Palmitoylation, membrane-proximal basic residues, and transmembrane glycine residues in the reovirus p10 protein are essential for syncytium formation

Virus-Induced Cell Fusion and Syncytia Formation

Most enveloped viruses encode viral fusion proteins to penetrate host cell by membrane fusion. Interestingly, many enveloped viruses can also use viral fusion proteins to induce cell-cell fusion, both in vitro and in vivo, leading to the formation of syncytia or multinucleated giant cells (MGCs). In addition, some non-enveloped viruses encode specialized viral proteins that induce cell-cell fusion to facilitate viral spread. Overall, viruses that can induce cell-cell fusion are nearly ubiquitous in mammals. Virus cell-to-cell spread by inducing cell-cell fusion may overcome entry and post-entry blocks in target cells and allow evasion of neutralizing antibodies. However, molecular mechanisms of virus-induced cell-cell fusion remain largely unknown. Here, I summarize the current understanding of virus-induced cell fusion and syncytia formation.

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.

Multiple rotavirus species encode fusion-associated small transmembrane (FAST) proteins with cell type-specific activity

Fusion-associated small transmembrane (FAST) proteins are viral nonstructural proteins that mediate cell-cell fusion to form multinucleated syncytia. We previously reported that human species B rotavirus NSP1-1 is a FAST protein that induces syncytia in primate epithelial cells but not rodent fibroblasts. We hypothesized that the NSP1-1 proteins of other rotavirus species could also mediate cell-cell fusion and that fusion activity might be limited to cell types derived from homologous hosts. To test this hypothesis, we predicted the structure and domain organization of NSP1-1 proteins of species B rotavirus from a human, goat, and pig, species G rotavirus from a pigeon and turkey, and species I rotavirus from a dog and cat. We cloned these sequences into plasmids and transiently expressed the NSP1-1 proteins in avian, canine, hamster, human, porcine, and simian cells. Regardless of host origin of the virus, each NSP1-1 protein induced syncytia in primate cells, while few induced syncytia in other cell types. To identify the domains that determined cell-specific fusion activity for human species B rotavirus NSP1-1, we engineered chimeric proteins containing domain exchanges with the p10 FAST protein from Nelson Bay orthoreovirus. Using the chimeric proteins, we found that the N-terminal and transmembrane domains determined the cell type specificity of fusion activity. Although the species and cell type criteria for fusion activity remain unclear, these findings suggest that rotavirus species B, G, and I NSP1-1 are functional FAST proteins whose N termini play a role in specifying the cells in which they mediate syncytia formation.

Oncolytic viruses-modulated immunogenic cell death, apoptosis and autophagy linking to virotherapy and cancer immune response

Recent reports have revealed that oncolytic viruses (OVs) play a significant role in cancer therapy. The infection of OVs such as oncolytic vaccinia virus (OVV), vesicular stomatitis virus (VSV), parvovirus, mammalian reovirus (MRV), human adenovirus, Newcastle disease virus (NDV), herpes simplex virus (HSV), avian reovirus (ARV), Orf virus (ORFV), inactivated Sendai virus (ISV), enterovirus, and coxsackievirus offer unique opportunities in immunotherapy through diverse and dynamic pathways. This mini-review focuses on the mechanisms of OVs-mediated virotherapy and their effects on immunogenic cell death (ICD), apoptosis, autophagy and regulation of the immune system.

Production of a replicating retroviral vector expressing Reovirus fast protein for cancer gene therapy

Reovirus fusion-associated small transmembrane (FAST) proteins induce syncytium formation. Recently, several studies have shown that the use of recombinant vectors engineered to express fusion proteins is becoming attractive for the development of enhanced oncolytic viruses. In this study, we investigated the cytotoxic effect of four different FAST proteins (p10 FAST of Avian reovirus [ARV], p10 FAST of Pulau virus [PuV], p13 FAST of Broome virus [BroV], and p14 FAST of reptilian reovirus [RRV]). Plasmids encoding FASTs were transfected into Vero cells. All FAST proteins induced syncytium formation at varying intensities. To achieve high levels of FAST expression, four different FAST genes were inserted into the murine leukemia virus (MLV)-based replication-competent retroviral (RCR) vector. Two days after transfection in 293 T cells, only the MoMLV-10A1-p10(PuV) RCR vector showed syncytia formation. Based on these results, p10(Puv) was selected from the four FASTs. Next, we investigated the cytotoxicity of p10(PuV) on HeLa cervical carcinoma cells, HT1080 human fibrosarcoma cells, and U87 human glioma cells. Although three human cancer cell lines induced syncytium formation, U87 cells were highly susceptible to syncytia formation by transfection with p10(PuV). In addition, the viral supernatants from MoMLV-10A-p10(PuV) RCR vector-transfected 293 T cells also induced syncytium formation in HT1080, TE671, and U87 cells. This RCR vector encoding p10(PuV) is a promising candidate for cancer gene therapy.

FAST Proteins: Development and Use of Reverse Genetics Systems for Reoviridae Viruses

Reverse genetics systems for viruses, the technology used to generate gene-engineered recombinant viruses from artificial genes, enable the study of the roles of the individual nucleotides and amino acids of viral genes and proteins in infectivity, replication, and pathogenicity. The successful development of a reverse genetics system for poliovirus in 1981 accelerated the establishment of protocols for other RNA viruses important for human health. Despite multiple efforts, rotavirus (RV), which causes severe gastroenteritis in infants, was refractory to reverse genetics analysis, and the first complete reverse genetics system for RV was established in 2017. This novel technique involves use of the fusogenic protein FAST (fusion-associated small transmembrane) derived from the bat-borne Nelson Bay orthoreovirus, which induces massive syncytium formation. Co-transfection of a FAST-expressing plasmid with complementary DNAs encoding RV genes enables rescue of recombinant RV. This review focuses on methodological insights into the reverse genetics system for RV and discusses applications and potential improvements to this system.

