Golgi complex-plasma membrane trafficking directed by an autonomous, tribasic Golgi export signal

Nonstructural protein NS17 of grass carp reovirus Honghu strain promotes virus infection by mediating cell-cell fusion and apoptosis

Fusion-associated small transmembrane (FAST) proteins can promote cell fusion, alter membrane permeability and trigger apoptosis to promote virus proliferation in orthoreoviruses. However, it is unknown whether FAST proteins perform these functions in aquareoviruses (AqRVs). Non-structural protein 17 (NS17) carried by grass carp reovirus Honghu strain (GCRV-HH196) belongs to the FAST protein family, and we preliminarily explored its relevance to virus infection. NS17 has similar domains to FAST protein NS16 of GCRV-873, comprising a transmembrane domain, a polybasic cluster, a hydrophobic patch and a polyproline motif. It was observed in the cytoplasm and the cell membrane. Overexpression of NS17 enhanced the efficiency of cell-cell fusion induced by GCRV-HH196 and promoted virus replication. Overexpression of NS17 also led to DNA fragmentation and reactive oxygen species (ROS) accumulation, and it triggered apoptosis. The findings illuminate the functions of NS17 in GCRV infection, and provide a reference for the development of novel antiviral strategies.

Conserved but not critical: Trafficking and function of NaV1.7 are independent of highly conserved polybasic motifs

Non-addictive treatment of chronic pain represents a major unmet clinical need. Peripheral voltage-gated sodium (NaV) channels are an attractive target for pain therapy because they initiate and propagate action potentials in primary afferents that detect and transduce noxious stimuli. NaV1.7 sets the gain on peripheral pain-signaling neurons and is the best validated peripheral ion channel involved in human pain, and previous work has shown that it is transported in vesicles in sensory axons which also carry Rab6a, a small GTPase known to be involved in vesicular packaging and axonal transport. Understanding the mechanism of the association between Rab6a and NaV1.7 could inform therapeutic modalities to decrease trafficking of NaV1.7 to the distal axonal membrane. Polybasic motifs (PBM) have been shown to regulate Rab-protein interactions in a variety of contexts. In this study, we explored whether two PBMs in the cytoplasmic loop that joins domains I and II of human NaV1.7 were responsible for association with Rab6a and regulate axonal trafficking of the channel. Using site-directed mutagenesis we generated NaV1.7 constructs with alanine substitutions in the two PBMs. Voltage-clamp recordings showed that the constructs retain wild-type like gating properties. Optical Pulse-chase Axonal Long-distance (OPAL) imaging in live sensory axons shows that mutations of these PBMs do not affect co-trafficking of Rab6a and NaV1.7, or the accumulation of the channel at the distal axonal surface. Thus, these polybasic motifs are not required for interaction of NaV1.7 with the Rab6a GTPase, or for trafficking of the channel to the plasma membrane.

Evaluating Cell Membrane Localization and Intracellular Transport of Proteins by Biotinylation

Protein translocation to the cell membrane and transport through intracellular compartments are dynamic processes frequently altered in cancer cells. Abnormal protein localization can affect key cell functions, including transduction of extracellular signals and organization of the cytoskeleton, significantly affecting oncogenicity and therapeutic responses. In this chapter, we describe a surface protein biotinylation method that allows the study of membrane localization and endosomal transport of membrane-associated proteins. Surface biotinylation can be used to evaluate baseline protein levels at the membrane, and other processes such as internalization, recycling, and degradation of proteins in response to different treatments or as a consequence of oncogenic mutations. Further, the combination of this technique with other strategies, such as treatments with transport inhibitors, allows investigation of specific steps of protein trafficking through the cell.

Synaptotagmin 7 is targeted to the axonal plasma membrane through γ-secretase processing to promote synaptic vesicle docking in mouse hippocampal neurons

Synaptotagmin 7 (SYT7) has emerged as a key regulator of presynaptic function, but its localization and precise role in the synaptic vesicle cycle remain the subject of debate. Here, we used iGluSnFR to optically interrogate glutamate release, at the single-bouton level, in SYT7KO-dissociated mouse hippocampal neurons. We analyzed asynchronous release, paired-pulse facilitation, and synaptic vesicle replenishment and found that SYT7 contributes to each of these processes to different degrees. 'Zap-and-freeze' electron microscopy revealed that a loss of SYT7 diminishes docking of synaptic vesicles after a stimulus and inhibits the recovery of depleted synaptic vesicles after a stimulus train. SYT7 supports these functions from the axonal plasma membrane, where its localization and stability require both γ-secretase-mediated cleavage and palmitoylation. In summary, SYT7 is a peripheral membrane protein that controls multiple modes of synaptic vesicle (SV) exocytosis and plasticity, in part, through enhancing activity-dependent docking of SVs.

