The COG and COPI complexes interact to control the abundance of GEARs, a subset of Golgi integral membrane proteins

Essential role of the conserved oligomeric Golgi complex in Toxoplasma gondii

IMPORTANCE

The Golgi is an essential eukaryotic organelle and a major place for protein sorting and glycosylation. Among apicomplexan parasites, Toxoplasma gondii retains the most developed Golgi structure and produces many glycosylated factors necessary for parasite survival. Despite its importance, Golgi function received little attention in the past. In the current study, we identified and characterized the conserved oligomeric Golgi complex and its novel partners critical for protein transport in T. gondii tachyzoites. Our results suggest that T. gondii broadened the role of the conserved elements and reinvented the missing components of the trafficking machinery to accommodate the specific needs of the opportunistic parasite T. gondii.

Genomic and AntiSMASH Analyses of Marine-Sponge-Derived Strain Aspergillus niger L14 Unveiling Its Vast Potential of Secondary Metabolites Biosynthesis

Aspergillus niger is one of the most important sources of secondary metabolites (SMs), with a wide array of pharmacological effects, including anti-inflammatory, antitumor, immunomodulatory and antioxidant effects. However, the biosynthetic analysis of these bioactive components has been rarely reported owing to the lack of high-quality genome sequences and comprehensive analysis. In this study, the whole genome of one marine-sponge-derived strain A. niger L14 was sequenced and assembled as well as in-depth bioinformatic analysis. The results indicated that the sequence assembly of strain L14 generated one high-quality genome with a total size of 36.1 Mb, a G + C content of 45.3% and an N50 scaffold of 4.2 Mb. Gene annotation was extensively deployed using various BLAST databases, including non-redudant (Nr) protein sequence, nucleotide (Nt) sequence, Swiss-Prot, Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Clusters of Orthologous Groups (COG) as well as Pathogen Host Interactions (PHI) and Carbohydrate-active enzymes (CAZy) databases. AntiSMASH analysis revealed that this marine strain harbors a total of 69 SMs biosynthesis gene clusters (BGCs), including 17 PKSs, 18 NRPSs, 21 NRPS-likes, 9 terpenes, 2 indoles, 1 betalactone and 1 siderophore, suggesting its biosynthetic potential to produce a wide variety of SMs. These findings will assist in future investigations on the genetic basis of strain L14 and provide insights into its new bioactive SMs for new drug discovery.

Proteomics of Two Thermotolerant Isolates of Trichoderma under High-Temperature Stress

Several species of the soil borne fungus of the genus Trichoderma are known to be versatile, opportunistic plant symbionts and are the most successful biocontrol agents used in today's agriculture. To be successful in field conditions, the fungus must endure varying climatic conditions. Studies have indicated that a high atmospheric temperature coupled with low humidity is a major factor in the inconsistent performance of Trichoderma under field conditions. Understanding the molecular modulations associated with Trichoderma that persist and deliver under abiotic stress conditions will aid in exploiting the value of these organisms for such uses. In this study, a comparative proteomic analysis, using two-dimensional gel electrophoresis (2DE) and matrix-assisted laser desorption/time-of-flight (MALDI-TOF-TOF) mass spectrometry, was used to identify proteins associated with thermotolerance in two thermotolerant isolates of Trichoderma: T. longibrachiatum 673, TaDOR673 and T. asperellum 7316, TaDOR7316; with 32 differentially expressed proteins being identified. Sequence homology and conserved domains were used to identify these proteins and to assign a probable function to them. The thermotolerant isolate, TaDOR673, seemed to employ the stress signaling MAPK pathways and heat shock response pathways to combat the stress condition, whereas the moderately tolerant isolate, TaDOR7316, seemed to adapt to high-temperature conditions by reducing the accumulation of misfolded proteins through an unfolded protein response pathway and autophagy. In addition, there were unique, as well as common, proteins that were differentially expressed in the two isolates studied.

Congenital disorder of glycosylation caused by starting site-specific variant in syntaxin-5

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein syntaxin-5 (Stx5) is essential for Golgi transport. In humans, the STX5 mRNA encodes two protein isoforms, Stx5 Long (Stx5L) from the first starting methionine and Stx5 Short (Stx5S) from an alternative starting methionine at position 55. In this study, we identify a human disorder caused by a single missense substitution in the second starting methionine (p.M55V), resulting in complete loss of the short isoform. Patients suffer from an early fatal multisystem disease, including severe liver disease, skeletal abnormalities and abnormal glycosylation. Primary human dermal fibroblasts isolated from these patients show defective glycosylation, altered Golgi morphology as measured by electron microscopy, mislocalization of glycosyltransferases, and compromised ER-Golgi trafficking. Measurements of cognate binding SNAREs, based on biotin-synchronizable forms of Stx5 (the RUSH system) and Förster resonance energy transfer (FRET), revealed that the short isoform of Stx5 is essential for intra-Golgi transport. Alternative starting codons of Stx5 are thus linked to human disease, demonstrating that the site of translation initiation is an important new layer of regulating protein trafficking.

Component of oligomeric Golgi complex 1 deficiency leads to hypoglycemia: a case report and literature review

Background

Congenital disorders of glycosylation (CDG) are a group of metabolic diseases with clinical and genetic heterogeneity, and CDG-IIg is one of the rare reported types of CDG. The aim of this study is to report the clinical manifestations and gene-phenotype characteristics of a rare case of CDG caused by a COG1 gene mutation and review literatures of CDG disease.

Case presentation

The patient was male, and the main clinical symptoms were developmental retardation, convulsion, strabismus, and hypoglycemia, which is rarely reported in CDG-IIg. We treated the patient with glucose infusion and he was recovered from hypoglycemia. Genetic analysis showed that the patient carried the heterozygous intron mutation c.1070 + 3A > G (splicing) in the coding region of the COG1 gene that was inherited from the mother, and the heterozygous mutation c.2492G > A (p. Arg831Gln) in exon 10 of the COG1 gene that was inherited from the father. The genes interacting with COG1 were mainly involved in the transport and composition of the Golgi. The clinical data and laboratory results from a patient diagnosed with CDG-IIg were analyzed, and the causative gene mutation was identified by high-throughput sequencing. The genes and signal pathways related to COG1 were analyzed by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses.

Conclusions

The c.2492G > A (p. Arg831Gln) mutation in exon 10 of the COG1 gene may be a potential pathogenetic variant for CDG-IIg. Because of the various manifestations of CDG in clinical practice, multidisciplinary collaboration is important for the diagnosis and treatment of this disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12887-021-02922-7.