Structural Insights into Membrane Fusion Mediated by Convergent Small Fusogens

From lifeless viral particles to complex multicellular organisms, membrane fusion is inarguably the important fundamental biological phenomena. Sitting at the heart of membrane fusion are protein mediators known as fusogens. Despite the extensive functional and structural characterization of these proteins in recent years, scientists are still grappling with the fundamental mechanisms underlying membrane fusion. From an evolutionary perspective, fusogens follow divergent evolutionary principles in that they are functionally independent and do not share any sequence identity; however, they possess structural similarity, raising the possibility that membrane fusion is mediated by essential motifs ubiquitous to all. In this review, we particularly emphasize structural characteristics of small-molecular-weight fusogens in the hope of uncovering the most fundamental aspects mediating membrane–membrane interactions. By identifying and elucidating fusion-dependent functional domains, this review paves the way for future research exploring novel fusogens in health and disease.

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.

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.

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.

Cell-cell fusion induced by reovirus FAST proteins enhances replication and pathogenicity of non-enveloped dsRNA viruses

Fusogenic reoviruses encode fusion-associated small transmembrane (FAST) protein, which induces cell-cell fusion. FAST protein is the only known fusogenic protein in non-enveloped viruses, and its role in virus replication is not yet known. We generated replication-competent, FAST protein-deficient pteropine orthoreovirus and demonstrated that FAST protein was not essential for viral replication, but enhanced viral replication in the early phase of infection. Addition of recombinant FAST protein enhanced replication of FAST-deficient virus and other non-fusogenic viruses in a fusion-dependent and FAST-species-independent manner. In a mouse model, replication and pathogenicity of FAST-deficient virus were severely impaired relative to wild-type virus, indicating that FAST protein is a major determinant of the high pathogenicity of fusogenic reovirus. FAST-deficient virus also conferred effective protection against challenge with lethal homologous virus strains in mice. Our results demonstrate a novel role of a viral fusogenic protein and the existence of a cell-cell fusion-dependent replication system in non-enveloped viruses.

Myoblast fusion confusion: the resolution begins

The fusion of muscle precursor cells is a required event for proper skeletal muscle development and regeneration. Numerous proteins have been implicated to function in myoblast fusion; however, the majority are expressed in diverse tissues and regulate numerous cellular processes. How myoblast fusion is triggered and coordinated in a muscle-specific manner has remained a mystery for decades. Through the discovery of two muscle-specific fusion proteins, Myomaker and Myomerger–Minion, we are now primed to make significant advances in our knowledge of myoblast fusion. This article reviews the latest findings regarding the biology of Myomaker and Minion–Myomerger, places these findings in the context of known pathways in mammalian myoblast fusion, and highlights areas that require further investigation. As our understanding of myoblast fusion matures so does our potential ability to manipulate cell fusion for therapeutic purposes.

Physicochemical sequence characteristics that influence S-palmitoylation propensity

Over the past 30 years, several hundred eukaryotic proteins spanning from yeast to man have been shown to be S-palmitoylated. This post-translational modification involves the reversible addition of a 16-carbon saturated fatty acyl chain onto the cysteine residue of a protein where it regulates protein membrane association and distribution, conformation, and stability. However, the large-scale proteome-wide discovery of new palmitoylated proteins has been hindered by the difficulty of identifying a palmitoylation consensus sequence. Using a bioinformatics approach, we show that the enrichment of hydrophobic and basic residues, the cellular context of the protein, and the structural features of the residues surrounding the palmitoylated cysteine all influence the likelihood of palmitoylation. We developed a new palmitoylation predictor that incorporates these identified features, and this predictor achieves a Matthews Correlation Coefficient of .74 using 10-fold cross validation, and significantly outperforms existing predictors on unbiased testing sets. This demonstrates that palmitoylation sites can be predicted with accuracy by taking into account not only physiochemical properties of the modified cysteine and its surrounding residues, but also structural parameters and the subcellular localization of the modified cysteine. This will allow for improved predictions of palmitoylated residues in uncharacterized proteins. A web-based version of this predictor is currently under development.

A critical role of LAMP-1 in avian reovirus P10 degradation associated with inhibition of apoptosis and virus release

Avian reovirus (ARV) causes viral arthritis, chronic respiratory diseases, retarded growth and malabsorption syndrome. The ARV p10 protein, a viroporin responsible for the induction of cell syncytium formation and apoptosis, is rapidly degraded in host cells. However, the mechanism of p10 degradation and its relevance are still unclear. We report here the identification of cellular lysosome-associated membrane protein 1 (LAMP-1) as an interaction partner of p10 by yeast two-hybrid screening, immunoprecipitation and confocal microscopy assays. We found that rapid degradation of p10 was associated with ubiquitination. Importantly, ARV p10 degradation in host cells could be completely abolished by knockdown of LAMP-1 by siRNA, indicating that LAMP-1 is required for ARV p10 degradation in host cells. In contrast, overexpression of LAMP-1 facilitated p10 degradation. Furthermore, knockdown of LAMP-1 allowed p10 accumulation, enhancing p10-induced apoptosis and viral release. Thus, LAMP-1 plays a critical role in ARV p10 degradation associated with inhibition of apoptosis and viral release.

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.

