research interest has primarily focused on neurons. However, there is accumulating evidence that suggests that astrocytes may be involved in the earliest changes, as well as the progression of Parkinson's disease (PD), though the role of α-syn in astrocytes has not been widely studied. In the present study, it was revealed that the mutant α-syn (A53T and A30P) in astrocytes triggered ER stress via the protein kinase RNA-like ER kinase/eukaryotic translation initiation factor 2α signaling pathway. Astrocyte apoptosis was induced through a CCAAT-enhancer-binding protein homologous protein-mediated pathway. In addition, Golgi fragmentation was observed in the process. On the other hand, it was also demonstrated, in a primary neuronal-astroglial co-culture system, that the overexpression of α-syn significantly decreased the levels of glia-derived neurotrophic factor (GDNF) and partly inhibited neurite outgrowth. Although direct evidence is currently lacking, it was proposed that dysfunction of the ER-Golgi compartment in astrocytes overexpressing α-syn may lead to a decline of GDNF levels, which in turn would suppress neurite outgrowth. Taken together, the results of the present study offer further insights into the pathogenesis of PD from the perspective of astrocytes, which may provide novel strategies for the diagnosis and treatment of PD in the future.

Golgi tethering factor golgin-97 suppresses breast cancer cell invasiveness by modulating NF-κB activity

Background

Golgin-97 is a tethering factor in the trans-Golgi network (TGN) and is crucial for vesicular trafficking and maintaining cell polarity. However, the significance of golgin-97 in human diseases such as cancer remains unclear.

Methods

We searched for a potential role of golgin-97 in cancers using Kaplan-Meier Plotter (http://kmplot.com) and Oncomine (www.oncomine.org) datasets. Specific functions of golgin-97 in migration and invasion were examined in golgin-97-knockdown and golgin-97-overexpressing cells. cDNA microarray, pathway analysis and qPCR were used to identify gene profiles regulated by golgin-97. The role of golgin-97 in NF-κB signaling pathway was examined by using subcellular fractionation, luciferase reporter assay, western blot analysis and immunofluorescence assay (IFA).

Results

We found that low expression of golgin-97 correlated with poor overall survival of cancer patients and was associated with invasiveness in breast cancer cells. Golgin-97 knockdown promoted cell migration and invasion, whereas re-expression of golgin-97 restored the above phenotypes in breast cancer cells. Microarray and pathway analyses revealed that golgin-97 knockdown induced the expression of several invasion-promoting genes that were transcriptionally regulated by NF-κB p65. Mechanistically, golgin-97 knockdown significantly reduced IκBα protein levels and activated NF-κB, whereas neither IκBα levels nor NF-κB activity was changed in TGN46- or GCC185-knockdown cells. Conversely, golgin-97 overexpression suppressed NF-κB activity and restored the levels of IκBα in golgin-97-knockdown cells. Interestingly, the results of Golgi-disturbing agent treatment revealed that the loss of Golgi integrity was not involved in the NF-κB activation induced by golgin-97 knockdown. Moreover, both TGN-bound and cytosolic golgin-97 inhibited NF-κB activation, indicating that golgin-97 functions as an NF-κB suppressor regardless of its subcellular localization.

Conclusion

Our results collectively demonstrate a novel and suppressive role of golgin-97 in cancer invasiveness. We also provide a new avenue for exploring the relationship between the TGN, golgin-97 and NF-κB signaling in tumor progression.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0230-5) contains supplementary material, which is available to authorized users.

A sophisticated, differentiated Golgi in the ancestor of eukaryotes

BACKGROUND

The Golgi apparatus is a central meeting point for the endocytic and exocytic systems in eukaryotic cells, and the organelle's dysfunction results in human disease. Its characteristic morphology of multiple differentiated compartments organized into stacked flattened cisternae is one of the most recognizable features of modern eukaryotic cells, and yet how this is maintained is not well understood. The Golgi is also an ancient aspect of eukaryotes, but the extent and nature of its complexity in the ancestor of eukaryotes is unclear. Various proteins have roles in organizing the Golgi, chief among them being the golgins.

RESULTS

We address Golgi evolution by analyzing genome sequences from organisms which have lost stacked cisternae as a feature of their Golgi and those that have not. Using genomics and immunomicroscopy, we first identify Golgi in the anaerobic amoeba Mastigamoeba balamuthi. We then searched 87 genomes spanning eukaryotic diversity for presence of the most prominent proteins implicated in Golgi structure, focusing on golgins. We show some candidates as animal specific and others as ancestral to eukaryotes.

