Mitotic phosphorylation of Golgi reassembly stacking protein 55 by mitogen-activated protein kinase ERK2

Golgi Reassembly Stacking Protein 2 Modulates Myometrial Contractility during Labor by Affecting ATP Production

The mechanism of maintaining myometrial contractions during labor remains unclear. Autophagy has been reported to be activated in laboring myometrium, along with the high expression of Golgi reassembly stacking protein 2 (GORASP2), a protein capable of regulating autophagy activation. This study aimed to investigate the role and mechanism of GORASP2 in uterine contractions during labor. Western blot confirmed the increased expression of GORASP2 in laboring myometrium. Furthermore, the knockdown of GORASP2 in primary human myometrial smooth muscle cells (hMSMCs) using siRNA resulted in reduced cell contractility. This phenomenon was independent of the contraction-associated protein and autophagy. Differential mRNAs were analyzed using RNA sequencing. Subsequently, KEGG pathway analysis identified that GORASP2 knockdown suppressed several energy metabolism pathways. Furthermore, reduced ATP levels and aerobic respiration impairment were observed in measuring the oxygen consumption rate (OCR). These findings suggest that GORASP2 is up-regulated in the myometrium during labor and modulates myometrial contractility mainly by maintaining ATP production.

Retro-2 alters Golgi structure

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

Lactate induces vascular permeability via disruption of VE-cadherin in endothelial cells during sepsis

Circulating lactate levels are a critical biomarker for sepsis and are positively correlated with sepsis-associated mortality. We investigated whether lactate plays a biological role in causing endothelial barrier dysfunction in sepsis. We showed that lactate causes vascular permeability and worsens organ dysfunction in CLP sepsis. Mechanistically, lactate induces ERK-dependent activation of calpain1/2 for VE-cadherin proteolytic cleavage, leading to the enhanced endocytosis of VE-cadherin in endothelial cells. In addition, we found that ERK2 interacts with VE-cadherin and stabilizes VE-cadherin complex in resting endothelial cells. Lactate-induced ERK2 phosphorylation promotes ERK2 disassociation from VE-cadherin. In vivo suppression of lactate production or genetic depletion of lactate receptor GPR81 mitigates vascular permeability and multiple organ injury and improves survival outcome in polymicrobial sepsis. Our study reveals that metabolic cross-talk between glycolysis-derived lactate and the endothelium plays a critical role in the pathophysiology of sepsis.

Regulation of the Golgi Apparatus by p38 and JNK Kinases during Cellular Stress Responses

p38 and c-Jun N-terninal kinase (JNK) are activated in response to acute stress and inflammatory signals. Through modification of a plethora of substrates, these kinases profoundly re-shape cellular physiology for the optimal response to a harmful environment and/or an inflammatory state. Here, we utilized phospho-proteomics to identify several hundred substrates for both kinases. Our results indicate that the scale of signaling from p38 and JNK are of a similar magnitude. Among the many new targets, we highlight the regulation of the transcriptional regulators grb10-interacting GYF protein 1 and 2 (GIGYF1/2) by p38-dependent MAP kinase-activated protein kinase 2 (MK2) phosphorylation and 14-3-3 binding. We also show that the Golgi apparatus contains numerous substrates, and is a major target for regulation by p38 and JNK. When activated, these kinases mediate structural rearrangement of the Golgi apparatus, which positively affects protein flux through the secretory system. Our work expands on our knowledge about p38 and JNK signaling with important biological ramifications.

Gβγ regulates mitotic Golgi fragmentation and G2/M cell cycle progression

Heterotrimeric G proteins (αβγ) function at the cytoplasmic surface of a cell’s plasma membrane to transduce extracellular signals into cellular responses. However, numerous studies indicate that G proteins also play noncanonical roles at unique intracellular locations. Previous work has established that G protein βγ subunits (Gβγ) regulate a signaling pathway on the cytoplasmic surface of Golgi membranes that controls the exit of select protein cargo. Now, we demonstrate a novel role for Gβγ in regulating mitotic Golgi fragmentation, a key checkpoint of the cell cycle that occurs in the late G2 phase. We show that small interfering RNA–mediated depletion of Gβ1 and Gβ2 in synchronized cells causes a decrease in the number of cells with fragmented Golgi in late G2 and a delay of entry into mitosis and progression through G2/M. We also demonstrate that during G2/M Gβγ acts upstream of protein kinase D and regulates the phosphorylation of the Golgi structural protein GRASP55. Expression of Golgi-targeted GRK2ct, a Gβγ-sequestering protein used to inhibit Gβγ signaling, also causes a decrease in Golgi fragmentation and a delay in mitotic progression. These results highlight a novel role for Gβγ in regulation of Golgi structure.

PsGRASP, a Golgi Reassembly Stacking Protein in Phytophthora sojae, Is Required for Mycelial Growth, Stress Responses, and Plant Infection

Golgi reassembly stacking proteins (GRASPs) play important roles in Golgi structure formation, ER stress response, and unconventional secretion in eukaryotic cells. However, GRASP functions in oomycetes haven’t been adequately characterized. Here, we report the identification and functional analysis of PsGRASP, a GRASP-encoding gene from the soybean-infecting oomycete Phytophthora sojae. Transcriptional profiling showed that PsGRASP expression is up-regulated at the infection stages. PsGRASP knockout mutants were created using the CRISPR/Cas9 system. These mutants exhibited impaired vegetative growth, zoospore release and virulence. PsGRASP was involved ER stress responses and altered laccase activity. Our work suggests that PsGRASP is crucial for P. sojae development and pathogenicity.

Mitotic HOOK3 phosphorylation by ERK1c drives microtubule-dependent Golgi destabilization and fragmentation

ERK1c is an alternatively spliced isoform of ERK1 that specifically regulates mitotic Golgi fragmentation, which allows division of the Golgi during mitosis. We have previously shown that ERK1c translocates to the Golgi during mitosis where it is activated by a resident MEK1b to induce Golgi fragmentation. However, the mechanism of ERK1c functions in the Golgi remained obscure. Here, we searched for ERK1c substrates and identified HOOK3 as a mediator of ERK1c-induced mitotic Golgi fragmentation, which requires a second phosphorylation by AuroraA for its function. In cycling cells, HOOK3 interacts with microtubules (MTs) and links them to the Golgi. Early in mitosis, HOOK3 is phosphorylated by ERK1c and later by AuroraA, resulting in HOOK3 detachment from the MTs, and elevated interaction with GM130. This detachment modulates Golgi stability and allows fragmentation of the Golgi. This study demonstrates a novel mechanism of Golgi apparatus destabilization early in mitosis to allow mitotic progression.