Glycans function as a Golgi export signal to promote the constitutive exocytic trafficking

Most proteins in the secretory pathway are glycosylated. However, the role of glycans in membrane trafficking is still unclear. Here, we discovered that transmembrane secretory cargos, such as interleukin 2 receptor α subunit or Tac, transferrin receptor, and cluster of differentiation 8a, unexpectedly displayed substantial Golgi localization when their O-glycosylation was compromised. By quantitatively measuring their Golgi residence times, we found that the observed Golgi localization of O-glycan–deficient cargos is due to their slow Golgi export. Using a superresolution microscopy method that we previously developed, we revealed that O-glycan–deficient Tac chimeras localize at the interior of the trans-Golgi cisternae. O-Glycans were observed to be both necessary and sufficient for the efficient Golgi export of Tac chimeras. By sequentially introducing O-glycosylation sites to ST6GAL1, we demonstrated that O-glycan's effect on Golgi export is probably additive. Finally, the finding that N-glycosylated GFP substantially reduces the Golgi residence time of a Tac chimera suggests that N-glycans might have a similar effect. Therefore, both O- and N-glycans might function as a generic Golgi export signal at the trans-Golgi to promote the constitutive exocytic trafficking.

Molecular mechanisms that regulate export of the planar cell-polarity protein Frizzled-6 out of the endoplasmic reticulum

Planar cell polarity (PCP) is a process during which cells are polarized along the plane of the epithelium and is regulated by several transmembrane signaling proteins. After their synthesis, these PCP proteins are delivered along the secretory transport pathway to the plasma membrane, where they perform their physiological functions. However, the molecular mechanisms that regulate PCP protein transport remain largely unclear. Here, we found that the delivery of a PCP protein, Frizzled-6, to the cell surface is regulated by two conserved polybasic motifs: one located in its first intracellular loop and the other in its C-terminal cytosolic domain. We observed that the polybasic motif of Frizzled is also important for its surface localization in the Drosophila wing. Results from a mechanistic analysis indicated that Frizzled-6 packaging into vesicles at the endoplasmic reticulum (ER) is regulated by a direct interaction between the polybasic motif and the Glu-62 and Glu-63 residues on the secretion-associated Ras-related GTPase 1A (SAR1A) subunit of coat protein complex II (COPII). Moreover, we found that newly synthesized Frizzled-6 is associated with another PCP protein, cadherin EGF LAG seven-pass G-type receptor 1 (CELSR1), in the secretory transport pathway, and that this association regulates their surface delivery. Our results reveal insights into the molecular machinery that regulates the ER export of Frizzled-6. They also suggest that the association of CELSR1 with Frizzled-6 is important, enabling efficient Frizzled-6 delivery to the cell surface, providing a quality control mechanism that ensures the appropriate stoichiometry of these two PCP proteins at cell boundaries.

Polycistronic Genome Segment Evolution and Gain and Loss of FAST Protein Function during Fusogenic Orthoreovirus Speciation

The Reoviridae family is the only non-enveloped virus family with members that use syncytium formation to promote cell–cell virus transmission. Syncytiogenesis is mediated by a fusion-associated small transmembrane (FAST) protein, a novel family of viral membrane fusion proteins. Previous evidence suggested the fusogenic reoviruses arose from an ancestral non-fusogenic virus, with the preponderance of fusogenic species suggesting positive evolutionary pressure to acquire and maintain the fusion phenotype. New phylogenetic analyses that included the atypical waterfowl subgroup of avian reoviruses and recently identified new orthoreovirus species indicate a more complex relationship between reovirus speciation and fusogenic capacity, with numerous predicted internal indels and 5’-terminal extensions driving the evolution of the orthoreovirus’ polycistronic genome segments and their encoded FAST and fiber proteins. These inferred recombination events generated bi- and tricistronic genome segments with diverse gene constellations, they occurred pre- and post-orthoreovirus speciation, and they directly contributed to the evolution of the four extant orthoreovirus FAST proteins by driving both the gain and loss of fusion capability. We further show that two distinct post-speciation genetic events led to the loss of fusion in the waterfowl isolates of avian reovirus, a recombination event that replaced the p10 FAST protein with a heterologous, non-fusogenic protein and point substitutions in a conserved motif that destroyed the p10 assembly into multimeric fusion platforms.