CONCLUSIONS

None of the proteins examined show a phyletic distribution that correlates with the morphology of stacked cisternae, suggesting the possibility of stacking as an emergent property. Strikingly, however, the combination of golgins conserved among diverse eukaryotes allows for the most detailed reconstruction of the organelle to date, showing a sophisticated Golgi with differentiated compartments and trafficking pathways in the common eukaryotic ancestor.

The Golgi ribbon in mammalian cells negatively regulates autophagy by modulating mTOR activity

In vertebrates, individual Golgi stacks are joined into a compact ribbon structure; however, the relevance of a ribbon structure has been elusive. Here, we exploit the finding that the membrane tether of the trans-Golgi network, GCC88 (encoded by GCC1), regulates the balance between Golgi mini-stacks and the Golgi ribbon. Loss of Golgi ribbons in stable cells overexpressing GCC88 resulted in compromised mechanistic target of rapamycin (mTOR) signaling and a dramatic increase in LC3-II-positive autophagosomes, whereas RNAi-mediated depletion of GCC88 restored the Golgi ribbon and reduced autophagy. mTOR was absent from dispersed Golgi mini-stacks whereas recruitment of mTOR to lysosomes was unaffected. We show that the Golgi ribbon is a site for localization and activation of mTOR, a process dependent on the ribbon structure. We demonstrate a strict temporal sequence of fragmentation of Golgi ribbon, loss of Golgi mTOR and subsequent increased autophagy. Golgi ribbon fragmentation has been reported in various neurodegenerative diseases and we demonstrate the potential relevance of our findings in neuronal cells using a model of neurodegeneration. Overall, this study highlights a role for the Golgi ribbon in pathways central to cellular homeostasis.This article has an associated First Person interview with the first author of the paper.

At the ends of their tethers! How coiled-coil proteins capture vesicles at the Golgi

Cells face a complex problem: how to transfer lipids and proteins between membrane compartments in an organized, timely fashion. Indeed, many thousands of membrane and secretory proteins must traffic out of the ER to different organelles to function, while others are retrieved from the plasma membrane having fulfilled their roles [Nat. Rev. Mol. Cell Biol. (2013) 14, 382-392]. This process is highly dynamic and failure to target cargo accurately leads to catastrophic consequences for the cell, as is clear from the numerous human diseases associated with defects in membrane trafficking [Int. J. Mol. Sci. (2013) 14, 18670-18681; Traffic (2000) 1, 836-851]. How then does the cell organize this enormous transfer of material in its crowded internal environment? And how specifically do vesicles carrying proteins and lipids recognize and fuse with the correct compartment?

The Function of the Golgi Ribbon Structure - An Enduring Mystery Unfolds!

The Golgi apparatus in vertebrate cells consists of individual Golgi stacks fused together in a continuous ribbon structure. The ribbon structure per se is not required to mediate the classical functions of this organelle and the relevance of the "ribbon" structure has been a mystery since first identified ultrastructurally in the 1950s. Recent advances recognize a role for the Golgi apparatus in a range of cellular processes, some mediated by signaling networks which are regulated at the Golgi. Here we review the cellular processes and signaling events regulated by the Golgi apparatus and, in particular, explore an emerging theme that the ribbon structure of the Golgi contributes directly to the regulation of these higher order functions.

Stacks off tracks: a role for the golgin AtCASP in plant endoplasmic reticulum-Golgi apparatus tethering

The plant Golgi apparatus modifies and sorts incoming proteins from the endoplasmic reticulum (ER) and synthesizes cell wall matrix material. Plant cells possess numerous motile Golgi bodies, which are connected to the ER by yet to be identified tethering factors. Previous studies indicated a role for cis-Golgi plant golgins, which are long coiled-coil domain proteins anchored to Golgi membranes, in Golgi biogenesis. Here we show a tethering role for the golgin AtCASP at the ER-Golgi interface. Using live-cell imaging, Golgi body dynamics were compared in Arabidopsis thaliana leaf epidermal cells expressing fluorescently tagged AtCASP, a truncated AtCASP-ΔCC lacking the coiled-coil domains, and the Golgi marker STtmd. Golgi body speed and displacement were significantly reduced in AtCASP-ΔCC lines. Using a dual-colour optical trapping system and a TIRF-tweezer system, individual Golgi bodies were captured in planta. Golgi bodies in AtCASP-ΔCC lines were easier to trap and the ER-Golgi connection was more easily disrupted. Occasionally, the ER tubule followed a trapped Golgi body with a gap, indicating the presence of other tethering factors. Our work confirms that the intimate ER-Golgi association can be disrupted or weakened by expression of truncated AtCASP-ΔCC and suggests that this connection is most likely maintained by a golgin-mediated tethering complex.