The ERK mitogen-activated protein kinase signaling network: the final frontier in RAS signal transduction

The RAF-MEK-ERK mitogen-activated protein kinase (MAPK) cascade is aberrantly activated in a diverse set of human cancers and the RASopathy group of genetic developmental disorders. This protein kinase cascade is one of the most intensely studied cellular signaling networks and has been frequently targeted by the pharmaceutical industry, with more than 30 inhibitors either approved or under clinical evaluation. The ERK-MAPK cascade was originally depicted as a serial and linear, unidirectional pathway that relays extracellular signals, such as mitogenic stimuli, through the cytoplasm to the nucleus. However, we now appreciate that this three-tiered protein kinase cascade is a central core of a complex network with dynamic signaling inputs and outputs and autoregulatory loops. Despite our considerable advances in understanding the ERK-MAPK network, the ability of cancer cells to adapt to the inhibition of key nodes reveals a level of complexity that remains to be fully understood. In this review, we summarize important developments in our understanding of the ERK-MAPK network and identify unresolved issues for ongoing and future study.

Signaling diversity enabled by Rap1-regulated plasma membrane ERK with distinct temporal dynamics

A variety of different signals induce specific responses through a common, extracellular-signal regulated kinase (ERK)-dependent cascade. It has been suggested that signaling specificity can be achieved through precise temporal regulation of ERK activity. Given the wide distrubtion of ERK susbtrates across different subcellular compartments, it is important to understand how ERK activity is temporally regulated at specific subcellular locations. To address this question, we have expanded the toolbox of Förster Resonance Energy Transfer (FRET)-based ERK biosensors by creating a series of improved biosensors targeted to various subcellular regions via sequence specific motifs to measure spatiotemporal changes in ERK activity. Using these sensors, we showed that EGF induces sustained ERK activity near the plasma membrane in sharp contrast to the transient activity observed in the cytoplasm and nucleus. Furthermore, EGF-induced plasma membrane ERK activity involves Rap1, a noncanonical activator, and controls cell morphology and EGF-induced membrane protrusion dynamics. Our work strongly supports that spatial and temporal regulation of ERK activity is integrated to control signaling specificity from a single extracellular signal to multiple cellular processes.

Cytosolic Ca2+ Modulates Golgi Structure Through PKCα-Mediated GRASP55 Phosphorylation

It has been well documented that the ER responds to cellular stresses through the unfolded protein response (UPR), but it is unknown how the Golgi responds to similar stresses. In this study, we treated HeLa cells with ER stress inducers, thapsigargin (TG), tunicamycin (Tm), and dithiothreitol (DTT), and found that only TG treatment resulted in Golgi fragmentation. TG induced Golgi fragmentation at a low dose and short time when UPR was undetectable, indicating that Golgi fragmentation occurs independently of ER stress. Further experiments demonstrated that TG induces Golgi fragmentation through elevating intracellular Ca²⁺ and protein kinase Cα (PKCα) activity, which phosphorylates the Golgi stacking protein GRASP55. Significantly, activation of PKCα with other activating or inflammatory agents, including phorbol 12-myristate 13-acetate and histamine, modulates Golgi structure in a similar fashion. Hence, our study revealed a novel mechanism through which increased cytosolic Ca²⁺ modulates Golgi structure and function.

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Thapsigargin (TG) treatment leads to Golgi fragmentation independent of ER stress

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TG induces Golgi fragmentation through elevated cytosolic Ca²⁺

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TG-induced cytosolic Ca²⁺ spikes activate PKCα that phosphorylates GRASP55

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Histamine modulates the Golgi structure and function by a similar mechanism

GRASP depletion-mediated Golgi destruction decreases cell adhesion and migration via the reduction of α5β1 integrin

The Golgi apparatus is a membrane-bound organelle that serves as the center for trafficking and processing of proteins and lipids. To perform these functions, the Golgi forms a multilayer stacked structure held by GRASP55 and GRASP65 trans-oligomers and perhaps their binding partners. Depletion of GRASP proteins disrupts Golgi stack formation and impairs critical functions of the Golgi, such as accurate protein glycosylation and sorting. However, how Golgi destruction affects other cellular activities is so far unknown. Here, we report that depletion of GRASP proteins reduces cell attachment and migration. Interestingly, GRASP depletion reduces the protein level of α5β1 integrin, the major cell adhesion molecule at the surface of HeLa and MDA-MB-231 cells, due to decreased integrin protein synthesis. GRASP depletion also increases cell growth and total protein synthesis. These new findings enrich our understanding on the role of the Golgi in cell physiology and provide a potential target for treating protein-trafficking disorders.

Tyrosine phosphorylation directs TACE into extracellular vesicles via unconventional secretion

When studying how HIV-1 Nef can promote packaging of the proinflammatory transmembrane protease TACE (tumor necrosis factor-α converting enzyme) into extracellular vesicles (EVs) we have revealed a novel tyrosine kinase-regulated unconventional protein secretion (UPS) pathway for TACE. When TACE was expressed without its trafficking cofactor iRhom allosteric Hck activation by Nef triggered translocation of TACE into EVs. This process was insensitive to blocking of classical secretion by inhibiting endoplasmic reticulum (ER) to Golgi transport, and involved a distinct form of TACE devoid of normal glycosylation and incompletely processed for prodomain removal. Like most other examples of UPS this process was Golgi reassembly stacking protein (GRASP)-dependent but was not associated with ER stress. These data indicate that Hck-activated UPS provides an alternative pathway for TACE secretion that can bypass iRhom-dependent ER to Golgi transfer, and suggest that tyrosine phosphorylation might have a more general role in regulating UPS.

