Sorting out glycosylation enzymes in the Golgi apparatus

Comprehensive Proteomic Characterization of the Intra-Golgi Trafficking Intermediates

Intracellular trafficking relies on small vesicular intermediates, though their specific role in Golgi function is still debated. To clarify this, we induced acute dysfunction of the Conserved Oligomeric Golgi (COG) complex and analyzed vesicles from cis, medial, and trans-Golgi compartments. Proteomic analysis of Golgi-derived vesicles from wild-type cells revealed distinct molecular profiles, indicating a robust recycling system for Golgi proteins. Notably, these vesicles retained various vesicular coats, while COG depletion accelerated uncoating. The increased overlap in molecular profiles with COG depletion suggests that persistent defects in vesicle tethering disrupt intra-Golgi sorting. Our findings reveal that the entire Golgi glycosylation machinery recycles within vesicles in a COG-dependent manner, whereas secretory and ER-Golgi trafficking proteins were not enriched. These results support a model in which the COG complex orchestrates multi-step recycling of glycosylation machinery, coordinated by specific Golgi coats, tethers, Rabs, and SNAREs.

Regulation of human GnT-IV family activity by the lectin domain

N-Glycan branching critically regulates glycoprotein functions and is involved in various diseases. Among the glycosyltransferases involved in N-glycan branching is the human N-acetylglucosaminyltransferase-IV (GnT-IV) family, which has four members: GnT-IVa, GnT-IVb, GnT-IVc, and GnT-IVd. GnT-IVa and GnT-IVb have glycosyltransferase activity that generates the type-2 diabetes-related β1,4-GlcNAc branch on the α1,3-Man arm of N-glycans, whereas GnT-IVc and GnT-IVd do not. Recently, this enzyme family was found to have a unique lectin domain in the C-terminal region, which is essential for enzyme activity toward glycoprotein substrates but not toward free N-glycans. Furthermore, interaction between the lectin domain of GnT-IV and N-glycan attached to GnT-IV enables self-regulation of GnT-IV activity, indicating that the lectin domain plays a unique and pivotal role in the regulation of GnT-IV activity. In this review, we summarize the GnT-IV family's biological functions, selectivity for glycoprotein substrates, and regulation of enzymatic activity, with a focus on its unique C-terminal lectin domain.

Near-Infrared Ratiometric Hemicyanine Fluorescent Probes for Monitoring Mitochondrial pH Dynamics in Live Cells during Oxidative Stress and Hypoxia

Novel near-infrared ratiometric molecules (probes A and B) produced by linking formyl-functionalized xanthene and methoxybenzene moieties, respectively, onto a xanthene-hemicyanine framework are detailed. Probe A exhibited a primary absorption peak at 780 nm and a shoulder peak at 730 nm and exhibited fluorescence at 740 nm↓ (signifies a downward shift in intensity upon acidification) in a pH 9.3 buffer and 780 nm↑ at pH 2.8 under excitation at 700 nm. Probe B featured absorptions at 618 and 668 nm at pH 3.2 and at 717 nm at pH 8.6, and fluorescence at 693 nm↑ at pH 3.2 and at 739 nm↓ at pH 8.6, in mostly the red to near-IR region. The ratiometric changes in the intensity of the fluorescent absorptions were reversed between A and B upon acidification as indicated by the arrows. Theoretical calculations confirmed that there were slight changes in conformation between probes and the protonated molecules, suggesting that the changes in emission spectra were due mostly to conjugation effects. Calculations at the APFD/6-311+g(d,p) level with a solvent described by the polarizable continuum model resulted in pK a values for A at 6.33 and B at 6.41, in good agreement with the experimentally determined value of 6.97 and an average of 6.40, respectively. The versatilities of the probes were demonstrated in various experimental contexts, including the effective detection of mitochondrial pH fluctuations. Live cell experiments involving exposure to different pH buffers in the presence of H+ ionophores, monitoring mitophagy processes during cell starvation, studying hypoxia induced by CoCl2 treatment, and investigating responses to various oxidative stresses are detailed. Our findings highlight the potential of attaching xanthene and methoxybenzaldehyde groups onto xanthene-hemicyanine structures as versatile tools for monitoring pH changes in a variety of cellular environments and processes.

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

Glycosylation-on-a-Chip: A Flow-Based Microfluidic System for Cell-Free Glycoprotein Biosynthesis

In recent years, cell-free synthetic glycobiology technologies have emerged that enable production and remodeling of glycoproteins outside the confines of the cell. However, many of these systems combine multiple synthesis steps into one pot where there can be competing reactions and side products that ultimately lead to low yield of the desired product. In this work, we describe a microfluidic platform that integrates cell-free protein synthesis, glycosylation, and purification of a model glycoprotein in separate compartments where each step can be individually optimized. Microfluidics offer advantages such as reaction compartmentalization, tunable residence time, the ability to tether enzymes for reuse, and the potential for continuous manufacturing. Moreover, it affords an opportunity for spatiotemporal control of glycosylation reactions that is difficult to achieve with existing cell-based and cell-free glycosylation systems. In this work, we demonstrate a flow-based glycoprotein synthesis system that promotes enhanced cell-free protein synthesis, efficient protein glycosylation with an immobilized oligosaccharyltransferase, and enrichment of the protein product from cell-free lysate. Overall, this work represents a first-in-kind glycosylation-on-a-chip prototype that could find use as a laboratory tool for mechanistic dissection of the protein glycosylation process as well as a biomanufacturing platform for small batch, decentralized glycoprotein production.

Recent progress of organic small molecule-based fluorescent probes for intracellular pH sensing

Fluorescent probes along with fluorescence microscopy are essential tools for biomedical research. Various cellular ubiquitous chemical factors such as pH, H₂O₂, and Ca²⁺ are labeled and traced using specific fluorescent probes, therefore helping us to explore their physiological function and pathological change. Among them, intracellular pH value is an important factor that governs biological processes, generally ∼7.2. Furthermore, specific organelles within cells possess unique acid-base homeostasis, involving the acidic lysosomes, alkalescent mitochondria, and neutral endoplasmic reticulum and Golgi apparatus, which undergo various physiological processes such as intracellular digestion, ATP production, and protein folding and processing. In this review, recently reported fluorescent probes targeted toward the lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, and cytoplasm for sensing pH change are discussed, which involves molecular structures, fluorescence behavior, and biological applications.