The cytoplasmic domain of the gamete membrane fusion protein HAP2 targets the protein to the fusion site in Chlamydomonas and regulates the fusion reaction

Cell-cell fusion between gametes is a defining step during development of eukaryotes, yet we know little about the cellular and molecular mechanisms of the gamete membrane fusion reaction. HAP2 is the sole gamete-specific protein in any system that is broadly conserved and shown by gene disruption to be essential for gamete fusion. The wide evolutionary distribution of HAP2 (also known as GCS1) indicates it was present in the last eukaryotic common ancestor and, therefore, dissecting its molecular properties should provide new insights into fundamental features of fertilization. HAP2 acts at a step after membrane adhesion, presumably directly in the merger of the lipid bilayers. Here, we use the unicellular alga Chlamydomonas to characterize contributions of key regions of HAP2 to protein location and function. We report that mutation of three strongly conserved residues in the ectodomain has no effect on targeting or fusion, although short deletions that include those residues block surface expression and fusion. Furthermore, HAP2 lacking a 237-residue segment of the cytoplasmic region is expressed at the cell surface, but fails to localize at the apical membrane patch specialized for fusion and fails to rescue fusion. Finally, we provide evidence that the ancient HAP2 contained a juxta-membrane, multi-cysteine motif in its cytoplasmic region, and that mutation of a cysteine dyad in this motif preserves protein localization, but substantially impairs HAP2 fusion activity. Thus, the ectodomain of HAP2 is essential for its surface expression, and the cytoplasmic region targets HAP2 to the site of fusion and regulates the fusion reaction.

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.

A compact, multifunctional fusion module directs cholesterol-dependent homomultimerization and syncytiogenic efficiency of reovirus p10 FAST proteins

The homologous p10 fusion-associated small transmembrane (FAST) proteins of the avian (ARV) and Nelson Bay (NBV) reoviruses are the smallest known viral membrane fusion proteins, and are virulence determinants of the fusogenic reoviruses. The small size of FAST proteins is incompatible with the paradigmatic membrane fusion pathway proposed for enveloped viral fusion proteins. Understanding how these diminutive viral fusogens mediate the complex process of membrane fusion is therefore of considerable interest, from both the pathogenesis and mechanism-of-action perspectives. Using chimeric ARV/NBV p10 constructs, the 36–40-residue ectodomain was identified as the major determinant of the differing fusion efficiencies of these homologous p10 proteins. Extensive mutagenic analysis determined the ectodomain comprises two distinct, essential functional motifs. Syncytiogenesis assays, thiol-specific surface biotinylation, and liposome lipid mixing assays identified an ∼25-residue, N-terminal motif that dictates formation of a cystine loop fusion peptide in both ARV and NBV p10. Surface immunofluorescence staining, FRET analysis and cholesterol depletion/repletion studies determined the cystine loop motif is connected through a two-residue linker to a 13-residue membrane-proximal ectodomain region (MPER). The MPER constitutes a second, independent motif governing reversible, cholesterol-dependent assembly of p10 multimers in the plasma membrane. Results further indicate that: (1) ARV and NBV homomultimers segregate to distinct, cholesterol-dependent microdomains in the plasma membrane; (2) p10 homomultimerization and cholesterol-dependent microdomain localization are co-dependent; and (3) the four juxtamembrane MPER residues present in the multimerization motif dictate species-specific microdomain association and homomultimerization. The p10 ectodomain therefore constitutes a remarkably compact, multifunctional fusion module that directs syncytiogenic efficiency and species-specific assembly of p10 homomultimers into cholesterol-dependent fusion platforms in the plasma membrane.

Natural infections by fusogenic orthoreoviruses can result in severe afflictions ranging from neuropathogenicity to pneumonia and death. The fusogenic capacity of these viruses, attributable to a unique family of fusion-associated small transmembrane (FAST) proteins, is a correlate of virulence. The FAST proteins are the only known examples of nonenveloped virus membrane fusion proteins, and they are the smallest known viral fusogens whose structural and functional attributes are incompatible with current models of protein-mediated membrane fusion. Exploiting the sequence divergence and distinct syncytiogenic rates of representative p10 FAST proteins from avian and bat reovirus isolates, we determined the p10 ectodomain is a compact, complex fusion module comprising two independent functional motifs. One motif determines species-specific p10 fusion efficiency by governing formation of a cystine loop fusion peptide, while the other directs reversible clustering and multimerization of p10 in cholesterol-dependent membrane microdomains. Remarkably, a juxtamembrane tetra-peptide is solely responsible for co-dependent clustering and multimerization of p10 in distinct, species-specific fusion platforms. This is the first example of a viral fusogen utilizing a membrane-proximal ectodomain region (MPER) to direct cholesterol-dependent multimerization and assembly into fusion platforms.

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

The first example of a cytosolic, membrane-proximal, tribasic motif required for Golgi export to the plasma membrane is identified and characterized. This novel Golgi export signal can also mediate trafficking of a heterologous Golgi-resident protein, indicating that it functions as an autonomous Golgi export signal.

Although numerous linear motifs that direct protein trafficking within cells have been identified, there are few examples of linear sorting signals mediating directed export of membrane proteins from the Golgi complex to the plasma membrane. The reovirus fusion-associated small transmembrane proteins are simple, single-pass transmembrane proteins that traffic through the endoplasmic reticulum–Golgi pathway to the plasma membrane, where they induce cell–cell membrane fusion. Here we show that a membrane-proximal, polybasic motif (PBM) in the cytosolic tail of p14 is essential for efficient export of p14 from the Golgi complex to the plasma membrane. Extensive mutagenic analysis reveals that the number, but not the identity or position, of basic residues present in the PBM dictates p14 export from the Golgi complex, with a minimum of three basic residues required for efficient Golgi export. Results further indicate that the tribasic motif does not affect plasma membrane retention of p14. Furthermore, introduction of the tribasic motif into a Golgi-localized, chimeric ERGIC-53 protein directs export from the Golgi complex to the plasma membrane. The p14 PBM is the first example of an autonomous, tribasic signal required for Golgi export to the plasma membrane.