Unconventional secretion of transmembrane proteins

Over the past 20 years it has become evident that eukaryotic cells utilize both conventional and unconventional pathways to deliver proteins to their target sites. Most proteins with a signal peptide and/or a transmembrane domain are conventionally transported through the endoplasmic reticulum to the Golgi apparatus and then to the plasma membrane. However, an increasing number of both soluble cargos (Type I, II, and III) and integral membrane proteins (Type IV) have been found to reach the plasma membrane via unconventional protein secretion (UPS) pathways that bypass the Golgi apparatus under certain conditions, such as cellular stress or development. Well-known examples of transmembrane proteins that undergo Type IV UPS pathways are position-specific antigen subunit alpha 1 integrin, cystic fibrosis transmembrane conductance regulator, myeloproliferative leukemia virus oncogene, and pendrin. Although we collectively refer to all Golgi-bypassing routes as UPS, individual trafficking pathways are diverse compared to the conventional pathways, and the molecular mechanisms of UPS pathways are not yet completely defined. This review summarizes the intracellular trafficking pathways of UPS cargo proteins, particularly those with transmembrane domains, and discusses the molecular machinery involved in the UPS of transmembrane proteins.

Golgi structure formation, function, and post-translational modifications in mammalian cells

The Golgi apparatus is a central membrane organelle for trafficking and post-translational modifications of proteins and lipids in cells. In mammalian cells, it is organized in the form of stacks of tightly aligned flattened cisternae, and dozens of stacks are often linked laterally into a ribbon-like structure located in the perinuclear region of the cell. Proper Golgi functionality requires an intact architecture, yet Golgi structure is dynamically regulated during the cell cycle and under disease conditions. In this review, we summarize our current understanding of the relationship between Golgi structure formation, function, and regulation, with focus on how post-translational modifications including phosphorylation and ubiquitination regulate Golgi structure and on how Golgi unstacking affects its functions, in particular, protein trafficking, glycosylation, and sorting in mammalian cells.

Knockout of the Golgi stacking proteins GRASP55 and GRASP65 impairs Golgi structure and function

GRASP55 and GRASP65 were knocked out, and it was found that double knockout of GRASP proteins disperses the Golgi stack into single cisternae and tubulovesicular structures, accelerates protein trafficking, and impairs accurate glycosylation of proteins and lipids.

Golgi reassembly stacking protein of 65 kDa (GRASP65) and Golgi reassembly stacking protein of 55 kDa (GRASP55) were originally identified as Golgi stacking proteins; however, subsequent GRASP knockdown experiments yielded inconsistent results with respect to the Golgi structure, indicating a limitation of RNAi-based depletion. In this study, we have applied the recently developed clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology to knock out GRASP55 and GRASP65, individually or in combination, in HeLa and HEK293 cells. We show that double knockout of GRASP proteins disperses the Golgi stack into single cisternae and tubulovesicular structures, accelerates protein trafficking, and impairs accurate glycosylation of proteins and lipids. These results demonstrate a critical role for GRASPs in maintaining the stacked structure of the Golgi, which is required for accurate posttranslational modifications in the Golgi. Additionally, the GRASP knockout cell lines developed in this study will be useful tools for studying the role of GRASP proteins in other important cellular processes.

Manganese-induced trafficking and turnover of GPP130 is mediated by sortilin

By binding and directing the cycling Golgi protein GPP130 to lysosomes, the sorting receptor sortilin mediates the manganese-induced GPP130 down-regulation that protects against Shiga toxicosis.

Elevated, nontoxic doses of manganese (Mn) protect against Shiga toxin-1–induced cell death via down-regulation of GPP130, a cycling Golgi membrane protein that serves as an endosome-to-Golgi trafficking receptor for the toxin. Mn binds to GPP130 in the Golgi and causes GPP130 to oligomerize/aggregate, and the complexes are diverted to lysosomes. In fact, based on experiments using the self-interacting FM domain, it appears generally true that aggregation of a Golgi protein leads to its lysosomal degradation. How such oligomers are selectively sorted out of the Golgi is unknown. Here we provide evidence that Mn-induced exit of GPP130 from the trans-Golgi network (TGN) toward lysosomes is mediated by the sorting receptor sortilin interacting with the lumenal stem domain of GPP130. In contrast, FM-induced lysosomal trafficking of the Golgi protein galactosyltransferase was sortilin independent and occurred even in the absence of its native lumenal domain. Thus sortilin-dependent as well as sortilin-independent sorting mechanisms target aggregated Golgi membrane proteins for lysosomal degradation.

Genetic, structural, and chemical insights into the dual function of GRASP55 in germ cell Golgi remodeling and JAM-C polarized localization during spermatogenesis

Spermatogenesis is a dynamic process that is regulated by adhesive interactions between germ and Sertoli cells. Germ cells express the Junctional Adhesion Molecule-C (JAM-C, encoded by Jam3), which localizes to germ/Sertoli cell contacts. JAM-C is involved in germ cell polarity and acrosome formation. Using a proteomic approach, we demonstrated that JAM-C interacted with the Golgi reassembly stacking protein of 55 kDa (GRASP55, encoded by Gorasp2) in developing germ cells. Generation and study of Gorasp2-/- mice revealed that knock-out mice suffered from spermatogenesis defects. Acrosome formation and polarized localization of JAM-C in spermatids were altered in Gorasp2-/- mice. In addition, Golgi morphology of spermatocytes was disturbed in Gorasp2-/- mice. Crystal structures of GRASP55 in complex with JAM-C or JAM-B revealed that GRASP55 interacted via PDZ-mediated interactions with JAMs and induced a conformational change in GRASP55 with respect of its free conformation. An in silico pharmacophore approach identified a chemical compound called Graspin that inhibited PDZ-mediated interactions of GRASP55 with JAMs. Treatment of mice with Graspin hampered the polarized localization of JAM-C in spermatids, induced the premature release of spermatids and affected the Golgi morphology of meiotic spermatocytes.

Golgi ribbon disassembly during mitosis, differentiation and disease progression

The Golgi apparatus is tightly integrated into the cellular system where it plays essential roles required for a variety of cellular processes. Its vital functions include not only processing and sorting of proteins and lipids, but also serving as a signaling hub and a microtubule-organizing center. Golgi stacks in mammalian cells are interconnected into a compact ribbon in the perinuclear region. However, the ribbon can undergo distinct disassembly processes that reflect the cellular state or environmental demands and stress. For instance, its most dramatic change takes place in mitosis when the ribbon is efficiently disassembled into vesicles through a combination of ribbon unlinking, cisternal unstacking and vesiculation. Furthermore, the ribbon can also be detached and positioned at specific cellular locations to gain additional functionalities during differentiation, or fragmented to different degrees along disease progression or upon cell death. Here, we describe the major morphological alterations of Golgi ribbon disassembly under physiological and pathological conditions and discuss the underlying mechanisms that drive these changes.