Acute disruption of the synaptic vesicle membrane protein synaptotagmin 1 using knockoff in mouse hippocampal neurons

The success of comparative cell biology for determining protein function relies on quality disruption techniques. Long-lived proteins, in postmitotic cells, are particularly difficult to eliminate. Moreover, cellular processes are notoriously adaptive; for example, neuronal synapses exhibit a high degree of plasticity. Ideally, protein disruption techniques should be both rapid and complete. Here, we describe knockoff, a generalizable method for the druggable control of membrane protein stability. We developed knockoff for neuronal use but show it also works in other cell types. Applying knockoff to synaptotagmin 1 (SYT1) results in acute disruption of this protein, resulting in loss of synchronous neurotransmitter release with a concomitant increase in the spontaneous release rate, measured optically. Thus, SYT1 is not only the proximal Ca²⁺ sensor for fast neurotransmitter release but also serves to clamp spontaneous release. Additionally, knockoff can be applied to protein domains as we show for another synaptic vesicle protein, synaptophysin 1.

Molecular Basis for Synaptotagmin-1-Associated Neurodevelopmental Disorder

At neuronal synapses, synaptotagmin-1 (syt1) acts as a Ca²⁺ sensor that synchronizes neurotransmitter release with Ca²⁺ influx during action potential firing. Heterozygous missense mutations in syt1 have recently been associated with a severe but heterogeneous developmental syndrome, termed syt1-associated neurodevelopmental disorder. Well-defined pathogenic mechanisms, and the basis for phenotypic heterogeneity in this disorder, remain unknown. Here, we report the clinical, physiological, and biophysical characterization of three syt1 mutations from human patients. Synaptic transmission was impaired in neurons expressing mutant variants, which demonstrated potent, graded dominant-negative effects. Biophysical interrogation of the mutant variants revealed novel mechanistic features concerning the cooperative action, and functional specialization, of the tandem Ca²⁺-sensing domains of syt1. These mechanistic studies led to the discovery that a clinically approved K⁺ channel antagonist is able to rescue the dominant-negative heterozygous phenotype. Our results establish a molecular cause, basis for phenotypic heterogeneity, and potential treatment approach for syt1-associated neurodevelopmental disorder.

Bluetongue Virus Nonstructural Protein 3 Orchestrates Virus Maturation and Drives Non-Lytic Egress via Two Polybasic Motifs

Bluetongue virus (BTV) is an arthropod-borne virus that infects domestic and wild ruminants. The virion is a non-enveloped double-layered particle with an outer capsid that encloses a core containing the segmented double-stranded RNA genome. Although BTV is canonically released by cell lysis, it also exits non-lytically. In infected cells, the BTV nonstructural glycoprotein 3 (NS3) is found to be associated with host membranes and traffics from the endoplasmic reticulum through the Golgi apparatus to the plasma membrane. This suggests a role for NS3 in BTV particle maturation and non-lytic egress. However, the mechanism by which NS3 coordinates these events has not yet been elucidated. Here, we identified two polybasic motifs (PMB1/PMB2), consistent with the membrane binding. Using site-directed mutagenesis, confocal and electron microscopy, and flow cytometry, we demonstrated that PBM1 and PBM2 mutant viruses retained NS3 either in the Golgi apparatus or in the endoplasmic reticulum, suggesting a distinct role for each motif. Mutation of PBM2 motif decreased NS3 export to the cell surface and virus production. However, both mutant viruses produced predominantly inner core particles that remained close to their site of assembly. Together, our data demonstrates that correct trafficking of the NS3 protein is required for virus maturation and release.

