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Viruses are obligatory intracellular parasites and utilize host elements to support key viral processes, including penetration of the plasma membrane, initiation of infection, replication, and suppression of the host's antiviral defenses. In this review, we focus on picornaviruses, a family of positive-strand RNA viruses, and discuss the mechanisms by which these viruses hijack the cellular machinery to form and operate membranous replication complexes. Studies aimed at revealing factors required for the establishment of viral replication structures identified several cellular-membrane-remodeling proteins and led to the development of models in which the virus used a preexisting cellular-membrane-shaping pathway "as is" for generating its replication organelles. However, as more data accumulate, this view is being increasingly questioned, and it is becoming clearer that viruses may utilize cellular factors in ways that are distinct from the normal functions of these proteins in uninfected cells. In addition, the proteincentric view is being supplemented by important new studies showing a previously unappreciated deep remodeling of lipid homeostasis, including extreme changes to phospholipid biosynthesis and cholesterol trafficking. The data on viral modifications of lipid biosynthetic pathways are still rudimentary, but it appears once again that the viruses may rewire existing pathways to generate novel functions. Despite remarkable progress, our understanding of how a handful of viral proteins can completely overrun the multilayered, complex mechanisms that control the membrane organization of a eukaryotic cell remains very limited. [ABSTRACT FROM AUTHOR]

Membrane traffic between secretory and endosomal compartments is vesicle‐mediated and must be tightly balanced to maintain a physiological compartment size. Vesicle formation is initiated by guanine nucleotide exchange factors (GEFs) that activate the ARF family of small GTPases. Regulatory mechanisms, including reversible phosphorylation, allow ARF‐GEFs to support vesicle formation only at the right time and place in response to cellular needs. Here, we review current knowledge of how the Golgi‐specific brefeldin A‐resistance factor 1 (GBF1)/brefeldin A‐inhibited guanine nucleotide exchange protein (BIG) family of ARF‐GEFs is influenced by phosphorylation and use predictive paradigms to propose new regulatory paradigms. We describe a conserved cluster of phosphorylation sites within the N‐terminal domains of the GBF1/BIG ARF‐GEFs and suggest that these sites may respond to homeostatic signals related to cell growth and division. In the C‐terminal region, GBF1 shows phosphorylation sites clustered differently as compared with the similar configuration found in both BIG1 and BIG2. Despite this similarity, BIG1 and BIG2 phosphorylation patterns are divergent in other domains. The different clustering of phosphorylation sites suggests that the nonconserved sites may represent distinct regulatory nodes and specify the function of GBF1, BIG1, and BIG2. [ABSTRACT FROM AUTHOR]

The cellular protein GBF1, an activator of Arf GTPases (ArfGEF: Arf guanine nucleotide exchange factor), is recruited to the replication organelles of enteroviruses through interaction with the viral protein 3A, and its ArfGEF activity is required for viral replication, however how GBF1-dependent Arf activation supports the infection remains enigmatic. Here, we investigated the development of resistance of poliovirus, a prototype enterovirus, to increasing concentrations of brefeldin A (BFA), an inhibitor of GBF1. High level of resistance required a gradual accumulation of multiple mutations in the viral protein 2C. The 2C mutations conferred BFA resistance even in the context of a 3A mutant previously shown to be defective in the recruitment of GBF1 to replication organelles, and in cells depleted of GBF1, suggesting a GBF1-independent replication mechanism. Still, activated Arfs accumulated on the replication organelles of this mutant even in the presence of BFA, its replication was inhibited by a pan-ArfGEF inhibitor LM11, and the BFA-resistant phenotype was compromised in Arf1-knockout cells. Importantly, the mutations strongly increased the interaction of 2C with the activated form of Arf1. Analysis of other enteroviruses revealed a particularly strong interaction of 2C of human rhinovirus 1A with activated Arf1. Accordingly, the replication of this virus was significantly less sensitive to BFA than that of poliovirus. Thus, our data demonstrate that enterovirus 2Cs may behave like Arf1 effector proteins and that GBF1 but not Arf activation can be dispensable for enterovirus replication. These findings have important implications for the development of host-targeted anti-viral therapeutics. Author summary: As ultimate intracellular parasites, viruses rely on cellular proteins for every step of their life cycle. This provides a possibility of controlling viral replication by targeting the cellular factors. Such therapeutics are expected to be more difficult to develop resistance to since cellular proteins do not change. Yet, enteroviruses, a large group of small positive RNA viruses that are associated with multiple human pathologies, rapidly develop resistance to virtually all experimental therapeutics including compounds targeting host factors essential for the viral replication. Here we investigated in detail the development of enterovirus resistance to one of such compounds targeting a host protein GBF1 that activates small GTPases of the Arf family. We found that while the mutations in a resistant virus make it insensitive to the inhibition of GBF1, the virus acquired the capability of obtaining activated Arfs from other cellular sources and that the interaction of one of the viral proteins with activated Arfs is likely essential for the functioning of the replication complex. Thus, our work revealed that it is not GBF1 per se, but activated Arfs that are indispensable for viral replication, which provides new opportunities for the development of anti-enterovirus therapeutics refractory to the development of resistance. [ABSTRACT FROM AUTHOR]