Development and Initial Characterization of Cellular Models for COG Complex-Related CDG-II Diseases

Conserved Oligomeric Golgi (COG) is an octameric protein complex that orchestrates intra-Golgi trafficking of glycosylation enzymes. Over a hundred individuals with 31 different COG mutations have been identified until now. The cellular phenotypes and clinical presentations of COG-CDGs are heterogeneous, and patients primarily represent neurological, skeletal, and hepatic abnormalities. The establishment of a cellular COG disease model will benefit the molecular study of the disease, explaining the detailed sequence of the interplay between the COG complex and the trafficking machinery. Moreover, patient fibroblasts are not a good representative of all the organ systems and cell types that are affected by COG mutations. We developed and characterized cellular models for human COG4 mutations, specifically in RPE1 and HEK293T cell lines. Using a combination of CRISPR/Cas9 and lentiviral transduction technologies, both myc-tagged wild-type and mutant (G516R and R729W) COG4 proteins were expressed under the endogenous COG4 promoter. Constructed isogenic cell lines were comprehensively characterized using biochemical, microscopy (superresolution and electron), and proteomics approaches. The analysis revealed similar stability and localization of COG complex subunits, wild-type cell growth, and normal Golgi morphology in all three cell lines. Importantly, COG4-G516R cells demonstrated increased HPA-647 binding to the plasma membrane glycoconjugates, while COG4-R729W cells revealed high GNL-647 binding, indicating specific defects in O- and N-glycosylation. Both mutant cell lines express an elevated level of heparin sulfate proteoglycans. Moreover, a quantitative mass-spectrometry analysis of proteins secreted by COG-deficient cell lines revealed abnormal secretion of SIL1 and ERGIC-53 proteins by COG4-G516R cells. Interestingly, the clinical phenotype of patients with congenital mutations in the SIL1 gene (Marinesco-Sjogren syndrome) overlaps with the phenotype of COG4-G516R patients (Saul-Wilson syndrome). Our work is the first compressive study involving the creation of different COG mutations in different cell lines other than the patient's fibroblast. It may help to address the underlying cause of the phenotypic defects leading to the discovery of a proper treatment guideline for COG-CDGs.

Glycosylation States on Intact Proteins Determined by NMR Spectroscopy

Protein glycosylation is important in many organisms for proper protein folding, signaling, cell adhesion, protein-protein interactions, and immune responses. Thus, effectively determining the extent of glycosylation in glycoprotein therapeutics is crucial. Up to now, characterizing protein glycosylation has been carried out mostly by liquid chromatography mass spectrometry (LC-MS), which requires careful sample processing, e.g., glycan removal or protein digestion and glycopeptide enrichment. Herein, we introduce an NMR-based method to better characterize intact glycoproteins in natural abundance. This non-destructive method relies on exploiting differences in nuclear relaxation to suppress the NMR signals of the protein while maintaining glycan signals. Using RNase B Man5 and RNase B Man9, we establish reference spectra that can be used to determine the different glycoforms present in heterogeneously glycosylated commercial RNase B.

The Close Relationship between the Golgi Trafficking Machinery and Protein Glycosylation

Glycosylation is the most common post-translational modification of proteins; it mediates their correct folding and stability, as well as their transport through the secretory transport. Changes in N- and O-linked glycans have been associated with multiple pathological conditions including congenital disorders of glycosylation, inflammatory diseases and cancer. Glycoprotein glycosylation at the Golgi involves the coordinated action of hundreds of glycosyltransferases and glycosidases, which are maintained at the correct location through retrograde vesicle trafficking between Golgi cisternae. In this review, we describe the molecular machinery involved in vesicle trafficking and tethering at the Golgi apparatus and the effects of mutations in the context of glycan biosynthesis and human diseases.

Golgi-Dependent Copper Homeostasis Sustains Synaptic Development and Mitochondrial Content

Rare genetic diseases preponderantly affect the nervous system causing neurodegeneration to neurodevelopmental disorders. This is the case for both Menkes and Wilson disease, arising from mutations in ATP7A and ATP7B, respectively. The ATP7A and ATP7B proteins localize to the Golgi and regulate copper homeostasis. We demonstrate genetic and biochemical interactions between ATP7 paralogs with the conserved oligomeric Golgi (COG) complex, a Golgi apparatus vesicular tether. Disruption of Drosophila copper homeostasis by ATP7 tissue-specific transgenic expression caused alterations in epidermis, aminergic, sensory, and motor neurons. Prominent among neuronal phenotypes was a decreased mitochondrial content at synapses, a phenotype that paralleled with alterations of synaptic morphology, transmission, and plasticity. These neuronal and synaptic phenotypes caused by transgenic expression of ATP7 were rescued by downregulation of COG complex subunits. We conclude that the integrity of Golgi-dependent copper homeostasis mechanisms, requiring ATP7 and COG, are necessary to maintain mitochondria functional integrity and localization to synapses.SIGNIFICANCE STATEMENT Menkes and Wilson disease affect copper homeostasis and characteristically afflict the nervous system. However, their molecular neuropathology mechanisms remain mostly unexplored. We demonstrate that copper homeostasis in neurons is maintained by two factors that localize to the Golgi apparatus, ATP7 and the conserved oligomeric Golgi (COG) complex. Disruption of these mechanisms affect mitochondrial function and localization to synapses as well as neurotransmission and synaptic plasticity. These findings suggest communication between the Golgi apparatus and mitochondria through homeostatically controlled cellular copper levels and copper-dependent enzymatic activities in both organelles.

Golgi inCOGnito: From vesicle tethering to human disease

The Conserved Oligomeric Golgi (COG) complex, a multi-subunit vesicle tethering complex of the CATCHR (Complexes Associated with Tethering Containing Helical Rods) family, controls several aspects of cellular homeostasis by orchestrating retrograde vesicle traffic within the Golgi. The COG complex interacts with all key players regulating intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, and vesicular coats. In cells, COG deficiencies result in the accumulation of non-tethered COG-complex dependent (CCD) vesicles, dramatic morphological and functional abnormalities of the Golgi and endosomes, severe defects in N- and O- glycosylation, Golgi retrograde trafficking, sorting and protein secretion. In humans, COG mutations lead to severe multi-systemic diseases known as COG-Congenital Disorders of Glycosylation (COG-CDG). In this report, we review the current knowledge of the COG complex and analyze COG-related trafficking and glycosylation defects in COG-CDG patients.

Sugary Logistics Gone Wrong: Membrane Trafficking and Congenital Disorders of Glycosylation

Glycosylation is an important post-translational modification for both intracellular and secreted proteins. For glycosylation to occur, cargo must be transported after synthesis through the different compartments of the Golgi apparatus where distinct monosaccharides are sequentially bound and trimmed, resulting in increasingly complex branched glycan structures. Of utmost importance for this process is the intraorganellar environment of the Golgi. Each Golgi compartment has a distinct pH, which is maintained by the vacuolar H+-ATPase (V-ATPase). Moreover, tethering factors such as Golgins and the conserved oligomeric Golgi (COG) complex, in concert with coatomer (COPI) and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion, efficiently deliver glycosylation enzymes to the right Golgi compartment. Together, these factors maintain intra-Golgi trafficking of proteins involved in glycosylation and thereby enable proper glycosylation. However, pathogenic mutations in these factors can cause defective glycosylation and lead to diseases with a wide variety of symptoms such as liver dysfunction and skin and bone disorders. Collectively, this group of disorders is known as congenital disorders of glycosylation (CDG). Recent technological advances have enabled the robust identification of novel CDGs related to membrane trafficking components. In this review, we highlight differences and similarities between membrane trafficking-related CDGs.