Inhibitor of ppGalNAc-T3-mediated O-glycosylation blocks cancer cell invasiveness and lowers FGF23 levels

Small molecule inhibitors of site-specific O-glycosylation by the polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T) family are currently unavailable but hold promise as therapeutics, especially if selective against individual ppGalNAc-T isozymes. To identify a compound targeting the ppGalNAc-T3 isozyme, we screened libraries to find compounds that act on a cell-based fluorescence sensor of ppGalNAc-T3 but not on a sensor of ppGalNAc-T2. This identified a hit that subsequent in vitro analysis showed directly binds and inhibits purified ppGalNAc-T3 with no detectable activity against either ppGalNAc-T2 or ppGalNAc-T6. Remarkably, the inhibitor was active in two medically relevant contexts. In cell culture, it opposed increased cancer cell invasiveness driven by upregulated ppGalNAc-T3 suggesting the inhibitor might be anti-metastatic. In cells and mice, it blocked ppGalNAc-T3-mediated glycan-masking of FGF23 thereby increasing its cleavage, a possible treatment of chronic kidney disease. These findings establish a pharmacological approach for the ppGalNAc-transferase family and suggest that targeting specific ppGalNAc-transferases will yield new therapeutics.

Glycosylation Quality Control by the Golgi Structure

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

The Golgin Family of Coiled-Coil Tethering Proteins

The golgins are a family of predominantly coiled-coil proteins that are localized to the Golgi apparatus. Golgins are present in all eukaryotes, suggesting an evolutionary conserved function. Golgins are anchored to the Golgi membrane by their carboxy terminus and are predicted to adopt an extended conformation that projects into the surrounding cytoplasm. This arrangement is ideal for the capture or tethering of nearby membranes or cytoskeletal elements. Golgin-mediated tethering is thought to be important for vesicular traffic at the Golgi apparatus, the maintenance of Golgi architecture, as well as the positioning of the Golgi apparatus within cells. In addition to acting as tethers, some golgins can also sequester various factors at the Golgi membrane, allowing for the spatiotemporal regulation of downstream cellular functions. Although it is now established that golgins are membrane and cytoskeleton tethers, the mechanisms underlying tethering remain poorly defined. Moreover, the importance of golgin-mediated tethering in a physiological context remains to be fully explored. This review will describe our current understanding of golgin function, highlighting recent progress that has been made, and goes on to discuss outstanding questions and potential avenues for future research with regard to this family of conserved Golgi-associated proteins.

GRASPs in Golgi Structure and Function

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

Mitotic Golgi translocation of ERK1c is mediated by a PI4KIIIβ-14-3-3γ shuttling complex

Golgi fragmentation is a highly regulated process that allows division of the Golgi complex between the two daughter cells. The mitotic reorganization of the Golgi is accompanied by a temporary block in Golgi functioning, as protein transport in and out of the Golgi stops. Our group has previously demonstrated the involvement of the alternatively spliced variants ERK1c and MEK1b (ERK1 is also known as MAPK3, and MEK1 as MAP2K1) in mitotic Golgi fragmentation. We had also found that ERK1c translocates to the Golgi at the G2 to M phase transition, but the molecular mechanism underlying this recruitment remains unknown. In this study, we narrowed the translocation timing to prophase and prometaphase, and elucidated its molecular mechanism. We found that CDK1 phosphorylates Ser343 of ERK1c, thereby allowing the binding of phosphorylated ERK1c to a complex that consists of PI4KIIIβ (also known as PI4KB) and the 14-3-3γ dimer (encoded by YWHAB). The stability of the complex is regulated by protein kinase D (PKD)-mediated phosphorylation of PI4KIIIβ. The complex assembly induces the Golgi shuttling of ERK1c, where it is activated by MEK1b, and induces Golgi fragmentation. Our work shows that protein shuttling to the Golgi is not completely abolished at the G2 to M phase transition, thus integrating several independent Golgi-regulating processes into one coherent pathway.

Systems biology of the secretory pathway: what have we learned so far?

Several RNAi screens were performed in search for regulators of the secretory pathway. These screens were performed in different organisms and cell lines and relied on different readouts. Therefore, they have only little overlap among their hits, leading to the question of what we have learned from this approach so far and how these screens contributed towards an integrative understanding of the endomembrane system. The aim of this review is to revisit these screens and discuss their strengths and weaknesses as well as potential reasons for their failure to overlap with each other. As with secretory trafficking, RNAi screens were also performed on other cellular processes such as cell migration and autophagy, both of which were shown to be intimately linked to secretion. Another aim of this review is to compare the outcome of the RNAi screens on secretion, autophagy and cell migration and ask whether the functional genomic approaches have uncovered potential mechanistic insights into the links between these processes.

Regulating the large Sec7 ARF guanine nucleotide exchange factors: the when, where and how of activation

Eukaryotic cells require selective sorting and transport of cargo between intracellular compartments. This is accomplished at least in part by vesicles that bud from a donor compartment, sequestering a subset of resident protein "cargos" destined for transport to an acceptor compartment. A key step in vesicle formation and targeting is the recruitment of specific proteins that form a coat on the outside of the vesicle in a process requiring the activation of regulatory GTPases of the ARF family. Like all such GTPases, ARFs cycle between inactive, GDP-bound, and membrane-associated active, GTP-bound, conformations. And like most regulatory GTPases the activating step is slow and thought to be rate limiting in cells, requiring the use of ARF guanine nucleotide exchange factor (GEFs). ARF GEFs are characterized by the presence of a conserved, catalytic Sec7 domain, though they also contain motifs or additional domains that confer specificity to localization and regulation of activity. These domains have been used to define and classify five different sub-families of ARF GEFs. One of these, the BIG/GBF1 family, includes three proteins that are each key regulators of the secretory pathway. GEF activity initiates the coating of nascent vesicles via the localized generation of activated ARFs and thus these GEFs are the upstream regulators that define the site and timing of vesicle production. Paradoxically, while we have detailed molecular knowledge of how GEFs activate ARFs, we know very little about how GEFs are recruited and/or activated at the right time and place to initiate transport. This review summarizes the current knowledge of GEF regulation and explores the still uncertain mechanisms that position GEFs at "budding ready" membrane sites to generate highly localized activated ARFs.