The NS16 protein of aquareovirus-C is a fusion-associated small transmembrane (FAST) protein, and its activity can be enhanced by the nonstructural protein NS26

Orthoreoviruses and aquareoviruses represent two different genera in the family Reoviridae, but they share many common characteristics in structural organization and pathogenesis. Similar to fusogenic orthoreoviruses, aquareoviruses can induce cell-cell fusion and multinucleated syncytium formation. Sequence analysis indicated that the nonstructural protein NS16 might be a fusion protein responsible for aquareovirus-C (AqRV-C) syncytiogenesis. To understand the basis of AqRV-C in syncytium formation, the properties of NS16 in mediating cell-cell fusion were investigated in this study. Bioinformatics analysis indicated that NS16 shares basic structural motifs with reovirus fusion-associated small transmembrane (FAST) proteins. However, the relative arrangement of these predicted structural motifs is different from the identified FAST proteins, suggesting that NS16 may present a new member of the FAST protein family. Further transfection assays showed that NS16 was able to induce cell-cell fusion. Nevertheless, the fusion activity was less efficient in comparison with that of the viral infection. In addition, NS16 was defined to display an N-terminus-outside/C-terminus-inside orientation, and the N-terminal ectodomain was critical for effective fusion. Moreover, immunofluorescence assays revealed that NS16 colocalized with nonstructural protein NS26 in cotransfected cells. And the enhanced fusion efficiency could be detected when NS16 was coexpressed with NS26, implying that NS26 may participate in cell-cell fusion through cooperation with NS16 in aquareovirus infection. Our study provided a basis for further characterization of cell-cell fusion mediated by AqRV-C.

Palmitoylation of virus proteins

The article summarises the results of more than 30 years of research on palmitoylation (S‐acylation) of viral proteins, the post‐translational attachment of fatty acids to cysteine residues of integral and peripheral membrane proteins. Analysing viral proteins is not only important to characterise the cellular pathogens but also instrumental to decipher the palmitoylation machinery of cells. This comprehensive review describes methods to identify S‐acylated proteins and covers the fundamental biochemistry of palmitoylation: the location of palmitoylation sites in viral proteins, the fatty acid species found in S‐acylated proteins, the intracellular site of palmitoylation and the enzymology of the reaction. Finally, the functional consequences of palmitoylation are discussed regarding binding of proteins to membranes or membrane rafts, entry of enveloped viruses into target cells by spike‐mediated membrane fusion as well as assembly and release of virus particles from infected cells. The topics are described mainly for palmitoylated proteins of influenza virus, but proteins of other important pathogens, such as the causative agents of AIDS and severe acute respiratory syndrome, and of model viruses are discussed.

Cell-cell membrane fusion induced by p15 fusion-associated small transmembrane (FAST) protein requires a novel fusion peptide motif containing a myristoylated polyproline type II helix

The p15 fusion-associated small transmembrane (FAST) protein is a nonstructural viral protein that induces cell-cell fusion and syncytium formation. The exceptionally small, myristoylated N-terminal ectodomain of p15 lacks any of the defining features of a typical viral fusion protein. NMR and CD spectroscopy indicate this small fusion module comprises a left-handed polyproline type II (PPII) helix flanked by small, unstructured N and C termini. Individual prolines in the 6-residue proline-rich motif are highly tolerant of alanine substitutions, but multiple substitutions that disrupt the PPII helix eliminate cell-cell fusion activity. A synthetic p15 ectodomain peptide induces lipid mixing between liposomes, but with unusual kinetics that involve a long lag phase before the onset of rapid lipid mixing, and the length of the lag phase correlates with the kinetics of peptide-induced liposome aggregation. Lipid mixing, liposome aggregation, and stable peptide-membrane interactions are all dependent on both the N-terminal myristate and the presence of the PPII helix. We present a model for the mechanism of action of this novel viral fusion peptide, whereby the N-terminal myristate mediates initial, reversible peptide-membrane binding that is stabilized by subsequent amino acid-membrane interactions. These interactions induce a biphasic membrane fusion reaction, with peptide-induced liposome aggregation representing a distinct, rate-limiting event that precedes membrane merger. Although the prolines in the proline-rich motif do not directly interact with membranes, the PPII helix may function to force solvent exposure of hydrophobic amino acid side chains in the regions flanking the helix to promote membrane binding, apposition, and fusion.

Helix-destabilizing, beta-branched, and polar residues in the baboon reovirus p15 transmembrane domain influence the modularity of FAST proteins

The fusogenic reoviruses induce syncytium formation using the fusion-associated small transmembrane (FAST) proteins. A recent study indicated the p14 FAST protein transmembrane domain (TMD) can be functionally replaced by the TMDs of the other FAST proteins but not by heterologous TMDs, suggesting that the FAST protein TMDs are modular fusion units. We now show that the p15 FAST protein is also a modular fusogen, as indicated by the functional replacement of the p15 ectodomain with the corresponding domain from the p14 FAST protein. Paradoxically, the p15 TMD is not interchangeable with the TMDs of the other FAST proteins, implying that unique attributes of the p15 TMD are required when this fusion module is functioning in the context of the p15 ecto- and/or endodomain. A series of point substitutions, truncations, and reextensions were created in the p15 TMD to define features that are specific to the functioning of the p15 TMD. Removal of only one or two residues from the N terminus or four residues from the C terminus of the p15 TMD eliminated membrane fusion activity, and there was a direct correlation between the fusion-promoting function of the p15 TMD and the presence of N-terminal, hydrophobic β-branched residues. Substitution of the glycine residues and triserine motif present in the p15 TMD also impaired or eliminated the fusion-promoting activity of the p15 TMD. The ability of the p15 TMD to function in an ecto- and endodomain-specific context is therefore influenced by stringent sequence requirements that reflect the importance of TMD polar residues and helix-destabilizing residues.