Diverse cellular processes, including membrane traffic, lipid homeostasis, cytokinesis, mitochondrial positioning, and cell motility are critically dependent on the Sec7 domain guanine nucleotide exchange factor GBF1. Yet, how the participation of GBF1 in a particular cellular function is regulated is unknown. Here, we show that the phosphorylation of specific highly conserved serine and tyrosine residues within the N-terminal domain of GBF1 differentially regulates its function in maintaining Golgi homeostasis and facilitating secretion versus its role in cytokinesis. Specifically, GBF1 mutants containing single amino acid substitutions that mimic a stably phosphorylated S233, S371, Y377, and Y515 or the S233A mutant that can't be phosphorylated are fully able to maintain Golgi architecture and support cargo traffic through the secretory pathway when assessed in multiple functional assays. However, the same mutants cause multi-nucleation when expressed in cells, and appear to inhibit the progression through mitosis and the resolution of cytokinetic bridges. Thus, GBF1 participates in distinct interactive networks when mediating Golgi homeostasis and secretion versus facilitating cytokinesis, and GBF1 integration into such networks is differentially regulated by the phosphorylation of specific GBF1 residues. [ABSTRACT FROM AUTHOR]

As ultimate parasites, viruses depend on host factors for every step of their life cycle. On the other hand, cells evolved multiple mechanisms of detecting and interfering with viral replication. Yet, our understanding of the complex ensembles of pro- and anti-viral factors is very limited in virtually every virus-cell system. Here we investigated the proteins recruited to the replication organelles of poliovirus, a representative of the genus Enterovirus of the Picornaviridae family. We took advantage of a strict dependence of enterovirus replication on a host protein GBF1, and established a stable cell line expressing a truncated GBF1 fused to APEX2 peroxidase that effectively supported viral replication upon inhibition of the endogenous GBF1. This construct biotinylated multiple host and viral proteins on the replication organelles. Among the viral proteins, the polyprotein cleavage intermediates were overrepresented, suggesting that the GBF1 environment is linked to viral polyprotein processing. The proteomics characterization of biotinylated host proteins identified multiple proteins previously associated with enterovirus replication, as well as more than 200 new factors recruited to the replication organelles. RNA metabolism proteins, many of which normally localize in the nucleus, constituted the largest group, underscoring the massive release of nuclear factors into the cytoplasm of infected cells and their involvement in viral replication. Functional analysis of several newly identified proteins revealed both pro- and anti-viral factors, including a novel component of infection-induced stress granules. Depletion of these proteins similarly affected the replication of diverse enteroviruses indicating broad conservation of the replication mechanisms. Thus, our data significantly expand the knowledge of the composition of enterovirus replication organelles, provide new insights into viral replication, and offer a novel resource for identifying targets for anti-viral interventions. Author summary: Enteroviruses are arguably the most numerous group of viral pathogens infecting humans. Enterovirus infection can cause multiple complications including the development of asthma, type I diabetes, myocarditis, encephalitis, paralysis, and many others. Yet, there are no approved drugs against any of the pathogenic enteroviruses. The lack of effective anti-enterovirus measures is due in large part to insufficient understanding of molecular mechanisms supporting the development of enterovirus infections. Here, we developed a novel probe based on a host protein GBF1 that is universally required for enterovirus RNA replication to identify the proteins localized on enterovirus replication organelles, specialized membranous structures harboring the viral RNA replication complexes. Among the host proteins, we identified factors that promote and suppress the replication of diverse enteroviruses. Thus, our research provides new insights into molecular mechanisms of enterovirus replication and presents an important resource for the development of host-targeted anti-viral interventions. [ABSTRACT FROM AUTHOR]