Modeling Glycan Processing Reveals Golgi-Enzyme Homeostasis upon Trafficking Defects and Cellular Differentiation

The decoration of proteins by carbohydrates is essential for eukaryotic life yet heterogeneous due to a lack of biosynthetic templates. This complex carbohydrate mixture—the glycan profile—is generated in the compartmentalized Golgi, in which level and localization of glycosylation enzymes are key determinants. Here, we develop and validate a computational model for glycan biosynthesis to probe how the biosynthetic machinery creates different glycan profiles. We combined stochastic modeling with Bayesian fitting that enables rigorous comparison to experimental data despite starting with uncertain initial parameters. This is an important development in the field of glycan modeling, which revealed biological insights about the glycosylation machinery in altered cellular states. We experimentally validated changes in N-linked glycan-modifying enzymes in cells with perturbed intra-Golgi-enzyme sorting and the predicted glycan-branching activity during osteogenesis. Our model can provide detailed information on altered biosynthetic paths, with potential for advancing treatments for glycosylation-related diseases and glyco-engineering of cells.

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Developed a stochastic model of N-glycosylation coupled with Bayesian fitting

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Validated predicted changes of Golgi organization in trafficking mutants

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Model pinpointed functionally relevant glycan alterations in osteogenesis

Arabidopsis COG6 is essential for pollen tube growth and Golgi structure maintenance

The conserved oligomeric Golgi (COG) complex, which consists of eight subunits named COG1-COG8, is highly conserved with homologous subunits present in most eukaryotic species. In yeast and mammalian, the COG complex has been implicated in the tethering of retrograde intra-Golgi vesicles. Although homologs of COG subunits have been identified in Arabidopsis, the functions of the complex and its subunits remain to be fully elucidated. In this study, we have utilized genetic and cytologic approaches to characterize the role of the COG6 subunit. We showed that a mutation in COG6 caused male transmission defect due to aberrant pollen tube growth. At the subcellular level, Golgi bodies exhibited altered morphology in cog6 pollen and cell wall components were incorrectly deposited in pollen tubes. COG6 fused to green fluorescent protein (GFP), which complemented the aberrant growth of cog6 pollen tubes, was localized to the Golgi apparatus. We propose that COG6, as a subunit of the COG complex, modulates Golgi morphology and vesicle trafficking homeostasis during pollen tube growth.

Maintaining order: COG complex controls Golgi trafficking, processing, and sorting

The conserved oligomeric Golgi (COG) complex, a multisubunit tethering complex of the CATCHR (complexes associated with tethering containing helical rods) family, controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle targeting within the Golgi. In humans, COG defects lead to severe multisystemic diseases known as COG‐congenital disorders of glycosylation (COG‐CDG). The COG complex both physically and functionally interacts with all classes of molecules maintaining intra‐Golgi trafficking, namely SNAREs, SNARE‐interacting proteins, Rabs, coiled‐coil tethers, and vesicular coats. Here, we review our current knowledge of COG‐related trafficking and glycosylation defects in humans and model organisms, and analyze possible scenarios for the molecular mechanism of the COG orchestrated vesicle targeting.

The N-Glycosylation Processing Potential of the Mammalian Golgi Apparatus

Heterogeneity is an inherent feature of the glycosylation process. Mammalian cells often produce a variety of glycan structures on separate molecules of the same protein, known as glycoforms. This heterogeneity is not random but is controlled by the organization of the glycosylation machinery in the Golgi cisternae. In this work, we use a computational model of the N-glycosylation process to probe how the organization of the glycosylation machinery into different cisternae drives N-glycan biosynthesis toward differing degrees of heterogeneity. Using this model, we demonstrate the N-glycosylation potential and limits of the mammalian Golgi apparatus, for example how the number of cisternae limits the goal of achieving near homogeneity for N-glycans. The production of specific glycoforms guided by this computational study could pave the way for “glycoform engineering,” which will find uses in the functional investigation of glycans, the modulation of glycan-mediated physiological functions, and in biotechnology.

The first case of antenatal presentation in COG8-congenital disorder of glycosylation with a novel splice site mutation and an extended phenotype

Congenital disorders of glycosylation (CDG) are an extremely rapidly growing and phenotypically versatile group of disorders. Conserved oligomeric Golgi (COG) complexes are hetero-octameric proteins involved in retrograde trafficking within the Golgi. Seven of its eight subunits have a causal role in CDG. To date, only three cases of COG8-CDG have been published but none in the antenatal period. We present the first case of antenatally diagnosed COG8-CDG with facial dysmorphism and additional features such as Dandy-Walker malformation and arthrogryposis multiplex congenita, thus expanding the phenotype of this rare disorder. Trio whole exome sequencing revealed a novel homozygous variant in COG8, which creates a new splice site in exon 5 and protein truncation after 12 amino acids downstream to the newly generated splice site. As the mutations of the previous three patients were also identified in exon 5, it is likely to be a potential mutational hotspot in COG8. An association between antenatally increased nuchal translucency and COG8-CDG is also established, which would alert clinicians to its diagnosis early in gestation. It remains to be seen if this observation can be extended to other COG-CDGs.

Selected Golgi-Localized Proteins and Carcinogenesis: What Do We Know?

The role of the Golgi apparatus in carcinogenesis still remains unclear. A number of structural and functional cis-, medial-, and trans-Golgi proteins as well as a complexity of metabolic pathways which they mediate may indicate a central role of the Golgi apparatus in the development and progression of cancer. Pleiotropy of cellular function of the Golgi apparatus makes it a "metabolic heart" or a relay station of a cell, which combines multiple signaling pathways involved in carcinogenesis. Therefore, any damage to or structural abnormality of the Golgi apparatus, causing its fragmentation and/or biochemical dysregulation, results in an up- or downregulation of signaling pathways and may in turn promote tumor progression, as well as local nodal and distant metastases. Three alternative or parallel models of spatial and functional Golgi organization within tumor cells were proposed: (1) compacted Golgi structure, (2) normal Golgi structure with its increased activity, and (3) the Golgi fragmentation with ministacks formation. Regardless of the assumed model, the increased activity of oncogenesis initiators and promoters with inhibition of suppressor proteins results in an increased cell motility and migration, increased angiogenesis, significantly activated trafficking kinetics, proliferation, EMT induction, decreased susceptibility to apoptosis-inducing factors, and modulating immune response to tumor cell antigens. Eventually, this will lead to the increased metastatic potential of cancer cells and an increased risk of lymph node and distant metastases. This chapter provided an overview of the current state of knowledge of selected Golgi proteins, their role in cytophysiology as well as potential involvement in tumorigenesis.