Golga5 is dispensable for mouse embryonic development and postnatal survival

Golgins are a family of coiled-coil proteins located at the cytoplasmic surface of the Golgi apparatus and have been implicated in maintaining Golgi structural integrity through acting as tethering factors for retrograde vesicle transport. Whereas knockdown of several individual golgins in cultured cells caused Golgi fragmentation and disruption of vesicle trafficking, analysis of mutant mouse models lacking individual golgins have discovered tissue-specific developmental functions. Recently, homozygous loss of function of GOLGA2, of which previous in vitro studies suggested an essential role in maintenance of Golgi structure and in mitosis, has been associated with a neuromuscular disorder in human patients, which highlights the need for understanding the developmental roles of the golgins in vivo. We report here generation of Golga5-deficient mice using CRISPR/Cas9-mediated genome editing. Although knockdown studies in cultured cells have implicated Golga5 in maintenance of Golgi organization, we show that Golga5 is not required for mouse embryonic development, postnatal survival, or fertility. Moreover, whereas Golga5 is structurally closely related to Golgb1, we show that inactivation of Golga5 does not enhance the severity of developmental defects in Golgb1-deficient mice. The Golga5-deficient mice enable further investigation of the roles and functional specificity of golgins in development and diseases.

Multiple activities of Arl1 GTPase in the trans-Golgi network

ADP-ribosylation factors (Arfs) and ADP-ribosylation factor-like proteins (Arls) are highly conserved small GTPases that function as main regulators of vesicular trafficking and cytoskeletal reorganization. Arl1, the first identified member of the large Arl family, is an important regulator of Golgi complex structure and function in organisms ranging from yeast to mammals. Together with its effectors, Arl1 has been shown to be involved in several cellular processes, including endosomal trans-Golgi network and secretory trafficking, lipid droplet and salivary granule formation, innate immunity and neuronal development, stress tolerance, as well as the response of the unfolded protein. In this Commentary, we provide a comprehensive summary of the Arl1-dependent cellular functions and a detailed characterization of several Arl1 effectors. We propose that involvement of Arl1 in these diverse cellular functions reflects the fact that Arl1 is activated at several late-Golgi sites, corresponding to specific molecular complexes that respond to and integrate multiple signals. We also provide insight into how the GTP-GDP cycle of Arl1 is regulated, and highlight a newly discovered mechanism that controls the sophisticated regulation of Arl1 activity at the Golgi complex.

Phosphorylation of Golgi Peripheral Membrane Protein Grasp65 Is an Integral Step in the Formation of the Human Cytomegalovirus Cytoplasmic Assembly Compartment

Human cytomegalovirus (HCMV) is the largest member of the Herpesviridae and represents a significant cause of disease. During virus replication, HCMV alters cellular functions to facilitate its replication, including significant reorganization of the secretory and endocytic pathways of the infected cell. A defining morphologic change of the infected cell is the formation of a membranous structure in the cytoplasm that is designated the virion assembly compartment (AC), which consists of virion structural proteins surrounded by cellular membranes. The loss of normal Golgi compartment morphology and its relocalization from a juxtanuclear ribbonlike structure to a series of concentric rings on the periphery of the AC represents a readily recognized reorganization of cellular membranes in the HCMV-infected cell. Although trafficking of viral proteins to this compartment is required for the assembly of infectious virions, the functional significance of the reorganization of intracellular membranes like the Golgi membranes into the AC in the assembly of infectious virus remains understudied. In this study, we determined that Golgi membrane ribbon fragmentation increased during the early cytoplasmic phase of virion assembly and that Golgi membrane fragmentation in infected cells was dependent on the phosphorylation of an integral cis-Golgi protein, Grasp65. Inhibition of Golgi membrane fragmentation and of its reorganization into the AC resulted in decreased production of infectious particles and alteration of the incorporation of an essential protein into the envelope of the mature virion. These results demonstrated the complexity of the virus-host cell interactions required for efficient assembly of this large DNA virus.