Tethering factors regulate the targeting of membrane-enclosed vesicles under the control of Rab GTPases. p115, a golgin family tether, has been shown to participate in multiple stages of ER/Golgi transport. Despite extensive study, the mechanism of action of p115 is poorly understood. SNARE proteins make up the machinery for membrane fusion, and strong evidence shows that function of p115 is directly linked to its interaction with SNAREs. Using a gel filtration binding assay, we have demonstrated that in solution p115 stably interacts with ER/Golgi SNAREs rbet1 and sec22b, but not membrin and syntaxin 5. These binding preferences stemmed from selectivity of p115 for monomeric SNARE motifs as opposed to SNARE oligomers. Soluble monomeric rbet1 can compete off p115 from coat protein II ( COPII) vesicles. Furthermore, excess p115 inhibits p115 function in trafficking. We conclude that monomeric SNAREs are a major binding site for p115 on COPII vesicles, and that p115 dissociates from its SNARE partners upon SNAREpin assembly. Our results suggest a model in which p115 forms a mixed p115/ SNARE helix bundle with a monomeric SNARE, facilitates the binding activity and/or concentration of the SNARE at prefusion sites and is subsequently ejected as SNARE complex formation and fusion proceed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Enterovirus replication requires the cellular protein GBF1, a guanine nucleotide exchange factor for small Arf GTPases. When activated, Arfs associate with membranes, where they regulate numerous steps of membrane homeostasis. The requirement for GBF1 implies that Arfs are important for replication, but which of the different Arfs function(s) during replication remains poorly understood. Here, we established cell lines expressing each of the human Arfs fused to a fluorescent tag and investigated their behavior during enterovirus infection. Arf1 was the first to be recruited to the replication organelles, where it strongly colocalized with the viral antigen 2B and mature virions but not double-stranded RNA. By the end of the infectious cycle, Arf3, Arf4, Arf5, and Arf6 were also concentrated on the replication organelles. Once on the replication membranes, all Arfs except Arf3 were no longer sensitive to inhibition of GBF1, suggesting that in infected cells they do not actively cycle between GTP- and GDP-bound states. Only the depletion of Arf1, but not other class 1 and 2 Arfs, significantly increased the sensitivity of replication to GBF1 inhibition. Surprisingly, depletion of Arf6, a class 3 Arf, normally implicated in plasma membrane events, also increased the sensitivity to GBF1 inhibition. Together, our results suggest that GBF1-dependent Arf1 activation directly supports the development and/or functioning of the replication complexes and that Arf6 plays a previously unappreciated role in viral replication. Our data reveal a complex pattern of Arf activation in enterovirus-infected cells that may contribute to the resilience of viral replication in different cellular environments. [ABSTRACT FROM AUTHOR]

The endoplasmic reticulum protein Jagunal (JAGN1) was first identified as a requirement for Drosophila melanogaster oocyte development. Subsequent studies in human patients linked mutations in JAGN1 to severe congenital neutropenia, as well as a broad range of additional symptoms, suggesting that JAGN1 function is required in many tissues. Moreover, JAGN1 orthologs are found throughout animal and plant phylogeny, suggesting that JAGN1 supports fundamental cellular processes not restricted to egg development or neutrophil function. JAGN1 lacks sequence similarity or recognizable domains other than a coatomer protein complex I-binding motif, and its cellular function is currently unknown. JAGN1 shares a tetraspanning membrane topology with two families of known cargo transporters: the tetraspanins and the endoplasmic reticulum vesicle (Erv) proteins. Herein, we discuss the similarities between JAGN1, tetraspanins, and Ervs and, based on those, suggest a role for JAGN1 in facilitating the traffic of cell-restricted and ubiquitously expressed proteins at the endoplasmic reticulum-Golgi interface. [ABSTRACT FROM AUTHOR]