TRAPPopathies: An emerging set of disorders linked to variations in the genes encoding transport protein particle (TRAPP)-associated proteins

The movement of proteins between cellular compartments requires the orchestrated actions of many factors including Rab family GTPases, Soluble NSF Attachment protein REceptors (SNAREs) and so-called tethering factors. One such tethering factor is called TRAnsport Protein Particle (TRAPP), and in humans, TRAPP proteins are distributed into two related complexes called TRAPP II and III. Although thought to act as a single unit within the complex, in the past few years it has become evident that some TRAPP proteins function independently of the complex. Consistent with this, variations in the genes encoding these proteins result in a spectrum of human diseases with diverse, but partially overlapping, phenotypes. This contrasts with other tethering factors such as COG, where variations in the genes that encode its subunits all result in an identical phenotype. In this review, we present an up-to-date summary of all the known disease-related variations of genes encoding TRAPP-associated proteins and the disorders linked to these variations which we now call TRAPPopathies.

Defective mucin-type glycosylation on α-dystroglycan in COG-deficient cells increases its susceptibility to bacterial proteases

Deficiency in subunits of the conserved oligomeric Golgi (COG) complex results in pleiotropic defects in glycosylation and causes congenital disorders in humans. Insight regarding the functional consequences of this defective glycosylation and the identity of specific glycoproteins affected is lacking. A chemical glycobiology strategy was adopted to identify the surface glycoproteins most sensitive to altered glycosylation in COG-deficient Chinese hamster ovary (CHO) cells. Following metabolic labeling, an unexpected increase in GalNAz incorporation into several glycoproteins, including α-dystroglycan (α-DG), was noted in cog1-deficient ldlB cells. Western blotting analysis showed a significantly lower molecular weight for α-DG in ldlB cells compared with WT CHO cells. The underglycosylated α-DG molecules on ldlB cells are highly vulnerable to bacterial proteases that co-purify with V. cholerae neuraminidase, leading to rapid removal of the protein from the cell surface. The purified bacterial mucinase StcE can cleave both WT and ldlB α-DG but did not cause rapid degradation of the fragments, implicating other V. cholerae proteases in the final proteolysis of the fragments. Extending terminal glycosylation on the existing mucin-type glycans of ldlB α-DG stabilized the resulting fragments, indicating that fragment stability, but not the initial fragmentation of the protein, is influenced by the glycosylation status of the cell. This discovery highlights a functional importance for mucin-type O-glycans found on α-DG and reinforces a growing role for these glycans as regulators of extracellular proteolysis and protein stability.

More than just sugars: Conserved oligomeric Golgi complex deficiency causes glycosylation-independent cellular defects

The conserved oligomeric Golgi (COG) complex controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle trafficking within the Golgi. Human COG defects lead to severe multisystemic diseases known as COG-congenital disorders of glycosylation (COG-CDG). To gain better understanding of COG-CDGs, we compared COG knockout cells with cells deficient to 2 key enzymes, Alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyltransferase and uridine diphosphate-glucose 4-epimerase (GALE), which contribute to proper N- and O-glycosylation. While all knockout cells share similar defects in glycosylation, these defects only account for a small fraction of observed COG knockout phenotypes. Glycosylation deficiencies were not associated with the fragmented Golgi, abnormal endolysosomes, defective sorting and secretion or delayed retrograde trafficking, indicating that these phenotypes are probably not due to hypoglycosylation, but to other specific interactions or roles of the COG complex. Importantly, these COG deficiency specific phenotypes were also apparent in COG7-CDG patient fibroblasts, proving the human disease relevance of our CRISPR knockout findings. The knowledge gained from this study has important implications, both for understanding the physiological role of COG complex in Golgi homeostasis in eukaryotic cells, and for better understanding human diseases associated with COG/Golgi impairment.

Membrane detachment is not essential for COG complex function

COG is a multisubunit vesicle tethering complex in the Golgi. We demonstrate that both COG subcomplexes can be permanently attached to Golgi membranes and that major COG functions do not require cycling between the membrane and cytosol.

The conserved oligomeric Golgi (COG) complex is a vesicle tether of the “complexes associated with tethering containing helical rods” family, which functions on the cytoplasmic side of Golgi. It is currently unknown whether COG function, or function of any multisubunit vesicular tether, depends on cycling between the membrane and cytosol. Therefore, we permanently anchored key subunits of COG subcomplexes (COG4, COG7, and COG8) to Golgi membranes using transmembrane protein TMEM115 (TMEM-COG). All TMEM-COG subunits tested were Golgi localized, integrated into the COG complex, and stabilized membrane association of endogenous subunits. Interestingly, TMEM-COG4 and TMEM-COG7 equally rescued COG function in organization of Golgi markers, glycosylation, and abundance of COG-sensitive proteins. In contrast, TMEM-COG8 was not as effective, indicating that N-terminal attachment of COG8 interfered with overall COG structure and function, and none of the TMEM-COG subunits rescued the abnormal Golgi architecture caused by COG knockout. Collectively, these data indicate that both subcomplexes of the COG complex can perform most of COG function when permanently attached to membranes and that the cytosolic pool of COG is not completely essential to COG function.

Glycans modify mesenchymal stem cell differentiation to impact on the function of resulting osteoblasts

Glycans are inherently heterogeneous, yet glycosylation is essential in eukaryotes, and glycans show characteristic cell type-dependent distributions. By using an immortalized human mesenchymal stromal cell (MSC) line model, we show that both N- and O-glycan processing in the Golgi functionally modulates early steps of osteogenic differentiation. We found that inhibiting O-glycan processing in the Golgi prior to the start of osteogenesis inhibited the mineralization capacity of the formed osteoblasts 3 weeks later. In contrast, inhibition of N-glycan processing in MSCs altered differentiation to enhance the mineralization capacity of the osteoblasts. The effect of N-glycans on MSC differentiation was mediated by the phosphoinositide-3-kinase (PI3K)/Akt pathway owing to reduced Akt phosphorylation. Interestingly, by inhibiting PI3K during the first 2 days of osteogenesis, we were able to phenocopy the effect of inhibiting N-glycan processing. Thus, glycan processing provides another layer of regulation that can modulate the functional outcome of differentiation. Glycan processing can thereby offer a novel set of targets for many therapeutically attractive processes.

Summary: Both N- and O-glycan processing modulate MSC differentiation early during osteogenesis to influence mineral formation. Inhibition of N-glycan processing increases mineralization.

trans-Golgi network-bound cargo traffic

Cargo following the retrograde trafficking are sorted at endosomes to be targeted the trans-Golgi network (TGN), a central receiving organelle. Though molecular requirements and their interaction networks have been somewhat established, the complete understanding of the intricate nature of their action mechanisms in every step of the retrograde traffic pathway remains unachieved. This review focuses on elucidating known functions of key regulators, including scission factors at the endosome and tethering/fusion mediators at the receiving dock, TGN, as well as a diverse range of cargo.

SNARE proteins in membrane trafficking

SNAREs are the core machinery mediating membrane fusion. In this review, we provide an update on the recent progress on SNAREs regulating membrane fusion events, especially the more detailed fusion processes dissected by well-developed biophysical methods and in vitro single molecule analysis approaches. We also briefly summarize the relevant research from Chinese laboratories and highlight the significant contributions on our understanding of SNARE-mediated membrane trafficking from scientists in China.