Rotavirus Species B Encodes a Functional Fusion-Associated Small Transmembrane Protein

Rotavirus is an important cause of diarrheal disease in young mammals. Rotavirus species A (RVA) causes most human rotavirus diarrheal disease and primarily affects infants and young children. Rotavirus species B (RVB) has been associated with sporadic outbreaks of human adult diarrheal disease. RVA and RVB are predicted to encode mostly homologous proteins but differ significantly in the proteins encoded by the NSP1 gene. In the case of RVB, the NSP1 gene encodes two putative protein products of unknown function, NSP1-1 and NSP1-2. We demonstrate that human RVB NSP1-1 mediates syncytium formation in cultured human cells. Based on sequence alignment, NSP1-1 proteins from species B, G, and I contain features consistent with fusion-associated small transmembrane (FAST) proteins, which have previously been identified in other genera of the Reoviridae family. Like some other FAST proteins, RVB NSP1-1 is predicted to have an N-terminal myristoyl modification. Addition of an N-terminal FLAG peptide disrupts NSP1-1-mediated fusion. NSP1-1 from a human RVB mediates fusion of human cells but not hamster cells and, thus, may serve as a species tropism determinant. NSP1-1 also can enhance RVA replication in human cells, both in single-cycle infection studies and during a multicycle time course in the presence of fetal bovine serum, which inhibits rotavirus spread. These findings suggest potential yet untested roles for NSP1-1 in RVB species tropism, immune evasion, and pathogenesis.IMPORTANCE While species A rotavirus is commonly associated with diarrheal disease in young children, species B rotavirus has caused sporadic outbreaks of adult diarrheal disease. A major genetic difference between species A and B rotaviruses is the NSP1 gene, which encodes two proteins for species B rotavirus. We demonstrate that the smaller of these proteins, NSP1-1, can mediate fusion of cultured human cells. Comparison with viral proteins of similar function provides insight into NSP1-1 domain organization and fusion mechanism. These comparisons suggest that there is a fatty acid modification at the amino terminus of the protein, and our results show that an intact amino terminus is required for NSP1-1-mediated fusion. NSP1-1 from a human virus mediates fusion of human cells, but not hamster cells, and enhances species A rotavirus replication in culture. These findings suggest potential, but currently untested, roles for NSP1-1 in RVB host species tropism, immune evasion, and pathogenesis.

Role of the cytosolic domain of occludin in trafficking and hepatitis C virus infection

The role of the tight-junction (TJ) protein occludin (OCLN) in hepatitis C virus (HCV) entry remains elusive. Here, we investigated the OCLN C-terminal cytosolic domain in HCV infection. We expressed a series of C-terminal deletion mutants in Huh-7 cells KO for OCLN and characterized their functionality in HCV infection and trafficking. Deleting the OCLN cytosolic domain led to protein instability and intracellular retention. The first 15 residues (OCLN-C15 mutant) of the cytosolic domain were sufficient for OCLN stability, but led to its accumulation in the trans-Golgi network (TGN) due to a deficient cell surface export after synthesis. In contrast, the OCLN-C18 mutant, containing the first 18 residues of the cytosolic domain, was expressed at the cell surface and could mediate HCV infection. Point mutations in the context of C18 showed that I279 and W281 are crucial residues for cell surface expression of OCLN-C18. However, in the context of full-length OCLN, mutation of these residues only partially affected infection and cell surface localization. Importantly, the characterization of OCLN-C18 in human-polarized hepatocytes revealed a defect in its TJ localization without affecting HCV infection. These data suggest that TJ localization of OCLN is not a prerequisite for HCV infection in polarized hepatocytes.

Fusogenic Reoviruses and Their Fusion-Associated Small Transmembrane (FAST) Proteins

With no limiting membrane surrounding virions, nonenveloped viruses have no need for membrane fusion to gain access to intracellular replication compartments. Consequently, nonenveloped viruses do not encode membrane fusion proteins. The only exception to this dogma is the fusogenic reoviruses that encode fusion-associated small transmembrane (FAST) proteins that induce syncytium formation. FAST proteins are the smallest viral membrane fusion proteins and, unlike their enveloped virus counterparts, are nonstructural proteins that evolved specifically to induce cell-to-cell, not virus-cell, membrane fusion. This distinct evolutionary imperative is reflected in structural and functional features that distinguish this singular family of viral fusogens from all other protein fusogens. These rudimentary fusogens comprise specific combinations of different membrane effector motifs assembled into small, modular membrane fusogens. FAST proteins offer a minimalist model to better understand the ubiquitous process of protein-mediated membrane fusion and to reveal novel mechanisms of nonenveloped virus dissemination that contribute to virulence.