The ADP-ribosylation factor (ARF) superfamily of regulatory GTPases, including both the ARF and ARF-like (ARL) proteins, control a multitude of cellular functions, including aspects of vesicular traffic, lipid metabolism, mitochondrial architecture, the assembly and dynamics of the microtubule and actin cytoskeletons, and other pathways in cell biology. Considering their general utility, it is perhaps not surprising that increasingly ARF/ARLs have been found in connection to primary cilia. Here, we critically evaluate the current knowledge of the roles four ARF/ARLs (ARF4, ARL3, ARL6, ARL13B) play in cilia and highlight key missing information that would help move our understanding forward. Importantly, these GTPases are themselves regulated by guanine nucleotide exchange factors (GEFs) that activate them and by GTPase-activating proteins (GAPs) that act as both effectors and terminators of signaling. We believe that the identification of the GEFs and GAPs and better models of the actions of these GTPases and their regulators will provide a much deeper understanding and appreciation of the mechanisms that underly ciliary functions and the causes of a number of human ciliopathies. [ABSTRACT FROM AUTHOR]

The replication of many positive-strand RNA viruses [(+)RNA viruses] depends on the cellular protein GBF1, but its role in the replication process is not clear. In uninfected cells, GBF1 activates small GTPases of the Arf family and coordinates multiple steps of membrane metabolism, including functioning of the cellular secretory pathway. The nonstructural protein 3A of poliovirus and related viruses has been shown to directly interact with GBF1, likely mediating its recruitment to the replication complexes. Surprisingly, viral mutants with a severely reduced level of 3A-GBF1 interaction demonstrate minimal replication defects in cell culture. Here, we systematically investigated the conserved elements of GBF1 to understand which determinants are important to support poliovirus replication. We demonstrate that multiple GBF1 mutants inactive in cellular metabolism could still be fully functional in the replication complexes. Our results show that the Arf-activating property, but not the primary structure of the Sec7 domain, is indispensable for viral replication. They also suggest a redundant mechanism of recruitment of GBF1 to the replication sites, which is dependent not only on direct interaction of the protein with the viral protein 3A but also on determinants located in the noncatalytic C-terminal domains of GBF1. Such a double-targeting mechanism explains the previous observations of the remarkable tolerance of different levels of GBF1-3A interaction by the virus and likely constitutes an important element of the resilience of viral replication. [ABSTRACT FROM AUTHOR]

Cellular and viral factors participate in the replication cycle of rotavirus. We report that the guanine nucleotide exchange factor GBF1, which activates the small GTPase Arf1 to induce COPI transport processes, is required for rotavirus replication since knocking down GBF1 expression by RNA interference or inhibiting its activity by treatment with brefeldin A (BFA) or Golgicide A (GCA) significantly reduces the yield of infectious viral progeny. This reduction in virus yield was related to a block in virus assembly, since in the presence of either BFA or GCA, the assembly of infectious mature triple-layered virions was significantly prevented and only double-layered particles were detected. We report that the catalytic activity of GBF1, but not the activation of Arf1, is essential for the assembly of the outer capsid of rotavirus. We show that both BFA and GCA, as well as interfering with the synthesis of GBF1, alter the electrophoretic mobility of glycoproteins VP7 and NSP4 and block the trimerization of the virus surface protein VP7, a step required for its incorporation into virus particles. Although a posttranslational modification of VP7 (other than glycosylation) could be related to the lack of trimerization, we found that NSP4 might also be involved in this process, since knocking down its expression reduces VP7 trimerization. In support, recombinant VP7 protein overexpressed in transfected cells formed trimers only when cotransfected with NSP4. [ABSTRACT FROM AUTHOR]