Homomultimerization of the reovirus p14 fusion-associated small transmembrane protein during transit through the ER-Golgi complex secretory pathway

The reovirus fusion-associated small transmembrane (FAST) proteins are the smallest known viral membrane-fusion proteins. How these diminutive fusogens mediate cell-cell fusion and syncytium formation is unclear. Ongoing efforts are aimed at defining the roles of the FAST protein ecto-, endo- and transmembrane domains in the membrane-fusion reaction. We now provide direct evidence for homomultimer formation by the FAST proteins by using an anti-haemagglutinin (HA) mAb to co-precipitate the untagged p14 FAST protein from cells co-transfected with HA-tagged p14. Disrupting the intracellular endoplasmic reticulum-Golgi complex vesicle transport pathway prevented p14 homomultimer formation, while lower pH disrupted p14 multimers. The p14 endodomain or transmembrane domains are not required for multimer formation, which, along with the pH sensitivity and the distribution of histidine residues, suggests the 36 aa p14 ectodomain is a multimerization motif.

Features of a spatially constrained cystine loop in the p10 FAST protein ectodomain define a new class of viral fusion peptides

The reovirus fusion-associated small transmembrane (FAST) proteins are the smallest known viral membrane fusion proteins. With ectodomains of only approximately 20-40 residues, it is unclear how such diminutive fusion proteins can mediate cell-cell fusion and syncytium formation. Contained within the 40-residue ectodomain of the p10 FAST protein resides an 11-residue sequence of moderately apolar residues, termed the hydrophobic patch (HP). Previous studies indicate the p10 HP shares operational features with the fusion peptide motifs found within the enveloped virus membrane fusion proteins. Using biotinylation assays, we now report that two highly conserved cysteine residues flanking the p10 HP form an essential intramolecular disulfide bond to create a cystine loop. Mutagenic analyses revealed that both formation of the cystine loop and p10 membrane fusion activity are highly sensitive to changes in the size and spatial arrangement of amino acids within the loop. The p10 cystine loop may therefore function as a cystine noose, where fusion peptide activity is dependent on structural constraints within the noose that force solvent exposure of key hydrophobic residues. Moreover, inhibitors of cell surface thioreductase activity indicate that disruption of the disulfide bridge is important for p10-mediated membrane fusion. This is the first example of a viral fusion peptide composed of a small, spatially constrained cystine loop whose function is dependent on altered loop formation, and it suggests the p10 cystine loop represents a new class of viral fusion peptides.

Aquareovirus effects syncytiogenesis by using a novel member of the FAST protein family translated from a noncanonical translation start site

As nonenveloped viruses, the aquareoviruses and orthoreoviruses are unusual in their ability to induce cell-cell fusion and syncytium formation. While an extraordinary family of fusion-associated small transmembrane (FAST) proteins is responsible for orthoreovirus syncytiogenesis, the basis for aquareovirus-induced syncytiogenesis is unknown. We now report that the S7 genome segment of an Atlantic salmon reovirus is polycistronic and uses a noncanonical CUG translation start codon to produce a 22-kDa integral membrane protein responsible for syncytiogenesis. The aquareovirus p22 protein represents a fourth distinct member of the FAST family with a unique repertoire and arrangement of structural motifs.

Reovirus FAST protein transmembrane domains function in a modular, primary sequence-independent manner to mediate cell-cell membrane fusion

The FAST proteins are a unique family of virus-encoded cell-cell membrane fusion proteins. In the absence of a cleavable N-terminal signal peptide, a single-pass transmembrane domain (TMD) functions as a reverse signal-anchor to direct the FAST proteins into the plasma membrane in an N(exo)/C(cyt) topology. There is little information available on the role of the FAST protein TMD in the cell-cell membrane fusion reaction. We show that in the absence of conservation in the length or primary amino acid sequence, the p14 TMD can be functionally exchanged with the TMDs of the p10 and p15 FAST proteins. This is not the case for chimeric p14 proteins containing the TMDs of two different enveloped viral fusion proteins or a cellular membrane protein; such chimeric proteins were defective for both pore formation and syncytiogenesis. TMD structural features that are conserved within members of the FAST protein family presumably play direct roles in the fusion reaction. Molecular modeling suggests that the funnel-shaped architecture of the FAST protein TMDs may represent such a conserved structural and functional motif. Interestingly, although heterologous TMDs exert diverse influences on the trafficking of the p14 FAST protein, these TMDs are capable of functioning as reverse signal-anchor sequences to direct p14 into lipid rafts in the correct membrane topology. The FAST protein TMDs are therefore not primary determinants of type III protein topology, but they do play a direct, sequence-independent role in the membrane fusion reaction.

Fusogenicity of Jaagsiekte sheep retrovirus envelope protein is dependent on low pH and is enhanced by cytoplasmic tail truncations

Jaagsiekte sheep retrovirus (JSRV) envelope (Env) is an active oncogene responsible for neoplastic transformation in animals and cultured cells. In this study, we used syncytium induction and fluorescence-based cell fusion assays to investigate JSRV Env fusion and its modulation by the cytoplasmic tail (CT). We found that JSRV Env induced syncytia in cells overexpressing the receptor for JSRV and that a low pH was required for this process to occur. Fusion kinetics studies revealed that cell-cell fusion by JSRV Env at neutral pH was poor, taking up to a day, in sharp contrast to fusion at low pH, which peaked within 2 min following a low-pH trigger. Deletion of the C-terminal 7 or 16 amino acids of the JSRV Env CT had no or little effect on fusion, yet additional truncation toward the membrane-spanning domain, resulting in mutants retaining as little as 1 amino acid of the CT, led to progressively increased syncytium formation at neutral pH that was further enhanced by low-pH treatment. Notably, the severely truncated mutants showed elevated levels of surface subunits in culture medium, suggesting that the CT truncations resulted in conformational changes in the ectodomain of Env that impaired surface subunit associations. Taken together, this study reveals for the first time that the fusion activity of the JSRV Env protein is dependent on a low pH and is modulated by the CT, whose truncation overcomes, at least partially, the low-pH requirement for fusion and enhances Env fusion activity and kinetics.