Protein Kinase C-epsilon in Membrane Delivery during Phagocytosis

During phagocytosis, internal membranes are recruited to the site of pathogen binding and fuse with the plasma membrane, providing the membrane needed for pseudopod extension and target uptake. The mechanism by which vesicles destined for the phagosome are generated, targeted, and fuse is unknown. We established that Golgi-associated protein kinase C-epsilon (PKC-ε) is necessary for the addition of membrane during FcyR-mediated phagocytosis. PKC-ε is tethered to the Golgi through interactions between its' regulatory domain and the Golgi lipids PI4P and diacylglycerol; disruption of these interactions prevents PKC-ε concentration at phagosomes and decreases phagocytosis. The accumulated evidence suggests that PKC-ε orchestrates vesicle formation at the Golgi by a mechanism requiring lipid binding but not enzymatic activity. This review discusses how PKC-ε might mediate vesicle formation at the level of budding and fission. Specifically, we discuss PKC-ε binding partners, the formation of lipid subdomains to generate membrane curvature, and PKC-ε mediated links to the actin and microtubule cytoskeleton to provide tension for vesicle fission. Assimilating information from several model systems, we propose a model for PKC-ε mediated vesicle formation for exocytosis during phagocytosis that may be applicable to other processes that require directed membrane delivery and fusion.

A mechanism for differential sorting of the planar cell polarity proteins Frizzled6 and Vangl2 at the trans-Golgi network

In planar cell polarity (PCP), the epithelial cells are polarized along the plane of the cell surface perpendicular to the classical apical-basal axis, a process mediated by several conserved signaling receptors. Two PCP-signaling proteins, VANGL planar cell polarity protein 2 (Vangl2) and Frizzled6 (Fzd6), are located asymmetrically on opposite boundaries of the cell. Examining sorting of these two proteins at the trans-Golgi network (TGN), we demonstrated previously that the GTP-binding protein ADP-ribosylation factor-related protein 1 (Arfrp1) and the clathrin-associated adaptor protein complex 1 (AP-1) are required for Vangl2 transport from the TGN. In contrast, TGN export of Frizzled6 does not depend on Arfrp1 or AP-1. Here, to further investigate the TGN sorting process in mammalian cells, we reconstituted release of Vangl2 and Frizzled6 from the TGN into vesicles in vitro Immunoblotting of released vesicles indicated that Vangl2 and Frizzled6 exit the TGN in separate compartments. Knockdown analysis revealed that a clathrin adaptor, epsinR, regulates TGN export of Frizzled6 but not of Vangl2. Protein interaction analysis suggested that epsinR forms a stable complex with clathrin and that this complex interacts with a conserved polybasic motif in the Frizzled6 cytosolic domain to package Frizzled6 into transport vesicles. Moreover, we found that Frizzled6-epsinR binding dissociates epsinR from AP-1, which may separate these two cargo adaptors from each other to perform distinct cargo-sorting functions. Our results suggest that Vangl2 and Frizzled6 are packaged into separate vesicles that are regulated by different clathrin adaptors at the TGN, which may contribute to their asymmetric localizations.

Reovirus FAST Protein Enhances Vesicular Stomatitis Virus Oncolytic Virotherapy in Primary and Metastatic Tumor Models