A Brucella Type IV Effector Targets the COG Tethering Complex to Remodel Host Secretory Traffic and Promote Intracellular Replication

Many intracellular pathogens exploit host secretory trafficking to support their intracellular cycle, but knowledge of these pathogenic processes is limited. The bacterium Brucella abortus uses a type IV secretion system (VirB T4SS) to generate a replication-permissive Brucella-containing vacuole (rBCV) derived from the host ER, a process that requires host early secretory trafficking. Here we show that the VirB T4SS effector BspB contributes to rBCV biogenesis and Brucella replication by interacting with the conserved oligomeric Golgi (COG) tethering complex, a major coordinator of Golgi vesicular trafficking, thus remodeling Golgi membrane traffic and redirecting Golgi-derived vesicles to the BCV. Altogether, these findings demonstrate that Brucella modulates COG-dependent trafficking via delivery of a T4SS effector to promote rBCV biogenesis and intracellular proliferation, providing mechanistic insight into how bacterial exploitation of host secretory functions promotes pathogenesis.

MALDI-MS profiling of serum O-glycosylation and N-glycosylation in COG5-CDG

Congenital disorders of glycosylation (CDG) are due to defective glycosylation of glycoconjugates. Conserved oligomeric Golgi (COG)-CDG are genetic diseases due to defects of the COG complex subunits 1-8 causing N-glycan and O-glycan processing abnormalities. In COG-CDG, isoelectric focusing separation of undersialylated glycoforms of serum transferrin and apolipoprotein C-III (apoC-III) allows to detect N-glycosylation and O-glycosylation defects, respectively. COG5-CDG (COG5 subunit deficiency) is a multisystem disease with dysmorphic features, intellectual disability of variable degree, seizures, acquired microcephaly, sensory defects and autistic behavior. We applied matrix-assisted laser desorption/ionization-MS for a high-throughput screening of differential serum O-glycoform and N-glycoform in five patients with COG5-CDG. When compared with age-matched controls, COG5-CDG showed a significant increase of apoC-III_0a (aglycosylated glycoform), whereas apoC-III₁ (mono-sialylated glycoform) decreased significantly. Serum N-glycome of COG5-CDG patients was characterized by the relative abundance of undersialylated and undergalactosylated biantennary and triantennary glycans as well as slight increase of high-mannose structures and hybrid glycans. Using advanced and well-established MS-based approaches, the present findings reveal novel aspects on O-glycan and N-glycan profiling in COG5-CDG patients, thus providing an increase of current knowledge on glycosylation defects caused by impairment of COG subunits, in support of clinical diagnosis. Copyright © 2017 John Wiley & Sons, Ltd.

The interactome of the copper transporter ATP7A belongs to a network of neurodevelopmental and neurodegeneration factors

Genetic and environmental factors, such as metals, interact to determine neurological traits. We reasoned that interactomes of molecules handling metals in neurons should include novel metal homeostasis pathways. We focused on copper and its transporter ATP7A because ATP7A null mutations cause neurodegeneration. We performed ATP7A immunoaffinity chromatography and identified 541 proteins co-isolating with ATP7A. The ATP7A interactome concentrated gene products implicated in neurodegeneration and neurodevelopmental disorders, including subunits of the Golgi-localized conserved oligomeric Golgi (COG) complex. COG null cells possess altered content and subcellular localization of ATP7A and CTR1 (SLC31A1), the transporter required for copper uptake, as well as decreased total cellular copper, and impaired copper-dependent metabolic responses. Changes in the expression of ATP7A and COG subunits in Drosophila neurons altered synapse development in larvae and copper-induced mortality of adult flies. We conclude that the ATP7A interactome encompasses a novel COG-dependent mechanism to specify neuronal development and survival.

A High Copy Suppressor Screen for Autophagy Defects in Saccharomyces arl1Δ and ypt6Δ Strains

In Saccharomyces cerevisiae, Arl1 and Ypt6, two small GTP-binding proteins that regulate membrane traffic in the secretory and endocytic pathways, are also necessary for autophagy. To gain information about potential partners of Arl1 and Ypt6 specifically in autophagy, we carried out a high copy number suppressor screen to identify genes that when overexpressed suppress the rapamycin sensitivity phenotype of arl1Δ and ypt6Δ strains at 37°. From the screen results, we selected COG4, SNX4, TAX4, IVY1, PEP3, SLT2, and ATG5, either membrane traffic or autophagy regulators, to further test whether they can suppress the specific autophagy defects of arl1Δ and ypt6Δ strains. As a result, we identified COG4, SNX4, and TAX4 to be specific suppressors for the arl1Δ strain, and IVY1 and ATG5 for the ypt6Δ strain. Through this screen, we were able to confirm several membrane traffic and autophagy regulators that have novel relationships with Arl1 and Ypt6 during autophagy.

The role of acroblast formation during Drosophila spermatogenesis

Protein recycling is important for maintaining homeostasis of the Golgi and its cisternae. The Vps54 (Scat) protein, a subunit of the GARP tethering complex, is a central factor in retrograde transport to the trans-Golgi. We found the scat¹ mutant to be male sterile in Drosophila with individualization problems occurring during spermatogenesis. Another typically observed phenotype was the abnormal nuclear structure in elongated mutant cysts. When examining the structure and function of the Golgi, a failure in acrosome formation and endosome-Golgi vesicular transport were found in the scat¹ mutant. This acrosome formation defect was due to a fault in the trans-Golgi side of the acroblast ribbon. When testing a mutation in a second retrograde transport protein, Fws, a subunit of the conserved oligomeric Golgi (COG) tethering complex, the acroblast structure, was again disrupted. fws^P caused a similar, albeit milder, acrosome and sperm individualization phenotype as the scat¹ mutant. In the case of fws^P the cis side of the acroblast ribbon was dispersed, in-line with the intra-Golgi retrograde function of COG. Our results highlight the importance of an intact acroblast for acrosome formation, nuclear elongation and therefore sperm maturation. Moreover, these results suggest the importance of retrograde tethering complexes in the formation of a functional Golgi ribbon.

Summary: This study demonstrates that retrograde tethering complexes are necessary to form a functional acroblast, which is essential for normal nuclear elongation and acrosome formation during Drosophila spermatogenesis.

COG lobe B sub-complex engages v-SNARE GS15 and functions via regulated interaction with lobe A sub-complex

The conserved oligomeric Golgi (COG) complex is a peripheral membrane protein complex which orchestrates tethering of intra-Golgi vesicles. We found that COG1-4 (lobe A) and 5-8 (lobe B) protein assemblies are present as independent sub-complexes on cell membranes. Super-resolution microscopy demonstrates that COG sub-complexes are spatially separated on the Golgi with lobe A preferential localization on Golgi stacks and the presence of lobe B on vesicle-like structures, where it physically interacts with v-SNARE GS15. The localization and specific interaction of the COG sub-complexes with the components of vesicle tethering/fusion machinery suggests their different roles in the vesicle tethering cycle. We propose and test a novel model that employs association/disassociation of COG sub-complexes as a mechanism that directs vesicle tethering at Golgi membranes. We demonstrate that defective COG assembly or restriction of tethering complex disassembly by a covalent COG1-COG8 linkage is inhibitory to COG complex activity, supporting the model.