N-Glycan Biosynthesis: Basic Principles and Factors Affecting Its Outcome

Carbohydrate chains are the most abundant and diverse of nature's biopolymers and represent one of the four fundamental macromolecular building blocks of life together with proteins, nucleic acids, and lipids. Indicative of their essential roles in cells and in multicellular organisms, genes encoding proteins associated with glycosylation account for approximately 2% of the human genome. It has been estimated that 50-80% of all human proteins carry carbohydrate chains-glycans-as part of their structure. Despite cells utilize only nine different monosaccharides for making their glycans, their order and conformational variation in glycan chains together with chain branching differences and frequent post-synthetic modifications can give rise to an enormous repertoire of different glycan structures of which few thousand is estimated to carry important structural or functional information for a cell. Thus, glycans are immensely versatile encoders of multicellular life. Yet, glycans do not represent a random collection of unpredictable structures but rather, a collection of predetermined but still dynamic entities that are present at defined quantities in each glycosylation site of a given protein in a cell, tissue, or organism.In this chapter, we will give an overview of what is currently known about N-glycan synthesis in higher eukaryotes, focusing not only on the processes themselves but also on factors that will affect or can affect the final outcome-the dynamicity and heterogeneity of the N-glycome. We hope that this review will help understand the molecular details underneath this diversity, and in addition, be helpful for those who plan to produce optimally glycosylated antibody-based therapeutics.

In Situ Fluorescent and Photoacoustic Imaging of Golgi pH to Elucidate the Function of Transmembrane Protein 165

Golgi pH homeostasis affects many different biological processes, including glycosylation. Recent studies have demonstrated that transmembrane protein 165 (TMEM165) deficiency leads to Golgi glycosylation abnormalities by disturbing Golgi pH homeostasis. However, due to the lack of specific tools to measure Golgi pH in situ, evidence for TMEM165 involvement in H⁺ transport in the Golgi apparatus is still absent. Herein, the photoacoustic and fluorescent dual-mode probe CPH was developed for ratiometric detection of Golgi pH. CPH was proved to accumulate in the Golgi apparatus and reversibly image Golgi pH in real-time with high sensitivity in cells. Furthermore, we found that the absence of TMEM165 influenced H⁺ equilibrium and caused Golgi apparatus acidification. Our work provides strong evidence that TMEM165 regulates Golgi pH homeostasis. Moreover, we believe that CPH has the potential to be a practical tool to monitor Golgi pH in various biological processes.

Golgi pH and Ion Homeostasis in Health and Disease

Maintenance of the main Golgi functions, glycosylation and sorting, is dependent on the unique Golgi pH microenvironment that is thought to be set by the balance between the rates of V-ATPase-mediated proton pumping and its leakage back to the cytoplasm via an unknown pathway. The concentration of other ions, such as chloride, potassium, calcium, magnesium, and manganese, is also important for Golgi homeostasis and dependent on the transport activity of other ion transporters present in the Golgi membranes. During the last decade, several new disorders have been identified that are caused by, or are associated with, dysregulated Golgi pH and ion homeostasis. Here, we will provide an updated overview on these disorders and the proteins involved. We will also discuss other disorders for which the molecular defects remain currently uncertain but which potentially involve proteins that regulate Golgi pH or ion homeostasis.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

Translation of genome to glycome: role of the Golgi apparatus

Glycans are one of the four biopolymers of the cell and they play important roles in cellular and organismal physiology. They consist of both linear and branched structures and are synthesized in a nontemplated manner in the secretory pathway of mammalian cells with the Golgi apparatus playing a key role in the process. In spite of the absence of a template, the glycans synthesized by a cell are not a random collection of possible glycan structures but a distribution of specific glycans in defined quantities that is unique to each cell type (Cell type here refers to distinct cell forms present in an organism that can be distinguished based on morphological, phenotypic and/or molecular criteria.) While information to produce cell type-specific glycans is encoded in the genome, how this information is translated into cell type-specific glycome (Glycome refers to the quantitative distribution of all glycan structures present in a given cell type.) is not completely understood. We summarize here the factors that are known to influence the fidelity of glycan biosynthesis and integrate them into known glycosylation pathways so as to rationalize the translation of genetic information to cell type-specific glycome.

Models of Intracellular Transport: Pros and Cons

Intracellular transport is one of the most confusing issues in the field of cell biology. Many different models and their combinations have been proposed to explain the experimental data on intracellular transport. Here, we analyse the data related to the mechanisms of endoplasmic reticulum-to-Golgi and intra-Golgi transport from the point of view of the main models of intracellular transport; namely: the vesicular model, the diffusion model, the compartment maturation–progression model, and the kiss-and-run model. This review initially describes our current understanding of Golgi function, while highlighting the recent progress that has been made. It then continues to discuss the outstanding questions and potential avenues for future research with regard to the models of these transport steps. To compare the power of these models, we have applied the method proposed by K. Popper; namely, the formulation of prohibitive observations according to, and the consecutive evaluation of, previous data, on the basis on the new models. The levels to which the different models can explain the experimental observations are different, and to date, the most powerful has been the kiss-and-run model, whereas the least powerful has been the diffusion model.

IgA1 hinge-region clustered glycan fidelity is established early during semi-ordered glycosylation by GalNAc-T2

GalNAc-type O-glycans are often added to proteins post-translationally in a clustered manner in repeat regions of proteins, such as mucins and IgA1. Observed IgA1 glycosylation patterns show that glycans occur at similar sites with similar structures. It is not clear how the sites and number of glycans added to IgA1, or other proteins, can follow a conservative process. GalNAc-transferases initiate GalNAc-type glycosylation. In IgA nephropathy, an autoimmune disease, the sites and O-glycan structures of IgA1 hinge-region are altered, giving rise to a glycan autoantigen. To better understand how GalNAc-transferases determine sites and densities of clustered O-glycans, we used IgA1 hinge-region (HR) segment as a probe. Using LC-MS, we demonstrated a semi-ordered process of glycosylation by GalNAc-T2 towards the IgA1 HR. The catalytic domain was responsible for selection of four initial sites based on amino-acid sequence recognition. Both catalytic and lectin domains were involved in multiple second site-selections, each dependent on initial site-selection. Our data demonstrated that multiple start-sites and follow-up pathways were key to increasing the number of glycans added. The lectin domain predominately enhanced IgA1 HR glycan density by increasing synthesis pathway exploration by GalNAc-T2. Our data indicated a link between site-specific glycan addition and clustered glycan density that defines a mechanism of how conserved clustered O-glycosylation patterns and glycoform populations of IgA1 can be controlled by GalNAc-T2. Together, these findings characterized a correlation between glycosylation pathway diversity and glycosylation density, revealing mechanisms by which a single GalNAc-T isozyme can limit and define glycan heterogeneity in a disease-relevant context.