The reovirus fusion-associated small transmembrane (FAST) proteins are the smallest known viral fusogens (∼100–150 amino acids) and efficiently induce cell-cell fusion and syncytium formation in multiple cell types. Syncytium formation enhances cell-cell virus transmission and may also induce immunogenic cell death, a form of apoptosis that stimulates immune recognition of tumor cells. These properties suggest that FAST proteins might serve to enhance oncolytic virotherapy. The oncolytic activity of recombinant VSVΔM51 (an interferon-sensitive vesicular stomatitis virus [VSV] mutant) encoding the p14 FAST protein (VSV-p14) was compared with a similar construct encoding GFP (VSV-GFP) in cell culture and syngeneic BALB/c tumor models. Compared with VSV-GFP, VSV-p14 exhibited increased oncolytic activity against MCF-7 and 4T1 breast cancer spheroids in culture and reduced primary 4T1 breast tumor growth in vivo. VSV-p14 prolonged survival in both primary and metastatic 4T1 breast cancer models, and in a CT26 metastatic colon cancer model. As with VSV-GFP, VSV-p14 preferentially replicated in vivo in tumors and was cleared rapidly from other sites. Furthermore, VSV-p14 increased the numbers of activated splenic CD4, CD8, natural killer (NK), and natural killer T (NKT) cells, and increased the number of activated CD4 and CD8 cells in tumors. FAST proteins may therefore provide a multi-pronged approach to improving oncolytic virotherapy via syncytium formation and enhanced immune stimulation.

The Prenylated Rab GTPase Receptor PRA1.F4 Contributes to Protein Exit from the Golgi Apparatus

Prenylated Rab acceptor1 (PRA1) functions in the recruitment of prenylated Rab proteins to their cognate organelles. Arabidopsis (Arabidopsis thaliana) contains a large number of proteins belonging to the AtPRA1 family. However, their physiological roles remain largely unknown. Here, we investigated the physiological role of AtPRA1.F4, a member of the AtPRA1 family. A T-DNA insertion knockdown mutant of AtPRA1.F4, atpra1.f4, was smaller in stature than parent plants and possessed shorter roots, whereas transgenic plants overexpressing HA:AtPRA1.F4 showed enhanced development of secondary roots and root hairs. However, both overexpression and knockdown plants exhibited increased sensitivity to high-salt stress, lower vacuolar Na⁺/K⁺-ATPase and plasma membrane ATPase activities, lower and higher pH in the vacuole and apoplast, respectively, and highly vesiculated Golgi apparatus. HA:AtPRA1.F4 localized to the Golgi apparatus and assembled into high-molecular-weight complexes. atpra1.f4 plants displayed a defect in vacuolar trafficking, which was complemented by low but not high levels of HA:AtPRA1.F4 Overexpression of HA:AtPRA1.F4 also inhibited protein trafficking at the Golgi apparatus, albeit differentially depending on the final destination or type of protein: trafficking of vacuolar proteins, plasma membrane proteins, and trans-Golgi network (TGN)-localized SYP61 was strongly inhibited; trafficking of TGN-localized SYP51 was slightly inhibited; and trafficking of secretory proteins and TGN-localized SYP41 was negligibly or not significantly inhibited. Based on these results, we propose that Golgi-localized AtPRA1.F4 is involved in the exit of many but not all types of post-Golgi proteins from the Golgi apparatus. Additionally, an appropriate level of AtPRA1.F4 is crucial for its function at the Golgi apparatus.

Topogenesis and cell surface trafficking of GPR34 are facilitated by positive-inside rule that effects through a tri-basic motif in the first intracellular loop

Protein folding, topogenesis and intracellular targeting of G protein-coupled receptors (GPCRs) must be precisely coordinated to ensure correct receptor localization. To elucidate how different steps of GPCR biosynthesis work together, we investigated the process of membrane topology determination and how it relates to the acquisition of cell surface trafficking competence in human GPR34. By monitoring a fused FLAG-tag and a conformation-sensitive native epitope during the expression of GPR34 mutant panel, a tri-basic motif in the first intracellular loop was identified as the key topogenic signal that dictates the orientation of transmembrane domain-1 (TM1). Charge disruption of the motif perturbed topogenic processes and resulted in the conformational epitope loss, post-translational processing alteration, and trafficking arrest in the Golgi. The placement of a cleavable N-terminal signal sequence as a surrogate topogenic determinant overcame the effects of tri-basic motif mutations and rectified the TM1 orientation; thereby restored the conformational epitope, post-translational modifications, and cell surface trafficking altogether. Progressive N-tail truncation and site-directed mutagenesis revealed that a proline-rich segment of the N-tail and all four cysteines individually located in the four separate extracellular regions must simultaneously reside in the ER lumen to muster the conformational epitope. Oxidation of all four cysteines was necessary for the epitope formation, but the cysteine residues themselves were not required for the trafficking event. The underlying biochemical properties of the conformational epitope was therefore the key to understand mechanistic processes propelled by positive-inside rule that simultaneously regulate the topogenesis and intracellular trafficking of GPR34.