Envelope protein palmitoylations are crucial for murine coronavirus assembly

The coronavirus assembly process encloses a ribonucleoprotein genome into vesicles containing the lipid-embedded proteins S (spike), E (envelope), and M (membrane). This process depends on interactions with membranes that may involve palmitoylation, a common posttranslational lipidation of cysteine residues. To determine whether specific palmitoylations influence coronavirus assembly, we introduced plasmid DNAs encoding mouse hepatitis coronavirus (MHV) S, E, M, and N (nucleocapsid) into 293T cells and found that virus-like particles (VLPs) were robustly assembled and secreted into culture medium. Palmitate adducts predicted on cysteines 40, 44, and 47 of the 83-residue E protein were then evaluated by constructing mutant cDNAs with alanine or glycine codon substitutions at one or more of these positions. Triple-substituted proteins (E.Ts) lacked palmitate adducts. Both native E and E.T proteins localized at identical perinuclear locations, and both copurified with M proteins, but E.T was entirely incompetent for VLP production. In the presence of the E.T proteins, the M protein subunits accumulated into detergent-insoluble complexes that failed to secrete from cells, while native E proteins mobilized M into detergent-soluble secreted forms. Many of these observations were corroborated in the context of natural MHV infections, with native E, but not E.T, complementing debilitated recombinant MHVs lacking E. Our findings suggest that palmitoylations are essential for E to act as a vesicle morphogenetic protein and further argue that palmitoylated E proteins operate by allowing the primary coronavirus assembly subunits to assume configurations that can mobilize into secreted lipid vesicles and virions.

Importance of conserved cysteine residues in the coronavirus envelope protein

Coronavirus envelope (E) proteins play an important, not fully understood role(s) in the virus life cycle. All E proteins have conserved cysteine residues located on the carboxy side of the long hydrophobic domain, suggesting functional significance. In this study, we confirmed that mouse hepatitis coronavirus A59 E protein is palmitoylated. To understand the role of the conserved residues and the necessity of palmitoylation, three cysteines at positions 40, 44, and 47 were changed singly and in various combinations to alanine. Double- and triple-mutant E proteins resulted in decreased virus-like particle output when coexpressed with the membrane (M) protein. Mutant E proteins were also studied in the context of a full-length infectious clone. Single-substitution viruses exhibited growth characteristics virtually identical to those of the wild-type virus, while the double-substitution mutations gave rise to viruses with less robust growth phenotypes indicated by smaller plaques and decreased virus yields. In contrast, replacement of all three cysteines resulted in crippled virus with significantly reduced yields. Triple-mutant viruses did not exhibit impairment in entry. Mutant E proteins localized properly in infected cells. A comparison of intracellular and extracellular virus yields suggested that release is only slightly impaired. E protein lacking all three cysteines exhibited an increased rate of degradation compared to that of the wild-type protein, suggesting that palmitoylation is important for the stability of the protein. Altogether, the results indicate that the conserved cysteines and presumably palmitoylation are functionally important for virus production.

Avian reovirus: Structure and biology

Avian reoviruses are important pathogens that cause considerable losses to the poultry industry, but they have been poorly characterized at the molecular level in the past, mostly because they have been considered to be very similar to the well-studied mammalian reoviruses. Studies performed over the last 20 years have revealed that avian reoviruses have unique properties and activities, different to those displayed by their mammalian counterparts, and of considerable interest to molecular virologists. Notably, the avian reovirus S1 gene is unique, in that it is a functional tricistronic gene that possesses three out-of-phase and partially overlapping open reading frames; the identification of the mechanisms that govern the initiation of translation of the three S1 cistrons, and the study of the properties and activities displayed by their encoded proteins, are particularly interesting areas of research. For instance, avian reoviruses are one of the few nonenveloped viruses that cause cell-cell fusion, and their fusogenic phenotype has been associated with a nonstructural 10 kDa transmembrane protein, which is expressed by the second cistron of the S1 gene; the small size of this atypical fusion protein offers an interesting model for studying the mechanisms of cell-cell fusion and for identifying fusogenic domains. Finally, avian reoviruses are highly resistant to interferon, and therefore they may be useful for investigating the mechanisms and strategies that viruses utilize to counteract the antiviral actions of interferons.

Early steps in avian reovirus morphogenesis

Avian reoviruses are important pathogens that may cause considerable economic losses in poultry farming. Their genome expresses at least eight structural and four nonstructural proteins, three of them encoded by the S1 gene. These viruses enter cells by receptor-mediated endocytosis, and acidification of virus-containing endosomes is necessary for the virus to uncoat and release transcriptionally active cores into the cytosol. Avian reoviruses replicate within cytoplasmic inclusions of globular morphology, termed viral factories, which are not microtubule-associated, and which are formed by the nonstructural protein muNS. This protein also mediates the association of some viral proteins (but not of others) with inclusions, suggesting that the recruitment of viral proteins into avian reovirus factories has specificity. Avian reovirus morphogenesis is a complex and temporally controlled process that takes place exclusively within viral factories of infected cells. Core assembly takes place within the first 30 min after the synthesis of their protein components, and fully formed cores are then coated by outer-capsid polypeptides over the next 30 min to generate mature infectious reovirions. Based on data from avian reovirus studies and on results reported for other members of the Reoviridae family, we present a model for avian reovirus gene expression and morphogenesis.