The human cytomegalovirus (HCMV)-induced reorganization of intracellular membranes that is required for the formation of the viral assembly compartment (AC) has been an area of study over the last 20 years. The significance of this virus-induced structure has been evinced by the results of several studies which showed that relocalization of viral proteins to the AC was required for efficient assembly of infectious virus. In this study, we have identified a mechanism for the fragmentation of the Golgi ribbon in the infected cell en route to AC morphogenesis. Identification of this fundamental process during HCMV replication allowed us to propose that the functional role of Golgi membrane reorganization during HCMV infection was the concentration of viral structural proteins and subviral structures into a single intracellular compartment in order to facilitate efficient protein-protein interactions and the virion protein trafficking required for the assembly of this large and structurally complex virus.

Glycosylation Quality Control by the Golgi Structure

Glycosylation is a ubiquitous modification that occurs on proteins and lipids in all living cells. Consistent with their high complexity, glycans play crucial biological roles in protein quality control and recognition events. Asparagine-linked protein N-glycosylation, the most complex glycosylation, initiates in the endoplasmic reticulum and matures in the Golgi apparatus. This process not only requires an accurate distribution of processing machineries, such as glycosyltransferases, glycosidases, and nucleotide sugar transporters, but also needs an efficient and well-organized factory that is responsible for the fidelity and quality control of sugar chain processing. In addition, accurate glycosylation must occur in coordination with protein trafficking and sorting. These activities are carried out by the Golgi apparatus, a membrane organelle in the center of the secretory pathway. To accomplish these tasks, the Golgi has developed into a unique stacked structure of closely aligned, flattened cisternae in which Golgi enzymes reside; in mammalian cells, dozens of Golgi stacks are often laterally linked into a ribbon-like structure. Here, we review our current knowledge of how the Golgi structure is formed and why its formation is required for accurate glycosylation, with the focus on how the Golgi stacking factors GRASP55 and GRASP65 generate the Golgi structure and how the conserved oligomeric Golgi complex maintains Golgi enzymes in different Golgi subcompartments by retrograde protein trafficking.

Arabidopsis COG Complex Subunits COG3 and COG8 Modulate Golgi Morphology, Vesicle Trafficking Homeostasis and Are Essential for Pollen Tube Growth

Spatially and temporally regulated membrane trafficking events incorporate membrane and cell wall materials into the pollen tube apex and are believed to underlie the rapid pollen tube growth. In plants, the molecular mechanisms and physiological functions of intra-Golgi transport and Golgi integrity maintenance remain largely unclear. The conserved oligomeric Golgi (COG) complex has been implicated in tethering of retrograde intra-Golgi vesicles in yeast and mammalian cells. Using genetic and cytologic approaches, we demonstrate that T-DNA insertions in Arabidopsis COG complex subunits, COG3 and COG8, cause an absolute, male-specific transmission defect that can be complemented by expression of COG3 and COG8 from the LAT52 pollen promoter, respectively. No obvious abnormalities in the microgametogenesis of the two mutants are observed, but in vitro and in vivo pollen tube growth are defective. COG3 or COG8 proteins fused to green fluorescent protein (GFP) label the Golgi apparatus. In pollen of both mutants, Golgi bodies exhibit altered morphology. Moreover, γ-COP and EMP12 proteins lose their tight association with the Golgi. These defects lead to the incorrect deposition of cell wall components and proteins during pollen tube growth. COG3 and COG8 interact directly with each other, and a structural model of the Arabidopsis COG complex is proposed. We believe that the COG complex helps to modulate Golgi morphology and vesicle trafficking homeostasis during pollen tube tip growth.

Transport Vesicle Tethering at the Trans Golgi Network: Coiled Coil Proteins in Action

The Golgi complex is decorated with so-called Golgin proteins that share a common feature: a large proportion of their amino acid sequences are predicted to form coiled-coil structures. The possible presence of extensive coiled coils implies that these proteins are highly elongated molecules that can extend a significant distance from the Golgi surface. This property would help them to capture or trap inbound transport vesicles and to tether Golgi mini-stacks together. This review will summarize our current understanding of coiled coil tethers that are needed for the receipt of transport vesicles at the trans Golgi network (TGN). How do long tethering proteins actually catch vesicles? Golgi-associated, coiled coil tethers contain numerous binding sites for small GTPases, SNARE proteins, and vesicle coat proteins. How are these interactions coordinated and are any or all of them important for the tethering process? Progress toward understanding these questions and remaining, unresolved mysteries will be discussed.