Functional implications of the Golgi and microtubular network in gonadotropes

In contrast to the widely accepted images of the Golgi apparatus as a cup-like shape, the Golgi in pituitary gonadotropes is organized as a spherical shape in which the outer and inner faces are cis- and trans-Golgi elements, respectively. At the center of the spherical Golgi, a pair of centrioles is situated as a microtubule-organizing center from which radiating microtubules isotropically extend toward the cell periphery. This review focuses on the significance of the characteristic organization of the Golgi and microtubule network in gonadotropes, considering the roles of microtubule-dependent membrane transport in the formation and maintenance of the Golgi structure. Because the highly symmetrical organization of the Golgi is possibly perturbed in response to experimental treatments of gonadotropes, monitoring of the Golgi structure in gonadotropes under various experimental conditions will be a novel in vivo approach to elucidate the biogenesis of the Golgi apparatus.

The role of GRASPs in morphological alterations of Golgi apparatus: mechanisms and effects

The Golgi apparatus (GA) is a pivotal organelle in cell metabolism, functioning not only in the processing and transportation of cargoes but also in ion homeostasis, cell apoptosis, and stress sensing. We are interested in the intricate role of GA and the recently present novel concept of 'GA stress'. GA shows various morphological alterations in many neurodegenerative diseases and cell apoptosis induced by biochemical reagents, mechanisms in which oxidative stress is strongly involved. In turn, the structural changes and morphological alterations of the GA could also transduce stress signals. Therefore, besides the biochemical changes, more attention should be paid to the morphological alterations of the GA itself during pathological processes and diseases. The Golgi reassembly and stacking proteins (GRASPs) have been identified as important components acting in the transformation of Golgi structure, and they may thus affect the Golgi functions and cell behavior. In this review, we will discuss the intricate role of the GRASPs in remodeling the GA morphology and focus on their mechanisms and effects in the processes of Golgi stacking, mitosis, cell apoptosis, and cargo secretion. We would also like to provide a further prospective of their potential biological values in neurodegenerative diseases.

Enzymatic reactions on immobilised substrates

This review gives an overview of enzymatic reactions that have been conducted on substrates attached to solid surfaces. Such biochemical reactions have become more important with the drive to miniaturisation and automation in chemistry, biology and medicine. Technical aspects such as choice of solid surface and analytical methods are discussed and examples of enzyme reactions that have been successful on these surfaces are provided.

Isoform-specific tethering links the Golgi ribbon to maintain compartmentalization

Use of photoinactivation, cisternae-specific fluorescence recovery, and high-resolution microscopy shows that the membrane tethers GRASP65 and GRASP55 on early and late Golgi membranes, respectively, are critical to the specific, homotypic fusion of the membranes on which they reside.

Homotypic membrane tethering by the Golgi reassembly and stacking proteins (GRASPs) is required for the lateral linkage of mammalian Golgi ministacks into a ribbon-like membrane network. Although GRASP65 and GRASP55 are specifically localized to cis and medial/trans cisternae, respectively, it is unknown whether each GRASP mediates cisternae-specific tethering and whether such specificity is necessary for Golgi compartmentalization. Here each GRASP was tagged with KillerRed (KR), expressed in HeLa cells, and inhibited by 1-min exposure to light. Significantly, inactivation of either GRASP unlinked the Golgi ribbon, and the immediate effect of GRASP65-KR inactivation was a loss of cis- rather than trans-Golgi integrity, whereas inactivation of GRASP55-KR first affected the trans- and not the cis-Golgi. Thus each GRASP appears to play a direct and cisternae-specific role in linking ministacks into a continuous membrane network. To test the consequence of loss of cisternae-specific tethering, we generated Golgi membranes with a single GRASP on all cisternae. Remarkably, the membranes exhibited the full connectivity of wild-type Golgi ribbons but were decompartmentalized and defective in glycan processing. Thus the GRASP isoforms specifically link analogous cisternae to ensure Golgi compartmentalization and proper processing.

Structural insight into Golgi membrane stacking by GRASP65 and GRASP55 proteins

The stacking of Golgi cisternae involves GRASP65 and GRASP55. The oligomerization of the N-terminal GRASP domain of these proteins, which consists of two tandem PDZ domains, is required to tether the Golgi membranes. However, the molecular basis for GRASP assembly is unclear. Here, we determined the crystal structures of the GRASP domain of GRASP65 and GRASP55. The structures reveal similar homotypic interactions: the GRASP domain forms a dimer in which the peptide-binding pockets of the two neighboring PDZ2 domains face each other, and the dimers are further connected by the C-terminal tail of one GRASP domain inserting into the binding pocket of the PDZ1 domain in another dimer. Biochemical analysis suggests that both types of contacts are relatively weak but are needed in combination for GRASP-mediated Golgi stacking. Our results unveil a novel mode of membrane tethering by GRASP proteins and provide insight into the mechanism of Golgi stacking.

Regulation of traffic and organelle architecture of the ER-Golgi interface by signal transduction

The components that control trafficking between organelles of the secretory pathway as well as their architecture were uncovered to a reasonable extent in the past decades. However, only recently did we begin to explore the regulation of the secretory pathway by cellular signaling. In the current review, we focus on trafficking between the endoplasmic reticulum and the Golgi apparatus. We highlight recent advances that have been made toward a better understanding of how the secretory pathway is regulated by signaling and discuss how this knowledge is important to obtain an integrative view of secretion in the context of other homeostatic processes such as growth and proliferation.

MEK1 inactivates Myt1 to regulate Golgi membrane fragmentation and mitotic entry in mammalian cells

The pericentriolar stacks of Golgi cisternae are separated from each other in G2 and fragmented extensively during mitosis. MEK1 is required for Golgi fragmentation in G2 and for the entry of cells into mitosis. We now report that Myt1 mediates MEK1's effects on the Golgi complex. Knockdown of Myt1 by siRNA increased the efficiency of Golgi complex fragmentation by mitotic cytosol in permeabilized and intact HeLa cells. Myt1 knockdown eliminated the requirement of MEK1 in Golgi fragmentation and alleviated the delay in mitotic entry due to MEK1 inhibition. The phosphorylation of Myt1 by MEK1 requires another kinase but is independent of RSK, Plk, and CDK1. Altogether our findings reveal that Myt1 is inactivated by MEK1 mediated phosphorylation to fragment the Golgi complex in G2 and for the entry of cells into mitosis. It is known that Myt1 inactivation is required for CDK1 activation. Myt1 therefore is an important link by which MEK1 dependent fragmentation of the Golgi complex in G2 is connected to the CDK1 mediated breakdown of Golgi into tubules and vesicles in mitosis.