Golgi pH, Ion and Redox Homeostasis: How Much Do They Really Matter?

Exocytic and endocytic compartments each have their own unique luminal ion and pH environment that is important for their normal functioning. A failure to maintain this environment - the loss of homeostasis - is not uncommon. In the worst case, all the main Golgi functions, including glycosylation, membrane trafficking and protein sorting, can be perturbed. Several factors contribute to Golgi homeostasis. These include not only ions such as H⁺, Ca²⁺, Mg²⁺, Mn²⁺, but also Golgi redox state and nitric oxide (NO) levels, both of which are dependent on the oxygen levels in the cells. Changes to any one of these factors have consequences on Golgi functions, the nature of which can be dissimilar or similar depending upon the defects themselves. For example, altered Golgi pH homeostasis gives rise to Cutis laxa disease, in which glycosylation and membrane trafficking are both affected, while altered Ca²⁺ homeostasis due to the mutated SCPA1 gene in Hailey-Hailey disease, perturbs various protein sorting, proteolytic cleavage and membrane trafficking events in the Golgi. This review gives an overview of the molecular machineries involved in the maintenance of Golgi ion, pH and redox homeostasis, followed by a discussion of the organelle dysfunction and disease that frequently result from their breakdown. Congenital disorders of glycosylation (CDGs) are discussed only when they contribute directly to Golgi pH, ion or redox homeostasis. Current evidence emphasizes that, rather than being mere supporting factors, Golgi pH, ion and redox homeostasis are in fact key players that orchestrate and maintain all Golgi functions.

Biosynthesis of O-N-acetylgalactosamine glycans in the human cell nucleus

Biological functions of nuclear proteins are regulated by post-translational modifications (PTMs) that modulate gene expression and cellular physiology. However, the role of O-linked glycosylation (O-GalNAc) as a PTM of nuclear proteins in the human cell has not been previously reported. Here, we examined in detail the initiation of O-GalNAc glycan biosynthesis, representing a novel PTM of nuclear proteins in the nucleus of human cells, with an emphasis on HeLa cells. Using soluble nuclear fractions from purified nuclei, enzymatic assays, fluorescence microscopy, affinity chromatography, MS, and FRET analyses, we identified all factors required for biosynthesis of O-GalNAc glycans in nuclei: the donor substrate (UDP-GalNAc), nuclear polypeptide GalNAc -transferase activity, and a GalNAc transferase (polypeptide GalNAc-T3). Moreover, we identified O-GalNAc glycosylated proteins in the nucleus and present solid evidence for O-GalNAc glycan synthesis in this organelle. The demonstration of O-GalNAc glycosylation of nuclear proteins in mammalian cells reported here has important implications for cell and chemical biology.

The extended cytoplasmic tail of the human B4GALNT2 is critical for its Golgi targeting and post-Golgi sorting

The Sd^a /Cad antigen reported on glycoconjugates of human tissues has an increasingly recognized wide impact on the physio-pathology of different biological systems. The last step of its biosynthesis relies on the enzymatic activity of the β1,4-N-acetylgalactosaminyltransferase-II (B4GALNT2), which shows the highest expression level in healthy colon. Previous studies reported the occurrence in human colonic cells of two B4GALNT2 protein isoforms that differ in the length of their cytoplasmic tail, the long isoform showing an extended 66-amino acid tail. We examined here, the subcellular distribution of the two B4GALNT2 protein isoforms in stably transfected colonic LS174T cells and in transiently transfected HeLa cells using fluorescence microscopy. While a similar subcellular distribution at the trans-Golgi cisternae level was observed for the two isoforms, our study pointed to an atypical subcellular localization of the long B4GALNT2 isoform into dynamic vesicles. We demonstrated a critical role of its extended cytoplasmic tail for its Golgi targeting and post-Golgi sorting and highlighted the existence of a newly described post-Golgi sorting signal as well as a previously undescribed fate of a Golgi glycosyltransferase.

DATABASE

The proteins β1,4GalNAcT II, β1,4-GalT1, FucT I, FucT VI and ST3Gal IV are noted B4GALNT2, B4GALT1, FUT1, FUT6 and ST3GAL4, whereas the corresponding human genes are noted B4GALNT2, B4GALT1, FUT1, FUT6 and ST3GAL4 according to the HUGO nomenclature.

The V-ATPase V1 subunit A1 is required for rhodopsin anterograde trafficking in Drosophila

Synthesis and maturation of the light sensor, rhodopsin, are critical for the maintenance of light sensitivity and for photoreceptor homeostasis. In Drosophila, the main rhodopsin, Rh1, is synthesized in the endoplasmic reticulum and transported to the rhabdomere through the secretory pathway. In an unbiased genetic screen for factors involved in rhodopsin homeostasis, we identified mutations in vha68-1, which encodes the vacuolar proton-translocating ATPase (V-ATPase) catalytic subunit A isoform 1 of the V1 component. Loss of vha68-1 in photoreceptor cells disrupted post-Golgi anterograde trafficking of Rh1, reduced light sensitivity, increased secretory vesicle pH, and resulted in incomplete Rh1 deglycosylation. In addition, vha68-1 was required for activity-independent photoreceptor cell survival. Importantly, vha68-1 mutants exhibited phenotypes similar to those exhibited by mutations in the V0 component of V-ATPase, vha100-1. These data demonstrate that the V1 and V0 components of V-ATPase play key roles in post-Golgi trafficking of Rh1 and that Drosophila may represent an important animal model system for studying diseases associated with V-ATPase dysfunction.