The Secret Life of Tethers: The Role of Tethering Factors in SNARE Complex Regulation

Trafficking in eukaryotic cells is a tightly regulated process to ensure correct cargo delivery to the proper destination organelle or plasma membrane. In this review, we focus on how the vesicle fusion machinery, the SNARE complex, is regulated by the interplay of the multisubunit tethering complexes (MTC) with the SNAREs and Sec1/Munc18 (SM) proteins. Although these factors are used in different stages of membrane trafficking, e.g., Golgi to plasma membrane transport vs. vacuolar fusion, and in a variety of diverse eukaryotic cell types, many commonalities between their functions are being revealed. We explore the various protein-protein interactions and findings from functional reconstitution studies in order to highlight both their common features and the differences in their modes of regulation. These studies serve as a starting point for mechanistic explorations in other systems.

COG Complex Complexities: Detailed Characterization of a Complete Set of HEK293T Cells Lacking Individual COG Subunits

The Conserved Oligomeric Golgi complex is an evolutionarily conserved multisubunit tethering complex (MTC) that is crucial for intracellular membrane trafficking and Golgi homeostasis. The COG complex interacts with core vesicle docking and fusion machinery at the Golgi; however, its exact mechanism of action is still an enigma. Previous studies of COG complex were limited to the use of CDGII (Congenital disorders of glycosylation type II)-COG patient fibroblasts, siRNA mediated knockdowns, or protein relocalization approaches. In this study we have used the CRISPR approach to generate HEK293T knock-out (KO) cell lines missing individual COG subunits. These cell lines were characterized for glycosylation and trafficking defects, cell proliferation rates, stability of COG subunits, localization of Golgi markers, changes in Golgi structure, and N-glycan profiling. We found that all KO cell lines were uniformly deficient in cis/medial-Golgi glycosylation and each had nearly abolished binding of Cholera toxin. In addition, all cell lines showed defects in Golgi morphology, retrograde trafficking and sorting, sialylation and fucosylation, but severities varied according to the affected subunit. Lobe A and Cog6 subunit KOs displayed a more severely distorted Golgi structure, while Cog2, 3, 4, 5, and 7 knock outs had the most hypo glycosylated form of Lamp2. These results led us to conclude that every subunit is essential for COG complex function in Golgi trafficking, though to varying extents. We believe that this study and further analyses of these cells will help further elucidate the roles of individual COG subunits and bring a greater understanding to the class of MTCs as a whole.

Post-Golgi anterograde transport requires GARP-dependent endosome-to-TGN retrograde transport

GARP (tethering factor)- and VAMP4 (v-SNARE)-dependent endosome-to-TGN retrograde transport is required for the efficient post-Golgi anterograde transport of cell-surface integral membrane proteins. Golgi-resident membrane proteins TMEM87A and TMEM87B are involved in endosome-to-TGN retrograde transport.

The importance of endosome-to–trans-Golgi network (TGN) retrograde transport in the anterograde transport of proteins is unclear. In this study, genome-wide screening of the factors necessary for efficient anterograde protein transport in human haploid cells identified subunits of the Golgi-associated retrograde protein (GARP) complex, a tethering factor involved in endosome-to-TGN transport. Knockout (KO) of each of the four GARP subunits, VPS51–VPS54, in HEK293 cells caused severely defective anterograde transport of both glycosylphosphatidylinositol (GPI)-anchored and transmembrane proteins from the TGN. Overexpression of VAMP4, v-SNARE, in VPS54-KO cells partially restored not only endosome-to-TGN retrograde transport, but also anterograde transport of both GPI-anchored and transmembrane proteins. Further screening for genes whose overexpression normalized the VPS54-KO phenotype identified TMEM87A, encoding an uncharacterized Golgi-resident membrane protein. Overexpression of TMEM87A or its close homologue TMEM87B in VPS54-KO cells partially restored endosome-to-TGN retrograde transport and anterograde transport. Therefore GARP- and VAMP4-dependent endosome-to-TGN retrograde transport is required for recycling of molecules critical for efficient post-Golgi anterograde transport of cell-surface integral membrane proteins. In addition, TMEM87A and TMEM87B are involved in endosome-to-TGN retrograde transport.

Congenital disorders of glycosylation: a concise chart of glycocalyx dysfunction

Glycosylation is a ubiquitous modification of lipids and proteins. Despite the essential contribution of glycoconjugates to the viability of all living organisms, diseases of glycosylation in humans have only been identified over the past few decades. The recent development of next-generation DNA sequencing techniques has accelerated the pace of discovery of novel glycosylation defects. The description of multiple mutations across glycosylation pathways not only revealed tremendous diversity in functional impairments, but also pointed to phenotypic similarities, emphasizing the interconnected flow of substrates underlying glycan assembly. The current list of 100 known glycosylation disorders provides an overview of the significance of glycosylation in human development and physiology.

Mutations in proteins of the Conserved Oligomeric Golgi Complex affect polarity, cell wall structure, and glycosylation in the filamentous fungus Aspergillus nidulans

We have described two Aspergillus nidulans gene mutations, designated podB1 (polarity defective) and swoP1 (swollen cell), which cause temperature-sensitive defects during polarization. Mutant strains also displayed unevenness and abnormal thickness of cell walls. Un-polarized or poorly-polarized mutant cells were capable of establishing normal polarity after a shift to a permissive temperature, and mutant hyphae shifted from permissive to restrictive temperature show wall and polarity abnormalities in subsequent growth. The mutated genes (podB=AN8226.3; swoP=AN7462.3) were identified as homologues of COG2 and COG4, respectively, each predicted to encode a subunit of the multi-protein COG (Conserved Oligomeric Golgi) Complex involved in retrograde vesicle trafficking in the Golgi apparatus. Down-regulation of COG2 or COG4 resulted in abnormal polarization and cell wall staining. The GFP-tagged COG2 and COG4 homologues displayed punctate, Golgi-like localization. Lectin-blotting indicated that protein glycosylation was altered in the mutant strains compared to the wild type. A multicopy expression experiment showed evidence for functional interactions between the homologues COG2 and COG4 as well as between COG2 and COG3. To date, this work is the first regarding a functional role of the COG proteins in the development of a filamentous fungus.