The high human cost of Zika virus infections and the rapid establishment of virus circulation in novel areas, including the United States, present an urgent need for countermeasures against this emerging threat. The development of an effective vaccine against Zika virus may be problematic because of the cross reactivity of the antibodies with other flaviviruses leading to antibody-dependent enhancement of infection. Moreover, rapidly replicating positive strand RNA viruses, including Zika virus, generate large spectrum of mutant genomes (quasi species) every replication round, allowing rapid selection of variants resistant to drugs targeting virus-specific proteins. On the other hand, viruses are ultimate cellular parasites and rely on the host metabolism for every step of their life cycle, thus presenting an opportunity to manipulate host processes as an alternative approach to suppress virus replication and spread. Zika and other flaviviruses critically depend on the cellular secretory pathway, which transfers proteins and membranes from the ER through the Golgi to the plasma membrane, for virion assembly, maturation and release. In this review, we summarize the current knowledge of interactions of Zika and similar arthropod-borne flaviviruses with the cellular secretory machinery with a special emphasis on virus-specific changes of the secretory pathway. Identification of the regulatory networks and effector proteins required to accommodate the trafficking of virions, which represent a highly unusual cargo for the secretory pathway, may open an attractive and virtually untapped reservoir of alternative targets for the development of superior anti-viral drugs. [ABSTRACT FROM AUTHOR]

In classical Cell Biology, fundamental cellular processes are revealed empirically, one experiment at a time. While this approach has been enormously fruitful, our understanding of cells is far from complete. In fact, the more we know, the more keenly we perceive our ignorance of the profoundly complex and dynamic molecular systems that underlie cell structure and function. Thus, it has become apparent to many cell biologists that experimentation alone is unlikely to yield major new paradigms, and that empiricism must be combined with theory and computational approaches to yield major new discoveries. To facilitate those discoveries, three workshops will convene annually for one day in three successive summers (2017–2019) to promote the use of computational modeling by cell biologists currently unconvinced of its utility or unsure how to apply it. The first of these workshops was held at the University of Illinois, Chicago in July 2017. Organized to facilitate interactions between traditional cell biologists and computational modelers, it provided a unique educational opportunity: a primer on how cell biologists with little or no relevant experience can incorporate computational modeling into their research. Here, we report on the workshop and describe how it addressed key issues that cell biologists face when considering modeling including: (1) Is my project appropriate for modeling? (2) What kind of data do I need to model my process? (3) How do I find a modeler to help me in integrating modeling approaches into my work? And, perhaps most importantly, (4) why should I bother? [ABSTRACT FROM PUBLISHER]

Cellular life requires the activation of the ADP-ribosylation factors (ARFs) by Golgi brefeldin A-resistant factor 1 (GBF1), a guanine nucleotide exchange factor (GEF) with a highly conserved catalytic Sec7 domain (Sec7d). In addition to the Sec7d, GBF1 contains other conserved domains whose functions remain unclear. Here, we focus on HDS2 (homology downstream of Sec7d 2) domain because the L1246R substitution within the HDS2 α-helix 5 of the zebrafish GBF1 ortholog causes vascular hemorrhaging and embryonic lethality (13). To dissect the structure/function relationships within HDS2, we generated six variants, in which the most conserved residues within α-helices 1, 2, 4, and 6 were mutated to alanines. Each HDS2 mutant was assessed in a cell-based "replacement" assay for its ability to support cellular functions normally supported by GBF1, such as maintaining Golgi homeostasis, facilitating COPI recruitment, supporting secretion, and sustaining cellular viability. We show that cells treated with the pharmacological GBF1 inhibitor brefeldin A (BFA) and expressing a BFA-resistant GBF1 variant with alanine substitutions of RDR1168 or LF1266 are compromised in Golgi homeostasis, impaired in ARF activation, unable to sustain secretion, and defective in maintaining cellular viability. To gain insight into the molecular mechanism of this dysfunction, we assessed the ability of each GBF1 mutant to target to Golgi membranes and found that mutations in RDR1168 and LF1266 significantly decrease targeting efficiency. Thus, these residues within α-helix 2 and α-helix 6 of the HDS2 domain in GBF1 are novel regulatory determinants that support GBF1 cellular function by impacting the Golgi-specific membrane association of GBF1. [ABSTRACT FROM AUTHOR]