A novel tribasic Golgi export signal directs cargo protein interaction with activated Rab11 and AP-1-dependent Golgi-plasma membrane trafficking

A novel sorting motif present in the reovirus p14 fusion–associated small transmembrane protein directs interaction with GTP-Rab11 at the TGN and sorting into AP-1–coated vesicles for trafficking to the plasma membrane. This is the first example of cargo protein interaction with activated Rab11 mediating anterograde trafficking from the TGN.

The reovirus fusion–associated small transmembrane (FAST) proteins comprise a unique family of viral membrane fusion proteins dedicated to inducing cell–cell fusion. We recently reported that a polybasic motif (PBM) in the cytosolic tail of reptilian reovirus p14 FAST protein functions as a novel tribasic Golgi export signal. Using coimmunoprecipitation and fluorescence resonance energy transfer (FRET) assays, we now show the PBM directs interaction of p14 with GTP-Rab11. Overexpression of dominant-negative Rab11 and RNA interference knockdown of endogenous Rab11 inhibited p14 plasma membrane trafficking and resulted in p14 accumulation in the Golgi complex. This is the first example of Golgi export to the plasma membrane that is dependent on the interaction of membrane protein cargo with activated Rab11. RNA interference and immunofluorescence microscopy further revealed that p14 Golgi export is dependent on AP-1 (but not AP-3 or AP-4) and that Rab11 and AP-1 both colocalize with p14 at the TGN. Together these results imply the PBM mediates interactions of p14 with activated Rab11 at the TGN, resulting in p14 sorting into AP1-coated vesicles for anterograde TGN–plasma membrane transport.

Reovirus FAST Proteins Drive Pore Formation and Syncytiogenesis Using a Novel Helix-Loop-Helix Fusion-Inducing Lipid Packing Sensor

Pore formation is the most energy-demanding step during virus-induced membrane fusion, where high curvature of the fusion pore rim increases the spacing between lipid headgroups, exposing the hydrophobic interior of the membrane to water. How protein fusogens breach this thermodynamic barrier to pore formation is unclear. We identified a novel fusion-inducing lipid packing sensor (FLiPS) in the cytosolic endodomain of the baboon reovirus p15 fusion-associated small transmembrane (FAST) protein that is essential for pore formation during cell-cell fusion and syncytiogenesis. NMR spectroscopy and mutational studies indicate the dependence of this FLiPS on a hydrophobic helix-loop-helix structure. Biochemical and biophysical assays reveal the p15 FLiPS preferentially partitions into membranes with high positive curvature, and this partitioning is impeded by bis-ANS, a small molecule that inserts into hydrophobic defects in membranes. Most notably, the p15 FLiPS can be functionally replaced by heterologous amphipathic lipid packing sensors (ALPS) but not by other membrane-interactive amphipathic helices. Furthermore, a previously unrecognized amphipathic helix in the cytosolic domain of the reptilian reovirus p14 FAST protein can functionally replace the p15 FLiPS, and is itself replaceable by a heterologous ALPS motif. Anchored near the cytoplasmic leaflet by the FAST protein transmembrane domain, the FLiPS is perfectly positioned to insert into hydrophobic defects that begin to appear in the highly curved rim of nascent fusion pores, thereby lowering the energy barrier to stable pore formation.

The fusogenic ortho- and aquareoviruses are the only known nonenveloped viruses that induce syncytium formation. Cell-cell fusion is a virulence determinant of fusogenic reoviruses, and is mediated by a singular family of fusion-associated small transmembrane (FAST) proteins, the smallest known viral fusogens. Unlike their enveloped virus counterparts, reovirus FAST proteins have exceptionally small ectodomains and considerable larger cytoplasmic endodomains, suggesting FAST protein interactions with the cytoplasmic leaflet of the plasma membrane likely play a prominent role in the fusion process. We determined that the baboon reovirus p15 FAST protein endodomain contains a novel type of helix-loop-helix lipid packing sensor that partitions into hydrophobic defects present in highly curved membranes. This fusion-inducing lipid packing sensor (FLiPS) is required for pore formation, and can be functionally replaced by heterologous lipid packing sensors. By masking hydrophobic defects appearing in the highly curved rim of nascent fusion pores, the FliPS would make the forward reaction to pore formation a more energetically favored means of resolving an unstable hemifusion intermediate. These results define a new role for curvature sensing motifs, and reveal how viral fusion proteins can drive pore formation without having to rely on membrane stresses induced by complex refolding of large ectodomains.