Target silencing of components of the conserved oligomeric Golgi complex impairs HIV-1 replication

All viruses require host cell factors to replicate. A large number of host factors have been identified that participate at numerous points of the human immunodeficiency virus 1 (HIV-1) life cycle. Recent evidence supports a role for components of the trans-Golgi network (TGN) in mediating early steps in the HIV-1 life cycle. The conserved oligomeric Golgi (COG) complex is a heteroctamer complex that functions in coat protein complex I (COPI)-mediated intra-Golgi retrograde trafficking and plays an important role in the maintenance of Golgi structure and integrity as well as glycosylation enzyme homeostasis. The targeted silencing of components of lobe B of the COG complex, namely COG5, COG6, COG7 and COG8, inhibited HIV-1 replication. This inhibition of HIV-1 replication preceded late reverse transcription (RT) but did not affect viral fusion. Silencing of the COG interacting protein the t-SNARE syntaxin 5, showed a similar defect in late RT product formation, strengthening the role of the TGN in HIV replication.

TMEM115 is an integral membrane protein of the Golgi complex involved in retrograde transport

Searching and evaluating the Human Protein Atlas for transmembrane proteins enabled us to identify an integral membrane protein, TMEM115, that is enriched in the Golgi complex. Biochemical and cell biological analysis suggested that TMEM115 has four candidate transmembrane domains located in the N-terminal region. Both the N- and C-terminal domains are oriented towards the cytoplasm. Immunofluorescence analysis supports that TMEM115 is enriched in the Golgi cisternae. Functionally, TMEM115 knockdown or overexpression delays Brefeldin-A-induced Golgi-to-ER retrograde transport, phenocopying cells with mutations or silencing of the conserved oligomeric Golgi (COG) complex. Co-immunoprecipitation and in vitro binding experiments reveals that TMEM115 interacts with the COG complex, and might self-interact to form dimers or oligomers. A short region (residues 206–229) immediately to the C-terminal side of the fourth transmembrane domain is both necessary and sufficient for Golgi targeting. Knockdown of TMEM115 also reduces the binding of the lectins peanut agglutinin (PNA) and Helix pomatia agglutinin (HPA), suggesting an altered O-linked glycosylation profile. These results establish that TMEM115 is an integral membrane protein of the Golgi stack regulating Golgi-to-ER retrograde transport and is likely to be part of the machinery of the COG complex.

From endosomes to the trans-Golgi network

The retrograde trafficking from endosomes to the trans-Golgi network (TGN) is one of the major endocytic pathways to divert proteins and lipids away from lysosomal degradation. Retrograde transported cargos enter the TGN via two itineraries from either the early endosome/recycling endosome or the late endosome and involve various machinery components such as retromer, sorting nexins, clathrin, small GTPases, tethering factors and SNAREs. Recently, the pathway has been recognized for its role in signal transduction, physiology and pathogenesis of human diseases.

Filter-aided N-glycan separation (FANGS): a convenient sample preparation method for mass spectrometric N-glycan profiling

We have developed a simple method for the release and isolation of glycoprotein N-glycans from whole-cell lysates using less than a million cells, for subsequent implementation with mass spectrometric analysis. Cellular protein extracts prepared using SDS solubilization were sequentially treated in a membrane filter device to ultimately release glycans enzymatically using PNGase F in the volatile buffer ammonium bicarbonate. The released glycans are recovered in the filtrate following centrifugation and typically permethylated prior to mass spectrometric analysis. We call our method “filter-aided N-glycan separation” and have successfully applied it to investigate N-glycan profiles of wild-type and mutant Chinese hamster ovary cells. This method is readily multiplexed and, because of the small numbers of cells needed, is compatible with the analysis of replicate samples to assess the true nature of glycan variability in tissue culture samples.

Dissecting functions of the conserved oligomeric Golgi tethering complex using a cell-free assay

Vesicle transport sorts proteins between compartments and is thereby responsible for generating the non-uniform protein distribution along the eukaryotic secretory and endocytic pathways. The mechanistic details of specific vesicle targeting are not yet well characterized at the molecular level. We have developed a cell-free assay that reconstitutes vesicle targeting utilizing the recycling of resident enzymes within the Golgi apparatus. The assay has physiological properties, and could be used to show that the two lobes of the conserved oligomeric Golgi tethering complex play antagonistic roles in trans-Golgi vesicle targeting. Moreover, we can show that the assay is sensitive to several different congenital defects that disrupt Golgi function and therefore cause glycosylation disorders. Consequently, this assay will allow mechanistic insight into the targeting step of vesicle transport at the Golgi, and could also be useful for characterizing some novel cases of congenital glycosylation disorders.

Oxysterol-binding protein (OSBP) is required for the perinuclear localization of intra-Golgi v-SNAREs

OSBP regulates the Golgi cholesterol level. This study demonstrates that OSBP and cholesterol are essential for localization of Golgi v-SNAREs. Knockdown of ArfGAP1 restores v-SNARE localization in OSBP-depleted cells, suggesting that OSBP-regulated cholesterol ensures proper COP-I vesicle transport.

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) have been implicated in the distribution of sterols among intracellular organelles. OSBP regulates the Golgi cholesterol level, but how it relates to Golgi function is elusive. Here we report that OSBP is essential for the localization of intra-Golgi soluble vesicle N-ethylmaleimide-sensitive fusion attachment protein receptors (v-SNAREs). Depletion of OSBP by small interfering RNA causes mislocalization of intra-Golgi v-SNAREs GS28 and GS15 throughout the cytoplasm without affecting the perinuclear localization of Golgi target-SNARE syntaxin5 and reduces the abundance of a Golgi enzyme, mannosidase II (Man II). GS28 mislocalization and Man II reduction are also induced by cellular cholesterol depletion. Three domains of OSBP—an endoplasmic reticulum–targeting domain, a Golgi-targeting domain, and a sterol-binding domain—are all required for Golgi localization of GS28. Finally, GS28 mislocalization and Man II reduction in OSBP-depleted cells are largely restored by depletion of ArfGAP1, a regulator of the budding of coat protein complex (COP)-I vesicles. From these results, we postulate that Golgi cholesterol level, which is controlled by OSBP, is essential for Golgi localization of intra-Golgi v-SNAREs by ensuring proper COP-I vesicle transport.

Deficiency of the Cog8 subunit in normal and CDG-derived cells impairs the assembly of the COG and Golgi SNARE complexes

Multiple mutations in different subunits of the tethering complex Conserved Oligomeric Golgi (COG) have been identified as a cause for Congenital Disorders of Glycosylation (CDG) in humans. Yet, the mechanisms by which COG mutations induce the pleiotropic CDG defects have not been fully defined. By detailed analysis of Cog8 deficiency in either HeLa cells or CDG-derived fibroblasts, we show that Cog8 is required for the assembly of both the COG complex and the Golgi Stx5-GS28-Ykt6-GS15 and Stx6-Stx16-Vti1a-VAMP4 SNARE complexes. The assembly of these SNARE complexes is also impaired in cells derived from a Cog7-deficient CDG patient. Likewise, the integrity of the COG complex is also impaired in Cog1-, Cog4- and Cog6-depleted cells. Significantly, deficiency of Cog1, Cog4, Cog6 or Cog8 distinctly influences the production of COG subcomplexes and their Golgi targeting. These results shed light on the structural organization of the COG complex and its subcellular localization, and suggest that its integrity is required for both tethering of transport vesicles to the Golgi apparatus and the assembly of Golgi SNARE complexes. We propose that these two key functions are generally and mechanistically impaired in COG-associated CDG patients, thereby exerting severe pleiotropic defects.