Targeting Cysteine Thiols for in Vitro Site-specific Glycosylation of Recombinant Proteins

Stromal interaction molecule-1 (STIM1) is a type-I transmembrane protein located on the endoplasmic reticulum (ER) and plasma membranes (PM). ER-resident STIM1 regulates the activity of PM Orai1 channels in a process known as store operated calcium (Ca²⁺) entry which is the principal Ca²⁺ signaling process that drives the immune response. STIM1 undergoes post-translational N-glycosylation at two luminal Asn sites within the Ca²⁺ sensing domain of the molecule. However, the biochemical, biophysical, and structure biological effects of N-glycosylated STIM1 were poorly understood until recently due to an inability to readily obtain high levels of homogeneous N-glycosylated protein. Here, we describe the implementation of an in vitro chemical approach which attaches glucose moieties to specific protein sites applicable to understanding the underlying effects of N-glycosylation on protein structure and mechanism. Using solution nuclear magnetic resonance spectroscopy we assess both efficiency of the modification as well as the structural consequences of the glucose attachment with a single sample. This approach can readily be adapted to study the myriad glycosylated proteins found in nature.

Land-locked mammalian Golgi reveals cargo transport between stable cisternae

The Golgi is composed of a stack of cis, medial, trans cisternae that are biochemically distinct. The stable compartments model postulates that permanent cisternae communicate through bi-directional vesicles, while the cisternal maturation model postulates that transient cisternae biochemically mature to ensure anterograde transport. Testing either model has been constrained by the diffraction limit of light microscopy, as the cisternae are only 10-20 nm thick and closely stacked in mammalian cells. We previously described the unstacking of Golgi by the ectopic adhesion of Golgi cisternae to mitochondria. Here, we show that cargo processing and transport continue-even when individual Golgi cisternae are separated and "land-locked" between mitochondria. With the increased spatial separation of cisternae, we show using three-dimensional live imaging that cis-Golgi and trans-Golgi remain stable in their composition and size. Hence, we provide new evidence in support of the stable compartments model in mammalian cells.The different composition of Golgi cisternae gave rise to two different models for intra-Golgi traffic: one where stable cisternae communicate via vesicles and another one where cisternae biochemically mature to ensure anterograde transport. Here, the authors provide evidence in support of the stable compartments model.

Sialylation of N-glycans: mechanism, cellular compartmentalization and function

Sialylated N-glycans play essential roles in the immune system, pathogen recognition and cancer. This review approaches the sialylation of N-glycans from three perspectives. The first section focuses on the sialyltransferases that add sialic acid to N-glycans. Included in the discussion is a description of these enzymes' glycan acceptors, conserved domain organization and sequences, molecular structure and catalytic mechanism. In addition, we discuss the protein interactions underlying the polysialylation of a select group of adhesion and signaling molecules. In the second section, the biosynthesis of sialic acid, CMP-sialic acid and sialylated N-glycans is discussed, with a special emphasis on the compartmentalization of these processes in the mammalian cell. The sequences and mechanisms maintaining the sialyltransferases and other glycosylation enzymes in the Golgi are also reviewed. In the final section, we have chosen to discuss processes in which sialylated glycans, both N- and O-linked, play a role. The first part of this section focuses on sialic acid-binding proteins including viral hemagglutinins, Siglecs and selectins. In the second half of this section, we comment on the role of sialylated N-glycans in cancer, including the roles of β1-integrin and Fas receptor N-glycan sialylation in cancer cell survival and drug resistance, and the role of these sialylated proteins and polysialic acid in cancer metastasis.

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.

Extrinsic Functions of Lectin Domains in O-N-Acetylgalactosamine Glycan Biosynthesis

Glycan biosynthesis occurs mainly in Golgi. Molecular organization and functional regulation of this process are not well understood. We evaluated the extrinsic effect of lectin domains (β-trefoil fold) of polypeptide GalNAc-transferases (ppGalNAc-Ts) on catalytic activity of glycosyltransferases during O-GalNAc glycan biosynthesis. The presence of lectin domain T3lec or T4lec during ppGalNAc-T2 and ppGalNAc-T3 catalytic reaction had a clear inhibitory effect on GalNAc-T activity. Interaction of T3lec or T4lec with ppGalNAc-T2 catalytic domain was not mediated by carbohydrate. T3lec, but not T2lec and T4lec, had a clear activating effect on Drosophila melanogaster core 1 galactosyltransferase enzyme activity and a predominant inhibitory effect on in vivo human core 1 glycan biosynthesis. The regulatory role of the β-trefoil fold of ppGalNAc-Ts in enzymatic activity of glycosyltransferases involved in the O-glycan biosynthesis pathway, described here for the first time, helps clarify the mechanism of biosynthesis of complex biopolymers (such as glycans) that is not template-driven.

Regulation of Golgi Cisternal Progression by Ypt/Rab GTPases

Current models entail that transport through the Golgi-the main sorting compartment of the cell-occurs via cisternal progression/maturation and that Ypt/Rab GTPases regulate this process. However, there is very limited evidence that cisternal progression is regulated, and no evidence for involvement of Ypt/Rab GTPases in such a regulation. Moreover, controversy about the placement of two of the founding members of the Ypt/Rab family, Ypt1 and Ypt31, to specific Golgi cisternae interferes with addressing this question in yeast, where cisternal progression has been extensively studied. Here, we establish the localization of Ypt1 and Ypt31 to opposite faces of the Golgi: early and late, respectively. Moreover, we show that they partially overlap on a transitional compartment. Finally, we determine that changes in Ypt1 and Ypt31 activity affect Golgi cisternal progression, early-to-transitional and transitional-to-late, respectively. These results show that Ypt/Rab GTPases regulate two separate steps of Golgi cisternal progression.

OsCYP21-4, a novel Golgi-resident cyclophilin, increases oxidative stress tolerance in rice

OsCYP21-4 is a rice cyclophilin protein that binds to cyclosporine A, an immunosuppressant drug. CYP21-4s in Arabidopsis and rice were previously shown to function as mitochondrial cyclophilins, as determined by TargetP analysis. In the current study, we found that OsCYP21-4-GFP localized to the Golgi, rather than mitochondria, in Nicotiana benthamiana leaves, which was confirmed based on its co-localization with cis Golgi α-ManI-mCherry protein. OsCYP21-4 transcript levels increased in response to treatments with various abiotic stresses and the phytohormone abscisic acid, revealing its stress-responsiveness. CYP21-4 homologs do not possess key peptidyl prolyl cis/trans isomerase (PPIase) activity/cyclosporine A (CsA) binding residues, and recombinant OsCYP21-4 protein did not convert the synthetic substrate Suc-AAPF-pNA via cis- trans- isomerization in vitro. In addition, transgenic plants overexpressing OsCYP21-4 exhibited increased tolerance to salinity and hydrogen peroxide treatment, along with increased peroxidase activity. These results demonstrate that OsCYP21-4 is a novel Golgi-localized cyclophilin that plays a role in oxidative stress tolerance, possibly by regulating peroxidase activity.