Emerging Insights into the Roles of Membrane Tethers from Analysis of Whole Organisms: The Tip of an Iceberg?

Membrane tethers have been identified throughout different compartments of the endomembrane system. It is now well established that a number of membrane tethers mediate docking of membrane carriers in anterograde and retrograde transport and in regulating the organization of membrane compartments. Much of our information on membrane tethers have been obtained from the analysis of individual membrane tethers in cultured cells. In the future it will be important to better appreciate the network of interactions mediated by tethers and the potential co-ordination of their collective functions in vivo. There are now a number of studies which have analyzed membrane tethers in tissues and organisms which are providing new insights into the role of this class of membrane protein at the physiological level. Here we review recent advances in the understanding of the function of membrane tethers from knock outs (or knock downs) in whole organisms and from mutations in tethers associated with disease.

GRASPs in Golgi Structure and Function

The Golgi apparatus is a central intracellular membrane organelle for trafficking and modification of proteins and lipids. Its basic structure is a stack of tightly aligned flat cisternae. In mammalian cells, dozens of stacks are concentrated in the pericentriolar region and laterally connected to form a ribbon. Despite extensive research in the last decades, how this unique structure is formed and why its formation is important for proper Golgi functioning remain largely unknown. The Golgi ReAssembly Stacking Proteins, GRASP65, and GRASP55, are so far the only proteins shown to function in Golgi stacking. They are peripheral membrane proteins on the cytoplasmic face of the Golgi cisternae that form trans-oligomers through their N-terminal GRASP domain, and thereby function as the “glue” to stick adjacent cisternae together into a stack and to link Golgi stacks into a ribbon. Depletion of GRASPs in cells disrupts the Golgi structure and results in accelerated protein trafficking and defective glycosylation. In this minireview we summarize our current knowledge on how GRASPs function in Golgi structure formation and discuss why Golgi structure formation is important for its function.

The Golgi complex in stress and death

The Golgi complex is a central organelle of the secretory pathway where sorting and processing of cargo occurs. While Golgi structure is important for the efficient processing of secretory cargo, the unusual organization suggests additional potential functions. The Golgi is disassembled after various cellular stresses, and we hypothesize that Golgi disassembly activates a stress signaling pathway. This pathway would function to correct the stress if possible, with irreparable stress resulting in apoptosis. Neurons appear to be particularly sensitive to Golgi stress; early disassembly of the organelle correlates with many neurodegenerative diseases. Here, Golgi stress and potential signaling pathways to the nucleus are reviewed.

Golgi fragmentation in Alzheimer's disease

The Golgi apparatus is an essential cellular organelle for post-translational modifications, sorting, and trafficking of membrane and secretory proteins. Proper functionality of the Golgi requires the formation of its unique cisternal-stacking morphology. The Golgi structure is disrupted in a variety of neurodegenerative diseases, suggesting a common mechanism and contribution of Golgi defects in neurodegenerative disorders. A recent study on Alzheimer's disease (AD) revealed that phosphorylation of the Golgi stacking protein GRASP65 disrupts its function in Golgi structure formation, resulting in Golgi fragmentation. Inhibiting GRASP65 phosphorylation restores the Golgi morphology from Aβ-induced fragmentation and reduces Aβ production. Perturbing Golgi structure and function in neurons may directly impact trafficking, processing, and sorting of a variety of proteins essential for synaptic and dendritic integrity. Therefore, Golgi defects may ultimately promote the development of AD. In the current review, we focus on the cellular impact of impaired Golgi morphology and its potential relationship to AD disease development.

Functional homologies in vesicle tethering

The HOPS multisubunit tethering factor (MTC) is a macromolecular protein complex composed of six different subunits. It is one of the key components in the perception and subsequent fusion of multivesicular bodies and vacuoles. Electron microscopy studies indicate structural flexibility of the purified HOPS complex. Inducing higher rigidity into HOPS by biochemically modifying the complex declines the potential to mediate SNARE-driven membrane fusion. Thus, we propose that integral flexibility seems to be not only a feature, but of essential need for the function of HOPS. This review focuses on the general features of membrane tethering and fusion. For this purpose, we compare the structure and mode of action of different tethering factors to highlight their common central features and mechanisms.