A model for the self-organization of vesicular flux and protein distributions in the Golgi apparatus

The generation of two non-identical membrane compartments via exchange of vesicles is considered to require two types of vesicles specified by distinct cytosolic coats that selectively recruit cargo, and two membrane-bound SNARE pairs that specify fusion and differ in their affinities for each type of vesicles. The mammalian Golgi complex is composed of 6-8 non-identical cisternae that undergo gradual maturation and replacement yet features only two SNARE pairs. We present a model that explains how distinct composition of Golgi cisternae can be generated with two and even a single SNARE pair and one vesicle coat. A decay of active SNARE concentration in aging cisternae provides the seed for a cis[Formula: see text]trans SNARE gradient that generates the predominantly retrograde vesicle flux which further enhances the gradient. This flux in turn yields the observed inhomogeneous steady-state distribution of Golgi enzymes, which compete with each other and with the SNAREs for incorporation into transport vesicles. We show analytically that the steady state SNARE concentration decays exponentially with the cisterna number. Numerical solutions of rate equations reproduce the experimentally observed SNARE gradients, overlapping enzyme peaks in cis, medial and trans and the reported change in vesicle nature across the Golgi: Vesicles originating from younger cisternae mostly contain Golgi enzymes and SNAREs enriched in these cisternae and extensively recycle through the Endoplasmic Reticulum (ER), while the other subpopulation of vesicles contains Golgi proteins prevalent in older cisternae and hardly reaches the ER.

The Golgi puppet master: COG complex at center stage of membrane trafficking interactions

The central organelle within the secretory pathway is the Golgi apparatus, a collection of flattened membranes organized into stacks. The cisternal maturation model of intra-Golgi transport depicts Golgi cisternae that mature from cis to medial to trans by receiving resident proteins, such as glycosylation enzymes via retrograde vesicle-mediated recycling. The conserved oligomeric Golgi (COG) complex, a multi-subunit tethering complex of the complexes associated with tethering containing helical rods family, organizes vesicle targeting during intra-Golgi retrograde transport. The COG complex, both physically and functionally, interacts with all classes of molecules maintaining intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, vesicular coats, and molecular motors. In this report, we will review the current state of the COG interactome and analyze possible scenarios for the molecular mechanism of the COG orchestrated vesicle targeting, which plays a central role in maintaining glycosylation homeostasis in all eukaryotic cells.

Molecular insights into vesicle tethering at the Golgi by the conserved oligomeric Golgi (COG) complex and the golgin TATA element modulatory factor (TMF)

Protein sorting between eukaryotic compartments requires vesicular transport, wherein tethering provides the first contact between vesicle and target membranes. Here we map and start to functionally analyze the interaction network of the conserved oligomeric Golgi (COG) complex that mediates retrograde tethering at the Golgi. The interactions of COG subunits with members of transport factor families assign the individual subunits as specific interaction hubs. Functional analysis of selected interactions suggests a mechanistic tethering model. We find that the COG complex interacts with two different Rabs in addition to each end of the golgin "TATA element modulatory factor" (TMF). This allows COG to potentially bridge the distance between the distal end of the golgin and the target membrane thereby promoting tighter docking. Concurrently we show that the central portion of TMF can bind to Golgi membranes that are liberated of their COPI cover. This latter interaction could serve to bring vesicle and target membranes into close apposition prior to fusion. A target selection mechanism, in which a hetero-oligomeric tethering factor organizes Rabs and coiled transport factors to enable protein sorting specificity, could be applicable to vesicle targeting throughout eukaryotic cells.

A sensor of protein O-glycosylation based on sequential processing in the Golgi apparatus

Protein O-glycosylation is important in numerous processes including the regulation of proteolytic processing sites by O-glycan masking in select newly synthesized proteins. To investigate O-glycan-mediated masking using an assay amenable to large-scale screens, we generated a fluorescent biosensor with an O-glycosylation site situated to mask a furin cleavage site. The sensor is activated when O-glycosylation fails to occur because furin cleavage releases a blocking domain allowing dye binding to a fluorogen activating protein. Thus, by design, glycosylation should block furin from activating the sensor only if it occurs first, which is predicted by the conventional view of Golgi organization. Indeed, and in contrast to the recently proposed rapid partitioning model, the sensor was non-fluorescent under normal conditions but became fluorescent when the Golgi complex was decompartmentalized. To test the utility of the sensor as a screening tool, cells expressing the sensor were exposed to a known inhibitor of O-glycosylation extension or siRNAs targeting factors known to alter glycosylation efficiency. These conditions activated the sensor substantiating its potential in identifying new inhibitors and cellular factors related to protein O-glycosylation. In summary, these findings confirm sequential processing in the Golgi, establish a new tool for studying the regulation of proteolytic processing by O-glycosylation, and demonstrate the sensor's potential usefulness for future screening projects.

Mutations in Cog7 affect Golgi structure, meiotic cytokinesis and sperm development during Drosophila spermatogenesis

The conserved oligomeric Golgi (COG) complex plays essential roles in Golgi function, vesicle trafficking and glycosylation. Deletions in the human COG7 gene are associated with a rare multisystemic congenital disorder of glycosylation that causes mortality within the first year of life. In this paper, we characterise the Drosophila orthologue of COG7 (Cog7). Loss-of-function Cog7 mutants are viable but male sterile. The Cog7 gene product is enriched in the Golgi stacks and in Golgi-derived structures throughout spermatogenesis. Mutations in the Cog7 gene disrupt Golgi architecture and reduce the number of Golgi stacks in primary spermatocytes. During spermiogenesis, loss of the Cog7 protein impairs the assembly of the Golgi-derived acroblast in spermatids and affects axoneme architecture. Similar to the Cog5 homologue, four way stop (Fws), Cog7 enables furrow ingression during cytokinesis. We show that the recruitment of the small GTPase Rab11 and the phosphatidylinositol transfer protein Giotto (Gio) to the cleavage site requires a functioning wild-type Cog7 gene. In addition, Gio coimmunoprecipitates with Cog7 and with Rab11 in the testes. Our results altogether implicate Cog7 as an upstream component in a gio-Rab11 pathway controlling membrane addition during cytokinesis.

Structures and mechanisms of vesicle coat components and multisubunit tethering complexes

Eukaryotic cells face a logistical challenge in ensuring prompt and precise delivery of vesicular cargo to specific organelles within the cell. Coat protein complexes select cargo and initiate vesicle formation, while multisubunit tethering complexes participate in the delivery of vesicles to target membranes. Understanding these macromolecular assemblies has greatly benefited from their structural characterization. Recent structural data highlight principles in coat recruitment and uncoating in both the endocytic and retrograde pathways, and studies on the architecture of tethering complexes provide a framework for how they might link vesicles to the respective acceptor compartments and the fusion machinery.