Organizational interplay of Golgi N-glycosyltransferases involves organelle microenvironment-dependent transitions between enzyme homo- and heteromers

Glycosylation of proteins and lipids takes place in the Golgi apparatus by the consecutive actions of functionally distinct glycosidases and glycosyltransferases. Current evidence indicates that they function as enzyme homomers and/or heteromers in the living cell. Here we investigate their organizational interplay and show that glycosyltransferase homomers are assembled in the endoplasmic reticulum. Upon transport to the Golgi, the majority of homomers are disassembled to allow the formation of enzyme heteromers between sequentially acting medial-Golgi enzymes GnT-I and GnT-II or trans-Golgi enzymes GalT-I and ST6Gal-I. This transition is driven by the acidic Golgi environment, as it was markedly inhibited by raising Golgi luminal pH with chloroquine. Our FRAP (fluorescence recovery after photobleaching) measurements showed that the complexes remain mobile Golgi membrane constituents that can relocate to the endoplasmic reticulum or to the scattered Golgi mini-stacks upon brefeldin A or nocodazole treatment, respectively. During this relocation, heteromers undergo a reverse transition back to enzyme homomers. These data unveil an unprecedented organizational interplay between Golgi N-glycosyltransferases that involves dynamic and organelle microenvironment-driven transitions between enzyme homomers and heteromers during their trafficking within the early secretory compartments.

Putative PmrA and PmcA are important for normal growth, morphogenesis and cell wall integrity, but not for viability in Aspergillus nidulans

P-type Ca(2+)-transporting ATPases are Ca(2+) pumps, extruding cytosolic Ca(2+) to the extracellular environment or the intracellular Ca(2+) store lumens. In budding yeast, Pmr1 (plasma membrane ATPase related), and Pmc1 (plasma membrane calcium-ATPase) cannot be deleted simultaneously for it to survive in standard medium. Here, we deleted two putative Ca(2+) pumps, designated AnPmrA and AnPmcA, from Aspergillus nidulans, and obtained the mutants ΔanpmrA and ΔanpmcA, respectively. Then, using ΔanpmrA as the starting strain, the promoter of its anpmcA was replaced with the alcA promoter to secure the mutant ΔanpmrAalcApmcA or its anpmcA was deleted completely to produce the mutant ΔanpmrAΔpmcA. Different from the case in Saccharomyces cerevisiae, double deletion of anpmrA and anpmcA was not lethal in A. nidulans. In addition, deletion of anpmrA and/or anpmcA had produced growth defects, although overexpression of AnPmc1 in ΔanpmrAalcApmcA could not restore the growth defects that resulted from the loss of AnPmrA. Moreover, we found AnPmrA was indispensable for maintenance of normal morphogenesis, especially in low-Ca(2+)/Mn(2+) environments. Thus, our findings suggest AnPmrA and AnPmcA might play important roles in growth, morphogenesis and cell wall integrity in A. nidulans in a different way from that in yeasts.

Signaling initiated by the secretory compartment

Classical signal transduction is initiated at the plasma membrane by extracellular signals and propagates to the cytosolic face of the same membrane. Multiple studies have shown that endomembranes can act as signaling platforms for this plasma-membrane-originated signaling. Recent evidence has indicated that endomembranes can also trigger their own signaling cascades that involve some of the molecular players that are classically engaged in signal transduction at the plasma membrane. Endomembrane-initiated signaling is important for synchronization of the functioning of the secretory pathway and coordination of the activities of the secretory organelles with other cellular machineries. However, these endomembrane-initiated regulatory circuits are only partially understood to date. This novel field is slowed by a lack of specific tools and the objective difficulties in the study of signal transduction of endomembrane-localized receptors, as their accessibility is limited. For example, the ligand-binding site of the KDEL receptor (that transduces endomembrane signaling) is positioned in the lumen of the Golgi complex. Here we report some approaches that are suitable for the study of endomembrane-initiated signaling.

Rab41 is a novel regulator of Golgi apparatus organization that is needed for ER-to-Golgi trafficking and cell growth

Background

The 60⁺ members of the mammalian Rab protein family group into subfamilies postulated to share common functionality. The Rab VI subfamily contains 5 Rab proteins, Rab6a/a’, Rab6b, Rab6c and Rab41. High-level knockdown of Rab6a/a’ has little effect on the tightly organized Golgi ribbon in HeLa cells as seen by fluorescence microscopy. In striking contrast, we found Rab41 was strongly required for normal Golgi ribbon organization.

Methods/Results

Treatment of HeLa cells with Rab41 siRNAs scattered the Golgi ribbon into clustered, punctate Golgi elements. Overexpression of GDP-locked Rab41, but not wild type or GTP-locked Rab41, produced a similar Golgi phenotype. By electron microscopy, Rab41 depletion produced short, isolated Golgi stacks. Golgi-associated vesicles accumulated. At low expression levels, wild type and GTP-locked Rab41 showed little concentration in the Golgi region, but puncta were observed and most were in ruffled regions at the cell periphery. There was 25% co-localization of GTP-locked Rab41 with the ER marker, Sec61p. GDP-locked Rab41, as expected, displayed an entirely diffuse cytoplasmic distribution. Depletion of Rab41 or overexpression of GDP-locked Rab41 partially inhibited ER-to-Golgi transport of VSV-G protein. However, Rab41 knockdown had little, if any, effect on endosome-to-Golgi transport of SLTB. Additionally, after a 2-day delay, treatment with Rab41 siRNA inhibited cell growth, while overexpression of GDP-locked Rab41, but not wild type or GTP-locked Rab41, produced a rapid, progressive cell loss. In double knockdown experiments with Rab6, the Golgi ribbon was fragmented, a result consistent with Rab41 and Rab6 acting in parallel.