The p14 fusion-associated small transmembrane (FAST) protein effects membrane fusion from a subset of membrane microdomains

The reovirus fusion-associated small transmembrane (FAST) proteins are a unique family of viral membrane fusion proteins. These nonstructural viral proteins induce efficient cell-cell rather than virus-cell membrane fusion. We analyzed the lipid environment in which the reptilian reovirus p14 FAST protein resides to determine the influence of the cell membrane on the fusion activity of the FAST proteins. Topographical mapping of the surface of fusogenic p14-containing liposomes by atomic force microscopy under aqueous conditions revealed that p14 resides almost exclusively in thickened membrane microdomains. In transfected cells, p14 was found in both Lubrol WX- and Triton X-100-resistant membrane complexes. Cholesterol depletion of donor cell membranes led to preferential disruption of p14 association with Lubrol WX (but not Triton X-100)-resistant membranes and decreased cell-cell fusion activity, both of which were reversed upon subsequent cholesterol repletion. Furthermore, co-patching analysis by fluorescence microscopy indicated that p14 did not co-localize with classical lipid-anchored raft markers. These data suggest that the p14 FAST protein associates with heterogeneous membrane microdomains, a distinct subset of which is defined by cholesterol-dependent Lubrol WX resistance and which may be more relevant to the membrane fusion process.

Recycling of MUC1 is dependent on its palmitoylation

MUC1 is a mucin-like transmembrane protein expressed on the apical surface of epithelia, where it protects the cell surface. The cytoplasmic domain has numerous sites for phosphorylation and docking of proteins involved in signal transduction. In a previous study, we showed that the cytoplasmic YXXphi motif Y20HPM and the tyrosine-phosphorylated Y60TNP motif are required for MUC1 clathrin-mediated endocytosis through binding AP-2 and Grb2, respectively (Kinlough, C. L., Poland, P. A., Bruns, J. B., Harkleroad, K. L., and Hughey, R. P. (2004) J. Biol. Chem. 279, 53071-53077). Palmitoylation of transmembrane proteins can affect their membrane trafficking, and the MUC1 sequence CQC3RRK at the boundary of the transmembrane and cytoplasmic domains mimics reported site(s) of S-palmitoylation. [3H]Palmitate labeling of Chinese hamster ovary cells expressing MUC1 with mutations in CQC3RRK revealed that MUC1 is dually palmitoylated at the CQC motif independent of RRK. Lack of palmitoylation did not affect the cold detergent solubility profile of a chimera (Tac ectodomain and MUC1 transmembrane and cytoplasmic domains), the rate of chimera delivery to the cell surface, or its half-life. Calculation of rate constants for membrane trafficking of wild-type and mutant Tac-MUC1 indicated that the lack of palmitoylation blocked recycling, but not endocytosis, and caused the chimera to accumulate in a EGFP-Rab11-positive endosomal compartment. Mutations CQC/AQA and Y20N inhibited Tac-MUC1 co-immunoprecipitation with AP-1, although mutant Y20N had reduced rates of both endocytosis and recycling, but a normal subcellular distribution. The double mutant chimera AQA+Y20N had reduced endocytosis and recycling rates and accumulated in EGFP-Rab11-positive endosomes, indicating that palmitoylation is the dominant feature modulating MUC1 recycling from endosomes back to the plasma membrane.

Liposome reconstitution of a minimal protein-mediated membrane fusion machine

Biological membrane fusion is dependent on protein catalysts to mediate localized restructuring of lipid bilayers. A central theme in current models of protein-mediated membrane fusion involves the sequential refolding of complex homomeric or heteromeric protein fusion machines. The structural features of a new family of fusion-associated small transmembrane (FAST) proteins appear incompatible with existing models of membrane fusion protein function. While the FAST proteins function to induce efficient cell-cell fusion when expressed in transfected cells, it was unclear whether they function on their own to mediate membrane fusion or are dependent on cellular protein cofactors. Using proteoliposomes containing the purified p14 FAST protein of reptilian reovirus, we now show via liposome-cell and liposome-liposome fusion assays that p14 is both necessary and sufficient for membrane fusion. Stoichiometric and kinetic analyses suggest that the relative efficiency of p14-mediated membrane fusion rivals that of the more complex cellular and viral fusion proteins, making the FAST proteins the simplest known membrane fusion machines.

Unusual topological arrangement of structural motifs in the baboon reovirus fusion-associated small transmembrane protein

Select members of the Reoviridae are the only nonenveloped viruses known to induce syncytium formation. The fusogenic orthoreoviruses accomplish cell-cell fusion through a distinct class of membrane fusion-inducing proteins referred to as the fusion-associated small transmembrane (FAST) proteins. The p15 membrane fusion protein of baboon reovirus is unique among the FAST proteins in that it contains two hydrophobic regions (H1 and H2) recognized as potential transmembrane (TM) domains, suggesting a polytopic topology. However, detailed topological analysis of p15 indicated only the H1 domain is membrane spanning. In the absence of an N-terminal signal peptide, the H1 TM domain serves as a reverse signal-anchor to direct p15 membrane insertion and a bitopic N(exoplasmic)/C(cytoplasmic) topology. This topology results in the translocation of the smallest ectodomain ( approximately 20 residues) of any known viral fusion protein, with the majority of p15 positioned on the cytosolic side of the membrane. Mutagenic analysis indicated the unusual presence of an N-terminal myristic acid on the small p15 ectodomain is essential to the fusion process. Furthermore, the only other hydrophobic region (H2) present in p15, aside from the TM domain, is located within the endodomain. Consequently, the p15 ectodomain is devoid of a fusion peptide motif, a hallmark feature of membrane fusion proteins. The exceedingly small, myristoylated ectodomain and the unusual topological distribution of structural motifs in this nonenveloped virus membrane fusion protein necessitate alternate models of protein-mediated membrane fusion.