Sequential phosphorylation of GRASP65 during mitotic Golgi disassembly

GRASP65 phosphorylation during mitosis and dephosphorylation after mitosis are required for Golgi disassembly and reassembly during the cell cycle. At least eight phosphorylation sites on GRASP65 have been identified, but whether they are modified in a coordinated fashion during mitosis is so far unknown. In this study, we raised phospho-specific antibodies that recognize phosphorylated T220/T224, S277 and S376 residues of GRASP65, respectively. Biochemical analysis showed that cdc2 phosphorylates all three sites, while plk1 enhances the phosphorylation. Microscopic studies using these antibodies for double and triple labeling demonstrate sequential phosphorylation and dephosphorylation during the cell cycle. S277 and S376 are phosphorylated from late G2 phase through metaphase until telophase when the new Golgi is reassembled. T220/224 is not modified until prophase, but is highly modified from prometaphase to anaphase. In metaphase, phospho-T220/224 signal localizes on both Golgi haze and mitotic Golgi clusters that represent dispersed Golgi vesicles and Golgi remnants, respectively, while phospho-S277 and S376 labeling is more concentrated on mitotic Golgi clusters. Expression of a phosphorylation-resistant GRASP65 mutant T220A/T224A inhibited mitotic Golgi fragmentation to a much larger extent than the expression of the S277A and S376A mutants. In cytokinesis, T220/224 dephosphorylation occurs prior to that of S277, but after S376. This study provides evidence that GRASP65 is sequentially phosphorylated and dephosphorylated during mitosis at different sites to orchestrate Golgi disassembly and reassembly during cell division, with phosphorylation of the T220/224 site being most critical in the process.

ERK1/2 regulate exocytosis through direct phosphorylation of the exocyst component Exo70

The exocyst is a multiprotein complex essential for exocytosis and plasma membrane remodeling. The assembly of the exocyst complex mediates the tethering of post-Golgi secretory vesicles to the plasma membrane prior to fusion. Elucidating the mechanisms regulating exocyst assembly is important for the understanding of exocytosis. Here we show that the exocyst component Exo70 is a direct substrate of the extracellular signal-regulated kinases 1/2 (ERK1/2). ERK1/2 phosphorylation enhances the binding of Exo70 to other exocyst components and promotes the assembly of the exocyst complex in response to epidermal growth factor (EGF) signaling. We further demonstrate that ERK1/2 regulates exocytosis, because blocking ERK1/2 signaling by a chemical inhibitor or the expression of an Exo70 mutant defective in ERK1/2 phosphorylation inhibited exocytosis. In tumor cells, blocking Exo70 phosphorylation inhibits matrix metalloproteinase secretion and invadopodia formation. ERK1/2 phosphorylation of Exo70 may thus coordinate exocytosis with other cellular events in response to growth factor signaling.

Golgi complex fragmentation in G2/M transition: An organelle-based cell-cycle checkpoint

In mammalian cells, the Golgi complex is organized into a continuous membranous system known as the Golgi ribbon, which is formed by individual Golgi stacks that are laterally connected by tubular bridges. During mitosis, the Golgi ribbon undergoes extensive fragmentation through a multistage process that is required for its correct partitioning into the daughter cells. Importantly, inhibition of this Golgi disassembly results in cell-cycle arrest at the G2 stage, suggesting that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Here, we discuss the mechanisms and regulation of the Golgi ribbon breakdown and briefly comment on how Golgi partitioning may inhibit G2/M transition.

Allosteric regulation of GRASP protein-dependent Golgi membrane tethering by mitotic phosphorylation

Mitotic phosphorylation of the conserved GRASP domain of GRASP65 disrupts its self-association, leading to a loss of Golgi membrane tethering, cisternal unlinking, and Golgi breakdown. Recently, the structural basis of the GRASP self-interaction was determined, yet the mechanism by which phosphorylation disrupts this activity is unknown. Here, we present the crystal structure of a GRASP phosphomimic containing an aspartic acid substitution for a serine residue (Ser-189) that in GRASP65 is phosphorylated by PLK1, causing a block in membrane tethering and Golgi ribbon formation. The structure revealed a conformational change in the GRASP internal ligand that prevented its insertion into the PDZ binding pocket, and gel filtration assays showed that this phosphomimic mutant exhibited a significant reduction in dimer formation. Interestingly, the structure also revealed an apparent propagation of conformational change from the site of phosphorylation to the shifted ligand, and alanine substitution of two residues (Glu-145 and Ser-146) at penultimate positions in this chain rescued dimer formation by the phosphomimic. These data reveal the structural basis of the phosphoinhibition of GRASP-mediated membrane tethering and provide a mechanism for its allosteric regulation.

The ERK Cascade: Distinct Functions within Various Subcellular Organelles

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is a central signaling pathway that regulates a wide variety of stimulated cellular processes, including mainly proliferation, differentiation, and survival, but apoptosis and stress response as well. The ability of this linear cascade to induce so many distinct and even opposing effects after various stimulations raises the question as to how the signaling specificity of the cascade is regulated. Over the past years, several specificity-mediating mechanisms have been elucidated, including temporal regulation, scaffolding interactions, crosstalks with other signaling components, substrate competition, and multiple components in each tier of the cascade. In addition, spatial regulation of various components of the cascade is probably one of the main ways by which signals can be directed to some downstream targets and not to others. In this review, we describe first the components of the ERK1/2 cascade and their mode of regulation by kinases, phosphatases, and scaffold proteins. In the second part, we focus on the role of MEK1/2 and ERK1/2 compartmentalization in the nucleus, mitochondria, endosomes, plasma membrane, cytoskeleton, and Golgi apparatus. We explain that this spatial distribution may direct ERK1/2 signals to regulate the organelles' activities. However, it can also direct the activity of the cascade's components to the outer surface of the organelles in order to bring them to close proximity to specific cytoplasmic targets. We conclude that the dynamic localization of the ERK1/2 cascade components is an important regulatory mechanism in determining the signaling specificity of the cascade, and its understanding should shed a new light on the understanding of many stimulus-dependent processes.