Conclusion

We provide the first evidence for distinctive Rab41 effects on Golgi organization, ER-to-Golgi trafficking and cell growth. When combined with the evidence that Rab6a/a’ and Rab6b have diverse roles in Golgi function, while Rab6c regulates mitotic function, our data indicate that Rab VI subfamily members, although related by homology and structure, share limited functional conservation.

Cell biology of the endoplasmic reticulum and the Golgi apparatus through proteomics

Enriched endoplasmic reticulum (ER) and Golgi membranes subjected to mass spectrometry have uncovered over a thousand different proteins assigned to the ER and Golgi apparatus of rat liver. This, in turn, led to the uncovering of several hundred proteins of poorly understood function and, through hierarchical clustering, showed that proteins distributed in patterns suggestive of microdomains in cognate organelles. This has led to new insights with respect to their intracellular localization and function. Another outcome has been the critical testing of the cisternal maturation hypothesis showing overwhelming support for a predominant role of COPI vesicles in the transport of resident proteins of the ER and Golgi apparatus (as opposed to biosynthetic cargo). Here we will discuss new insights gained and also highlight new avenues undertaken to further explore the cell biology of the ER and the Golgi apparatus through tandem mass spectrometry.

Golgi-localized enzyme complexes for plant cell wall biosynthesis

The plant cell wall mostly comprises complex glycans, which are synthesized by numerous enzymes located in the Golgi apparatus and plasma membrane. Protein-protein interactions have been shown to constitute an important organizing principle for glycan biosynthetic enzymes in mammals and yeast. Recent genetic and biochemical data also indicate that such interactions could be common in plant cell wall biosynthesis. In this review, we examine the new findings in protein-protein interactions among plant cell wall biosynthetic enzymes and discuss the possibilities for enzyme complexes in the Golgi apparatus. These new insights in the field may contribute to novel strategies for molecular engineering of the cell wall.

Division of the intermediate compartment at the onset of mitosis provides a mechanism for Golgi inheritance

As mammalian cells prepare for mitosis, the Golgi ribbon is first unlinked into its constituent stacks and then transformed into spindle-associated, pleiomorphic membrane clusters in a process that remains enigmatic. Also, it remains unclear whether Golgi inheritance involves the incorporation of Golgi enzymes into a pool of coat protein I (COPI) vesicles, or their COPI-independent transfer to the endoplasmic reticulum (ER). Based on the observation that the intermediate compartment (IC) at the ER-Golgi boundary is connected to the centrosome, we examined its mitotic fate and possible role in Golgi breakdown. The use of multiple imaging techniques and markers revealed that the IC elements persist during the M phase, maintain their compositional and structural properties and remain associated with the mitotic spindle, forming circular arrays at the spindle poles. At G2/M transition, the movement of the pericentrosomal domain of the IC (pcIC) to the cell centre and its expansion coincide with the unlinking of the Golgi ribbon. At prophase, coupled to centrosome separation, the pcIC divides together with recycling endosomes, providing novel landmarks for mitotic entry. We provide evidence that the permanent IC elements function as way stations during the COPI-dependent dispersal of Golgi components at prometa- and metaphase, indicating that they correspond to the previously described Golgi clusters. In addition, they continue to communicate with the vesicular 'Golgi haze' and thus are likely to provide templates for Golgi reassembly. These results implicate the IC in mitotic Golgi inheritance, resulting in a model that integrates key features of the two previously proposed pathways.

The origin and function of platelet glycosyltransferases

Platelets are megakaryocyte subfragments that participate in hemostatic and host defense reactions and deliver pro- and antiangiogenic factors throughout the vascular system. Although they are anucleated cells that lack a complex secretory apparatus with distinct Golgi/endoplasmic reticulum compartments, past studies have shown that platelets have glycosyltransferase activities. In the present study, we show that members of 3 distinct glycosyltransferase families are found within and on the surface of platelets. Immunocytology and flow cytometry results indicated that megakaryocytes package these Golgi-derived glycosyltransferases into vesicles that are sent via proplatelets to nascent platelets, where they accumulate. These glycosyltransferases are active, and intact platelets glycosylate large exogenous substrates. Furthermore, we show that activation of platelets results in the release of soluble glycosyltransferase activities and that platelets contain sufficient levels of sugar nucleotides for detection of glycosylation of exogenously added substrates. Therefore, the results of the present study show that blood platelets are a rich source of both glycosyltransferases and donor sugar substrates that can be released to function in the extracellular space. This platelet-glycosylation machinery offers a pathway to a simple glycoengineering strategy improving storage of platelets and may serve hitherto unknown biologic functions.

Golgi and related vesicle proteomics: simplify to identify

Despite more than six decades of successful Golgi research, the fundamental question as to how biosynthetic material is transported through the secretory pathway remains unanswered. New technologies such as live cell imaging and correlative microscopy have highlighted the plastic nature of the Golgi, one that is sensitive to perturbation yet highly efficient in regaining both structure and function. Single molecule-microscopy and super resolution-microscopy further adds to this picture. Various models for protein transport have been put forward, each with its own merits and pitfalls but we are far from resolving whether one is more correct than the other. As such, our laboratory considers multiple mechanisms of Golgi transport until proven otherwise. This includes the two classical modes of transport, vesicular transport and cisternal progression/maturation as well as more recent models such as tubular inter- and intra-cisternal connections (long lasting or transient) and inter-Golgi stack transport. In this article, we focus on an emerging inductive technology, mass spectrometry-based proteomics that has already enabled insight into the relative composition of compartments and subcompartments of the secretory pathway including mechanistic aspects of protein transport. We note that proteomics, as with any other technology, is not a stand-alone technology but one that works best alongside complementary approaches.

Models for Golgi traffic: a critical assessment

A variety of secretory cargoes move through the Golgi, but the pathways and mechanisms of this traffic are still being debated. Here, we evaluate the strengths and weaknesses of five current models for Golgi traffic: (1) anterograde vesicular transport between stable compartments, (2) cisternal progression/maturation, (3) cisternal progression/maturation with heterotypic tubular transport, (4) rapid partitioning in a mixed Golgi, and (5) stable compartments as cisternal progenitors. Each model is assessed for its ability to explain a set of key observations encompassing multiple cell types. No single model can easily explain all of the observations from diverse organisms. However, we propose that cisternal progression/maturation is the best candidate for a conserved core mechanism of Golgi traffic, and that some cells elaborate this core mechanism by means of heterotypic tubular transport between cisternae.