Reovirus FAST proteins: virus-encoded cellular fusogens

Reovirus fusion-associated small transmembrane (FAST) proteins are the only known nonenveloped virus fusogens and are dedicated to inducing cell-to-cell, not virus-cell, membrane fusion. Numerous structural and functional attributes distinguish this novel family of viral fusogens from all enveloped virus membrane fusion proteins. Both families of viral fusogens play key roles in virus dissemination and pathogenicity, but employ different mechanisms to mediate membrane apposition and merger. However, convergence of these distinct families of viral membrane fusion proteins on common pathways needed for pore expansion and syncytium formation suggests syncytiogenesis represents a cellular response to the presence of cell-cell fusion pores. Together, FAST proteins and enveloped virus fusion proteins provide exceptional insights into the ubiquitous process of cell-cell membrane fusion and syncytium formation.

Protein folding and secretion: mechanistic insights advancing recombinant protein production in S. cerevisiae

The emergence of genomic approaches coupled to recombinant DNA technologies have identified the quality control systems that regulate proteostasis - biological pathways that modulate protein biogenesis, maturation, trafficking, and degradation. The elucidation of these pathways has become of growing importance in therapeutics as loss of proteostasis has been suggested to lead to a number of human diseases including Alzheimer's, Parkinson's Disease and Type II Diabetes. We anticipate that the most successful strategies for protein expression and therapeutics development may involve integration of protein engineering strategies with host manipulation, to exploit the cell's native stress response pathways and trafficking mechanisms. This review will highlight recent findings and mechanistic detail correlated to quality control in the early secretory pathway of Saccharomyces cerevisiae.

Golgi compartmentation and identity

Recent work supports the idea that cisternae of the Golgi apparatus can be assigned to three classes, which correspond to discrete stages of cisternal maturation. Each stage has a unique pattern of membrane traffic. At the first stage, cisternae form in association with the ER at multifunctional membrane assembly stations. At the second stage, cisternae synthesize carbohydrates while exchanging material via COPI vesicles. At the third stage, cisternae of the trans-Golgi network segregate into domains and produce transport carriers with the aid of specific lipids and the actin cytoskeleton. These processes are coordinated by cascades of Rab and Arf/Arl GTPases.

Polybasic trafficking signal mediates golgi export, ER retention or ER export and retrieval based on membrane-proximity

Trafficking of integral membrane proteins between the ER and Golgi complex, and protein sorting and trafficking between the TGN and endosomal/lysosomal compartments or plasma membranes, are dependent on cis-acting, linear amino acid sorting signals. Numerous sorting signals of this type have been identified in the cytoplasmic domains of membrane proteins, several of which rely on basic residues. A novel Golgi export signal that relies on a membrane-proximal polybasic motif (PBM) was recently identified in the reptilian reovirus p14 protein, a representative of an unusual group of bitopic fusion-associated small transmembrane (FAST) proteins encoded by fusogenic orthoreoviruses and responsible for cell-cell fusion and syncytium formation. Using immunofluorescence microscopy, cell surface immunofluorescence, and endoglycosidase H assays, we now show the p14 PBM can mediate several distinct trafficking functions depending on its proximity to the transmembrane domain (TMD). When present within 4-residues of the TMD it serves as a Golgi export signal, but when located at the C-terminus of the 68-residue p14 cytoplasmic endodomain it functions as an ER retention signal. The PBM has no effect on protein trafficking when located at an internal position in the cytoplasmic domain. When present in both membrane-proximal and -distal locations, the PBMs promote export to, and efficient retrieval from, the Golgi complex. Interestingly, the conflicting trafficking signals provided by two PBMs induces extensive ER tubulation and segregation of ER components. These studies highlight how a single trafficking signal in a simple transmembrane protein can have remarkably diverse, position-dependent effects on protein trafficking and ER morphogenesis.