Rescue of ΔF508-CFTR trafficking via a GRASP-dependent unconventional secretion pathway

The most prevalent disease-causing mutation of CFTR is the deletion of Phe508 (ΔF508), which leads to defects in conventional Golgi-mediated exocytosis and cell surface expression. We report that ΔF508-CFTR surface expression can be rescued in vitro and in vivo by directing it to an unconventional GRASP-dependent secretion pathway. An integrated molecular and physiological analysis indicates that mechanisms associated with ER stress induce cell surface trafficking of the ER core-glycosylated wild-type and ΔF508-CFTR via the GRASP-dependent pathway. Phosphorylation of a specific site of GRASP and the PDZ-based interaction between GRASP and CFTR are critical for this unconventional surface trafficking. Remarkably, transgenic expression of GRASP in ΔF508-CFTR mice restores CFTR function and rescues mouse survival without apparent toxicity. These findings provide insight into how unconventional protein secretion is activated, and offer a potential therapeutic strategy for the treatment of cystic fibrosis and perhaps diseases stemming from other misfolded proteins.

Golgi biogenesis

The Golgi is an essential membrane-bound organelle in the secretary pathway of eukaryotic cells. In mammalian cells, the Golgi stacks are integrated into a continuous perinuclear ribbon, which poses a challenge for the daughter cells to inherit this membrane organelle during cell division. To facilitate proper partitioning, the mammalian Golgi ribbon is disassembled into vesicles in early mitosis. Following segregation into the daughter cells, a functional Golgi is reformed. Here we summarize our current understanding of the molecular mechanisms that control the mitotic Golgi disassembly and postmitotic reassembly cycle in mammalian cells.

Structure of the membrane-tethering GRASP domain reveals a unique PDZ ligand interaction that mediates Golgi biogenesis

Biogenesis of the ribbon-like membrane network of the mammalian Golgi requires membrane tethering by the conserved GRASP domain in GRASP65 and GRASP55, yet the tethering mechanism is not fully understood. Here, we report the crystal structure of the GRASP55 GRASP domain, which revealed an unusual arrangement of two tandem PDZ folds that more closely resemble prokaryotic PDZ domains. Biochemical and functional data indicated that the interaction between the ligand-binding pocket of PDZ1 and an internal ligand on PDZ2 mediates the GRASP self-interaction, and structural analyses suggest that this occurs via a unique mode of internal PDZ ligand recognition. Our data uncover the structural basis for ligand specificity and provide insight into the mechanism of GRASP-dependent membrane tethering of analogous Golgi cisternae.

New components of the Golgi matrix

The eukaryotic Golgi apparatus is characterized by a stack of flattened cisternae that are surrounded by transport vesicles. The organization and function of the Golgi require Golgi matrix proteins, including GRASPs and golgins, which exist primarily as fiber-like bridges between Golgi cisternae or between cisternae and vesicles. In this review, we highlight recent findings on Golgi matrix proteins, including their roles in maintaining the Golgi structure, vesicle tethering, and novel, unexpected functions. These new discoveries further our understanding of the molecular mechanisms that maintain the structure and the function of the Golgi, as well as its relationship with other cellular organelles such as the centrosome.

The multiple facets of the Golgi reassembly stacking proteins

The mammalian GRASPs (Golgi reassembly stacking proteins) GRASP65 and GRASP55 were first discovered more than a decade ago as factors involved in the stacking of Golgi cisternae. Since then, orthologues have been identified in many different organisms and GRASPs have been assigned new roles that may seem disconnected. In vitro, GRASPs have been shown to have the biochemical properties of Golgi stacking factors, but the jury is still out as to whether they act as such in vivo. In mammalian cells, GRASP65 and GRASP55 are required for formation of the Golgi ribbon, a structure which is fragmented in mitosis owing to the phosphorylation of a number of serine and threonine residues situated in its C-terminus. Golgi ribbon unlinking is in turn shown to be part of a mitotic checkpoint. GRASP65 also seems to be the key target of signalling events leading to re-orientation of the Golgi during cell migration and its breakdown during apoptosis. Interestingly, the Golgi ribbon is not a feature of lower eukaryotes, yet a GRASP homologue is present in the genome of Encephalitozoon cuniculi, suggesting they have other roles. GRASPs have no identified function in bulk anterograde protein transport along the secretory pathway, but some cargo-specific trafficking roles for GRASPs have been discovered. Furthermore, GRASP orthologues have recently been shown to mediate the unconventional secretion of the cytoplasmic proteins AcbA/Acb1, in both Dictyostelium discoideum and yeast, and the Golgi bypass of a number of transmembrane proteins during Drosophila development. In the present paper, we review the multiple roles of GRASPs.

Mitotic inhibition of GRASP65 organelle tethering involves Polo-like kinase 1 (PLK1) phosphorylation proximate to an internal PDZ ligand

GRASP65 links cis-Golgi cisternae via a homotypic, N-terminal PDZ interaction, and its mitotic phosphorylation disrupts this activity. Neither the identity of the PDZ ligand involved in the GRASP65 self-interaction nor the mechanism by which phosphorylation inhibits its interaction is known. Phospho-mimetic mutation of known cyclin-dependent kinase 1/cyclin B sites, all of which are in the C-terminal "regulatory domain" of the molecule, failed to block organelle tethering. However, we identified a site phosphorylated by Polo-like kinase 1 (PLK1) in the GRASP65 N-terminal domain for which mutation to aspartic acid blocked tethering and alanine substitution prevented mitotic Golgi unlinking. Further, using interaction assays, we discovered an internal PDZ ligand adjacent to the PLK phosphorylation site that was required for tethering. These results reveal the mechanism of phosphoinhibition as direct inhibition by PLK1 of the PDZ ligand underlying the GRASP65 self-interaction.