A dynamic mathematical model for monoclonal antibody N-linked glycosylation and nucleotide sugar donor transport within a maturing Golgi apparatus

Monoclonal antibodies (mAbs) are one of the most important products of the biopharmaceutical industry. Their therapeutic efficacy depends on the post-translational process of glycosylation, which is influenced by manufacturing process conditions. Herein, we present a dynamic mathematical model for mAb glycosylation that considers cisternal maturation by approximating the Golgi apparatus to a plug flow reactor and by including recycling of Golgi-resident proteins (glycosylation enzymes and transport proteins [TPs]). The glycosylation reaction rate expressions were derived based on the reported kinetic mechanisms for each enzyme, and transport of nucleotide sugar donors [NSDs] from the cytosol to the Golgi lumen was modeled to serve as a link between glycosylation and cellular metabolism. Optimization-based methodologies were developed for estimating unknown enzyme and TP concentration profile parameters. The resulting model is capable of reproducing glycosylation profiles of commercial mAbs. It can further reproduce the effect gene silencing of the FucT glycosylation enzyme and cytosolic NSD depletion have on the mAb oligosaccharide profile. All novel elements of our model are based on biological evidence and generate more accurate results than previous reports. We therefore believe that the improvements contribute to a more detailed representation of the N-linked glycosylation process. The overall results show the potential of our model toward evaluating cell engineering strategies that yield desired glycosylation profiles. Additionally, when coupled to cellular metabolism, this model could be used to assess the effect of process conditions on glycosylation and aid in the design, control, and optimization of biopharmaceutical manufacturing processes.

Functional organization of Golgi N- and O-glycosylation pathways involves pH-dependent complex formation that is impaired in cancer cells

Glycosylation is one of the most common modifications of proteins and lipids and also a major source of biological diversity in eukaryotes. It is critical for many basic cellular functions and recognition events that range from protein folding to cell signaling, immunological defense, and the development of multicellular organisms. Glycosylation takes place mainly in the endoplasmic reticulum and Golgi apparatus and involves dozens of functionally distinct glycosidases and glycosyltransferases. How the functions of these enzymes, which act sequentially and often competitively, are coordinated to faithfully synthesize a vast array of different glycan structures is currently unclear. Here, we investigate the supramolecular organization of the Golgi N- and O-glycosylation pathways in live cells using a FRET flow cytometric quantification approach. We show that the enzymes form enzymatically active homo- and/or heteromeric complexes within each pathway. However, no complexes composed of enzymes that operate in different pathways, were detected, which suggests that the pathways are physically distinct. In addition, we show that complex formation is mediated almost exclusively by the catalytic domains of the interacting enzymes. Our data also suggest that the heteromeric complexes are functionally more important than enzyme homomers. Heteromeric complex formation was found to be dependent on Golgi acidity, markedly impaired in acidification-defective cancer cells, and required for the efficient synthesis of cell surface glycans. Collectively, the results emphasize that the Golgi glycosylation pathways are functionally organized into complexes that are important for glycan synthesis.

Golgi glycosylation and human inherited diseases

The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.

Evolutionary forces shaping the Golgi glycosylation machinery: why cell surface glycans are universal to living cells

Despite more than 3 billion years since the origin of life on earth, the powerful forces of biological evolution seem to have failed to generate any living cell that is devoid of a dense and complex array of cell surface glycans. Thus, cell surface glycans seem to be as essential for life as having a DNA genetic code, diverse RNAs, structural/functional proteins, lipid-based membranes, and metabolites that mediate energy flux and signaling. The likely reasons for this apparently universal law of biology are considered here, and include the fact that glycans have the greatest potential for generating diversity, and thus evading recognition by pathogens. This may also explain why in striking contrast to the genetic code, glycans show widely divergent patterns between taxa. On the other hand, glycans have also been coopted for myriad intrinsic functions, which can vary in their importance for organismal survival. In keeping with these considerations, a significant percentage of the genes in the typical genome are dedicated to the generation and/or turnover of glycans. Among eukaryotes, the Golgi is the subcellular organelle that serves to generate much of the diversity of cell surface glycans, carrying out various glycan modifications of glycoconjugates that transit through the Golgi, en route to the cell surface or extracellular destinations. Here I present an overview of general considerations regarding the selective forces shaping evolution of the Golgi glycosylation machinery, and then briefly discuss the common types of variations seen in each major class of glycans, finally focusing on sialic acids as an extreme example of evolutionary glycan diversity generated by the Golgi. Future studies need to address both the phylogenetic diversity the Golgi and the molecular mechanisms for its rapid responses to intrinsic and environmental stimuli.

The Ras signaling inhibitor LOX-PP interacts with Hsp70 and c-Raf to reduce Erk activation and transformed phenotype of breast cancer cells

The lysyl oxidase gene (LOX) inhibits Ras signaling in transformed fibroblasts and breast cancer cells. Its activity was mapped to the 162-amino-acid propeptide domain (LOX-PP) of the lysyl oxidase precursor protein. LOX-PP inhibits Erk signaling, motility, and tumor formation in a breast cancer xenograft model; however, its mechanism of action is largely unknown. Here, a copurification-mass spectrometry approach was taken using ectopically expressed LOX-PP in HEK293T cells and the heat shock/chaperone protein Hsp70 identified. Hsp70 interaction with LOX-PP was confirmed using coimmunoprecipitation of intracellularly and bacterially expressed and endogenous proteins. The interaction was mapped to the Hsp70 peptide-binding domain and to LOX-PP amino acids 26 to 100. LOX-PP association reduced Hsp70 chaperone activities of protein refolding and survival after heat shock. LOX-PP interacted with the Hsp70 chaperoned protein c-Raf. With the use of ectopic expression of LOX-PP wild-type and deletion proteins, small interfering RNA (siRNA) knockdown, and Lox(-/-) mouse embryo fibroblasts, LOX-PP interaction with c-Raf was shown to decrease downstream activation of MEK and NF-κB, migration, and anchorage-independent growth and reduce its mitochondrial localization. Thus, the interaction of LOX-PP with Hsp70 and c-Raf inhibits a critical intermediate in Ras-induced MEK signaling and plays an important role in the function of this tumor suppressor.