Role of the specific amino acid sequence of the membrane-spanning domain of human immunodeficiency virus type 1 in membrane fusion

Fusion between cell and virus membranes mediated by gp41 initiates the life cycle of human immunodeficiency virus type 1. In contrast to the many studies that have elucidated the structure-function relationship of the ectodomain, the study of the membrane-spanning domain (MSD) has been rather limited. In particular, the role that the MSD's specific amino acid sequences may have in membrane fusion as well as other gp41 functions is not well understood. The MSD of gp41 contains well-conserved glycine residues that form the GXXXG motif (G, glycine; X, other amino acid residues), a motif often found at the helix-helix interface of membrane spanning alpha-helices. Here we examined the role that the specific amino acid sequence of the gp41 MSD has in gp41 function, particularly in membrane fusion, by making two types of MSD mutants: (i) glycine substitution mutants in which glycine residues of the MSD were mutated to alanine or leucine residues, and (ii) replacement mutants in which the entire MSD was replaced with one derived from glycophorin A or from vesicular stomatitis virus G. The substitution of glycines did not affect gp41 function. MSD-replacement mutants, however, showed severely impaired fusion activity. The assay using the Env expression vector revealed defects in membrane fusion after CD4 binding steps in the MSD-replacement mutants. In addition, the change in Env processing was noted for MSD-replacement mutants. These results suggest that the MSD of gp41 has a relatively wide but not unlimited tolerance for mutations and plays a critical role in membrane fusion as well as in other steps of Env biogenesis.

De novo design of conformationally flexible transmembrane peptides driving membrane fusion

Fusion of biological membranes is mediated by distinct integral membrane proteins, e.g., soluble N-ethylmaleimide-sensitive factor attachment protein receptors and viral fusion proteins. Previous work has indicated that the transmembrane segments (TMSs) of such integral membrane proteins play an important role in fusion. Furthermore, peptide mimics of the transmembrane part can drive the fusion of liposomes, and evidence had been obtained that fusogenicity depends on their conformational flexibility. To test this hypothesis, we present a series of unnatural TMSs that were designed de novo based on the structural properties of hydrophobic residues. We find that the fusogenicity of these peptides depends on the ratio of alpha-helix-promoting Leu and beta-sheet-promoting Val residues and is enhanced by helix-destabilizing Pro and Gly residues within their hydrophobic cores. The ability of these peptides to refold from an alpha-helical state to a beta-sheet conformation and backwards was determined under different conditions. Membrane fusogenic peptides with mixed Leu/Val sequences tend to switch more readily between different conformations than a nonfusogenic peptide with an oligo-Leu core. We propose that structural flexibility of these TMSs is a prerequisite of fusogenicity.

Cell-cell fusion induced by the avian reovirus membrane fusion protein is regulated by protein degradation

The p10 fusion-associated small transmembrane protein of avian reovirus induces extensive syncytium formation in transfected cells. Here we show that p10-induced cell-cell fusion is restricted by rapid degradation of the majority of newly synthesized p10. The small ectodomain of p10 targets the protein for degradation following p10 insertion into an early membrane compartment. Paradoxically, conservative amino acid substitutions in the p10 ectodomain hydrophobic patch that eliminate fusion activity also increase p10 stability. The small amount of p10 that escapes intracellular degradation accumulates at the cell surface in a relatively stable form, where it mediates cell-cell fusion as a late-stage event in the virus replication cycle. The unusual relationship between a nonstructural viral membrane fusion protein and the replication cycle of a nonenveloped virus has apparently contributed to the evolution of a novel mechanism for restricting the extent of virus-induced cell-cell fusion.

Reptilian reovirus utilizes a small type III protein with an external myristylated amino terminus to mediate cell-cell fusion

Reptilian reovirus is one of a limited number of nonenveloped viruses that are capable of inducing cell-cell fusion. A small, hydrophobic, basic, 125-amino-acid fusion protein encoded by the first open reading frame of a bicistronic viral mRNA is responsible for this fusion activity. Sequence comparisons to previously characterized reovirus fusion proteins indicated that p14 represents a new member of the fusion-associated small transmembrane (FAST) protein family. Topological analysis revealed that p14 is a representative of a minor subset of integral membrane proteins, the type III proteins N(exoplasmic)/C(cytoplasmic) (N(exo)/C(cyt)), that lack a cleavable signal sequence and use an internal reverse signal-anchor sequence to direct membrane insertion and protein topology. This topology results in the unexpected, cotranslational translocation of the essential myristylated N-terminal domain of p14 across the cell membrane. The topology and structural motifs present in this novel reovirus membrane fusion protein further accentuate the diversity and unusual properties of the FAST protein family and clearly indicate that the FAST proteins represent a third distinct class of viral membrane fusion proteins.

Structural and functional properties of an unusual internal fusion peptide in a nonenveloped virus membrane fusion protein

The avian and Nelson Bay reoviruses are two of only a limited number of nonenveloped viruses capable of inducing cell-cell membrane fusion. These viruses encode the smallest known membrane fusion proteins (p10). We now show that a region of moderate hydrophobicity we call the hydrophobic patch (HP), present in the small N-terminal ectodomain of p10, shares the following characteristics with the fusion peptides of enveloped virus fusion proteins: (i) an abundance of glycine and alanine residues, (ii) a potential amphipathic secondary structure, (iii) membrane-seeking characteristics that correspond to the degree of hydrophobicity, and (iv) the ability to induce lipid mixing in a liposome fusion assay. The p10 HP is therefore predicted to provide a function in the mechanism of membrane fusion similar to those of the fusion peptides of enveloped virus fusion peptides, namely, association with and destabilization of opposing lipid bilayers. Mutational and biophysical analysis suggested that the internal fusion peptide of p10 lacks alpha-helical content and exists as a disulfide-stabilized loop structure. Similar kinked structures have been reported in the fusion peptides of several enveloped virus fusion proteins. The preservation of a predicted loop structure in the fusion peptide of this unusual nonenveloped virus membrane fusion protein supports an imperative role for a kinked fusion peptide motif in biological membrane fusion.