Tethering function of the caspase cleavage fragment of Golgi protein p115 promotes apoptosis via a p53-dependent pathway

The Golgi apparatus undergoes extensive fragmentation during apoptosis due in part to caspase-mediated cleavage of its structural proteins. Significantly, the Golgi-vesicle-tethering protein p115 is cleaved at Asp(757) early during apoptosis and the nuclear translocation of its 205 amino acid C-terminal fragment (CTF) precedes observable Golgi fragmentation. Nuclear localization of the p115 CTF induces apoptosis. The regulation of CTF nuclear translocation and the mechanism of its apoptotic activity however, remain unknown. Here, we demonstrate that nuclear translocation of the CTF is regulated by SUMOylation. CTF-induced apoptosis is transcription dependent and mediated by the tumor suppressor, p53. Expression of the CTF led to the phosphorylation and stabilization of p53 and results in the expression of PUMA, a pro-apoptotic target of p53. CTF-induced stabilization of p53 is sensitive to the MEK/ERK inhibitor U0126. Co-immunoprecipitation studies indicate that the p115 CTF can bind to both p53 and ERK1. The CTF is also able to form dimers and its dimerization is dependent on residues 859-884, previously determined to be required for apoptosis. Indeed, CTF expression promotes p53-ERK interaction, which is diminished upon deletion of residues 859-884. Together, our results indicate a conserved tethering function of the Golgi protein p115 CTF which promotes p53-ERK interaction for the amplification of the apoptotic signal.

Golgi reassembly stacking protein 55 interacts with membrane-type (MT) 1-matrix metalloprotease (MMP) and furin and plays a role in the activation of the MT1-MMP zymogen

Membrane-type 1 matrix metalloproteinase (MT1-MMP) is a proteinase involved in the remodelling of extracellular matrix and the cleavage of a number of substrates. MT1-MMP is synthesized as a zymogen that requires intracellular post-translational cleavage to gain biological activity. Furin, a member of the pro-protein convertase family, has been implicated in the proteolytic removal of the MT1-MMP prodomain sequence. In the present study, we demonstrate a role for the peripheral Golgi matrix protein GRASP55 in the furin-dependent activation of MT1-MMP. MT1-MMP and furin were found to co-localize with Golgi reassembly stacking protein 55 (GRASP55). Further analysis revealed that GRASP55 associated with the cytoplasmic domain of both proteases and that the LLY(573) motif in the MT1-MMP intracellular domain was crucial for the interaction with GRASP55. Overexpression of GRASP55 was found to enhance the formation of a complex between MT1-MMP and furin. Finally, we report that disruption of the interaction between GRASP55 and furin led to a reduction in pro-MT1-MMP activation. Taken together, these data suggest that GRASP55 may function as an adaptor protein coupling MT1-MMP with furin, thus leading to the activation of the zymogen.

The yeast GRASP Grh1 colocalizes with COPII and is dispensable for organizing the secretory pathway

In mammalian cells, the 'Golgi reassembly and stacking protein' (GRASP) family has been implicated in Golgi stacking, but the broader functions of GRASP proteins are still unclear. The yeast Saccharomyces cerevisiae contains a single non-essential GRASP homolog called Grh1. However, Golgi cisternae in S. cerevisiae are not organized into stacks, so a possible structural role for Grh1 has been difficult to test. Here, we examined the localization and function of Grh1 in S. cerevisiae and in the related yeast Pichia pastoris, which has stacked Golgi cisternae. In agreement with earlier studies indicating that Grh1 interacts with coat protein II (COPII) vesicle coat proteins, we find that Grh1 colocalizes with COPII at transitional endoplasmic reticulum (tER) sites in both yeasts. Deletion of P. pastoris Grh1 had no obvious effect on the structure of tER-Golgi units. To test the role of S. cerevisiae Grh1, we exploited the observation that inhibiting ER export in S. cerevisiae generates enlarged tER sites that are often associated with the cis Golgi. This tER-Golgi association was preserved in the absence of Grh1. The combined data suggest that Grh1 acts early in the secretory pathway, but is dispensable for the organization of secretory compartments.

Emerging new roles of GM130, a cis-Golgi matrix protein, in higher order cell functions

GM130 is a peripheral membrane protein strongly attached to the Golgi membrane and is isolated from the detergent and salt resistant Golgi matrix. GM130 is rich in coiled-coil structures and predicted to take a rod-like shape. Together with p115, giantin, and GRASP65, GM130 facilitates vesicle fusion to the Golgi membrane as a vesicle "tethering factor". GM130 is also involved in the maintenance of the Golgi structure and plays a major role in the disassembly and reassembly of the Golgi apparatus during mitosis. Emerging evidence suggests that GM130 is involved in the control of glycosylation, cell cycle progression, and higher order cell functions such as cell polarization and directed cell migration. This creates the potential for novel Golgi-targeted drugs and treatments for various diseases including glycosylation defects, immune diseases, and cancer.

GRASP55 and GRASP65 play complementary and essential roles in Golgi cisternal stacking

Two peripheral GRASP membrane proteins work together to keep the Golgi from falling apart.

In vitro studies have suggested that Golgi stack formation involves two homologous peripheral Golgi proteins, GRASP65 and GRASP55, which localize to the cis and medial-trans cisternae, respectively. However, no mechanism has been provided on how these two GRASP proteins work together to stack Golgi cisternae. Here, we show that depletion of either GRASP55 or GRASP65 by siRNA reduces the number of cisternae per Golgi stack, whereas simultaneous knockdown of both GRASP proteins leads to disassembly of the entire stack. GRASP55 stacks Golgi membranes by forming oligomers through its N-terminal GRASP domain. This process is regulated by phosphorylation within the C-terminal serine/proline-rich domain. Expression of nonphosphorylatable GRASP55 mutants enhances Golgi stacking in interphase cells and inhibits Golgi disassembly during mitosis. These results demonstrate that GRASP55 and GRASP65 stack mammalian Golgi cisternae via a common mechanism.

Mitotic division of the mammalian Golgi apparatus

Successful cell reproduction requires faithful duplication and proper segregation of cellular contents, including not only the genome but also intracellular organelles. Since the Golgi apparatus is an essential organelle of the secretory pathway, its accurate inheritance is therefore of importance to sustain cellular function. Regulation of Golgi division and its coordination with cell cycle progression involves a series of sequential events that are subjected to a precise spatiotemporal control. Here, we summarize the current knowledge about the underlying mechanisms, the molecular players and the biological relevance of this process, particularly in mammalian cells, and discuss the unsolved problems and future perspectives opened by the recent studies.