ADP-ribosylation factors (ARF) GTPases are activated by guanine nucleotide exchange factors (GEFs) to support cellular homeostasis. Key to understanding spatio-temporal regulation of ARF signaling is the mechanism of GEF recruitment to membranes. Small GEFs are recruited through phosphoinositide (PIP) binding by a pleckstrin homology (PH) domain downstream from the catalytic Sec7 domain (Sec7d). The large GEFs lack PH domains, and their recruitment mechanisms are poorly understood. We probed Golgi recruitment of GBF1, a GEF catalyzing ARF activation required for Golgi homeostasis. We show that the homology downstream of Sec7d-1 (HDS1) regulates Golgi recruitment of GBF1. We document that GBF1 binds phosphoinositides, preferentially PI3P, PI4P and PI(4,5)P2, and that lipid binding requires the HDS1 domain. Mutations within HDS1 that reduce GBF1 binding to specific PIPs in vitro inhibit GBF1 targeting to Golgi membranes in cells. Our data imply that HDS1 and PH domains are functionally analogous in that each uses lipid-based membrane information to regulate GEF recruitment. Lipid-based recruitment of GBF1 extends the paradigm of lipid regulation to small and large GEFs and suggests that lipid-based mechanisms evolved early during GEF diversification. [ABSTRACT FROM AUTHOR]

Many secretory cells increase the synthesis and secretion of cargo proteins in response to specific stimuli. How cells couple increased cargo load with a coordinate rise in secretory capacity to ensure efficient transport is not well understood. We used thyroid cells stimulated with thyrotropin (TSH) to demonstrate a coordinate increase in the production of thyroid-specific cargo proteins and ER-Golgi transport factors, and a parallel expansion of the Golgi complex. TSH also increased expression of the CREB3L1 transcription factor, which alone caused amplified transport factor levels and Golgi enlargement. Furthermore, CREB3L1 potentiated the TSH-induced increase in Golgi volume. A dominant-negative CREB3L1 construct hampered the ability of TSH to induce Golgi expansion, implying that this transcription factor contributes to Golgi expansion. Our findings support a model in which CREB3L1 acts as a downstream effector of TSH to regulate the expression of cargo proteins, and simultaneously increases the synthesis of transport factors and the expansion of the Golgi to synchronize the rise in cargo load with the amplified capacity of the secretory pathway. [ABSTRACT FROM AUTHOR]

Members of the large Sec7 domain- containing Arf guanine nucleotide exchange factor (GEF) family have been shown to dimerize through their NH2-terminal dimerization and cyclophilin binding (DCB) and homology upstream of Sec7 (HUS) domains. However, the importance of dimerization in GEF localization and function has not been assessed. We generated a GBF1 mutant (91/130) in which two residues required for oligomerization (K91 and E130 within the DCB domain) were replaced with A and assessed the effects of these mutations on GBF1 localization and cellular functions. We show that 91/130 is compromised in oligomerization but that it targets to the Golgi in a manner indistinguishable from wild-type GBF1 and that it rapidly exchanges between the cytosolic and membrane-bound pools. The 91/130 mutant appears active as it integrates within the functional network at the Golgi, supports Arf activation and COPI recruitment, and sustains Golgi homeostasis and cargo secretion when provided as a sole copy of functional GBF1 in cells. In addition, like wild-type GBF1, the 91/130 mutant supports poliovirus RNA replication, a process requiring GBF1 but believed to be independent of GBF1 catalytic activity. However, oligomerization appears to stabilize GBF1 in cells, and the 91/130 mutant is degraded faster than the wild-type GBF1. Our data support a model in which oligomerization is not a key regulator of GBF1 activity but impacts its function by regulating the cellular levels of GBF1. [ABSTRACT FROM AUTHOR]