Cellular and molecular biology of glycosphingolipid glycosylation

Brain tissue is characterized by its high glycosphingolipid content, particularly those containing sialic acid (gangliosides). As a result of this observation, brain tissue was a focus for studies leading to the characterization of the enzymes participating in ganglioside biosynthesis, and their participation in driving the compositional changes that occur in glycolipid expression during brain development. Later on, this focus shifted to the study of cellular aspects of the synthesis, which lead to the identification of the site of synthesis in the neuronal soma and their axonal transport toward the periphery. In this review article, we will focus in subcellular aspects of the biosynthesis of glycosphingolipid oligosaccharides, particularly the mechanisms underlying the trafficking of glycosphingolipid glycosyltransferases from the endoplasmic reticulum to the Golgi, those that promote their retention in the Golgi and those that participate in their topological organization as part of the complex membrane bound machinery for the synthesis of glycosphingolipids.

Organization of the synthesis of glycolipid oligosaccharides in the Golgi complex

Glycolipids constitute a complex family of amphipathic molecules structurally characterized by a hydrophilic mono- or oligo-saccharide moiety linked to a hydrophobic ceramide moiety. Due to their asymmetric distribution in cell membranes, exposing the saccharide moiety to the extracytoplasmic side of the cell, glycolipids participate in a variety of cell-cell and cell-ligand interactions. Here we summarize aspects of the cell biology of the stepwise synthesis of the saccharide moiety in the Golgi complex of cells from vertebrates. In particular we refer to the participant glycosyltransferases, with emphasis on their trafficking along the secretory pathway, their retention and organization in the Golgi complex membranes and their dependence on the Golgi complex ultra structural organization for proper function.

Conserved oligomeric Golgi complex specifically regulates the maintenance of Golgi glycosylation machinery

Cell surface lectin staining, examination of Golgi glycosyltransferases stability and localization, and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) analysis were employed to investigate conserved oligomeric Golgi (COG)-dependent glycosylation defects in HeLa cells. Both Griffonia simplicifolia lectin-II and Galanthus nivalus lectins were specifically bound to the plasma membrane glycoconjugates of COG-depleted cells, indicating defects in activity of medial- and trans-Golgi-localized enzymes. In response to siRNA-induced depletion of COG complex subunits, several key components of Golgi glycosylation machinery, including MAN2A1, MGAT1, B4GALT1 and ST6GAL1, were severely mislocalized. MALDI-TOF analysis of total N-linked glycoconjugates indicated a decrease in the relative amount of sialylated glycans in both COG3 KD and COG4 KD cells. In agreement to a proposed role of the COG complex in retrograde membrane trafficking, all types of COG-depleted HeLa cells were deficient in the Brefeldin A- and Sar1 DN-induced redistribution of Golgi resident glycosyltransferases to the endoplasmic reticulum. The retrograde trafficking of medial- and trans-Golgi-localized glycosylation enzymes was affected to a larger extent, strongly indicating that the COG complex regulates the intra-Golgi protein movement. COG complex-deficient cells were not defective in Golgi re-assembly after the Brefeldin A washout, confirming specificity in the retrograde trafficking block. The lobe B COG subcomplex subunits COG6 and COG8 were localized on trafficking intermediates that carry Golgi glycosyltransferases, indicating that the COG complex is directly involved in trafficking and maintenance of Golgi glycosylation machinery.

Sub-compartmental organization of Golgi-resident N-glycan processing enzymes in plants

In all eukaryotes, the Golgi apparatus is the main site of protein glycosylation. It is widely accepted that the glycosidases and glycosyltransferases involved in N-glycan processing are found concentrated within the Golgi stack where they provide their function. This means that enzymes catalyzing early steps in the processing pathway are located mainly at the cis-side, whereas late-acting enzymes mostly locate to the trans-side of the stacks, creating a non-uniform distribution along the cis-trans axis of the Golgi. There is compelling evidence that the information for their sorting to specific Golgi cisternae depends on signals encoded in the proteins themselves as well as on the trafficking machinery that recognizes these signals and it is believed that cisternal sub-compartmentalization is achieved and maintained by a combination of retention and retrieval mechanisms. Yet, the signals, mechanism(s), and molecular factors involved are still unknown. Here, we address recent findings and summarize the current understanding of this fundamental process in plant cell biology.

The Golgi apparatus: an organelle with multiple complex functions

Remarkable advances have been made during the last few decades in defining the organizational principles of the secretory pathway. The Golgi complex in particular has attracted special attention due to its central position in the pathway, as well as for its fascinating and complex structure. Analytical studies of this organelle have produced significant advances in our understanding of its function, although some aspects still seem to elude our comprehension. In more recent years a level of complexity surrounding this organelle has emerged with the discovery that the Golgi complex is involved in cellular processes other than the 'classical' trafficking and biosynthetic pathways. The resulting picture is that the Golgi complex can be considered as a cellular headquarters where cargo sorting/processing, basic metabolism, signalling and cell-fate decisional processes converge.

Sugar-free frosting, a homolog of SAD kinase, drives neural-specific glycan expression in the Drosophila embryo

Precise glycan structures on specific glycoproteins impart functionalities essential for neural development. However, mechanisms controlling embryonic neural-specific glycosylation are unknown. A genetic screen for relevant mutations in Drosophila generated the sugar-free frosting (sff) mutant that reveals a new function for protein kinases in regulating substrate flux through specific Golgi processing pathways. Sff is the Drosophila homolog of SAD kinase, which regulates synaptic vesicle tethering and neuronal polarity in nematodes and vertebrates. Our Drosophila sff mutant phenotype has features in common with SAD kinase mutant phenotypes in these other organisms, but we detect altered neural glycosylation well before the initiation of embryonic synaptogenesis. Characterization of Golgi compartmentation markers indicates altered colocalization that is consistent with the detected shift in glycan complexity in sff mutant embryos. Therefore, in analogy to synaptic vesicle tethering, we propose that Sff regulates vesicle tethering at Golgi membranes in the developing Drosophila embryo. Furthermore, neuronal sff expression is dependent on transcellular signaling through a non-neural toll-like receptor, linking neural-specific glycan expression to a kinase activity that is induced in response to environmental cues.