Liposome reconstitution of a minimal protein-mediated membrane fusion machine

Rab11 Binding Promotes the p14 FAST Protein-Induced Syncytium Formation

Reptile reoviruses encode the p14 fusion-associated small transmembrane (FAST) protein, which induces cell-cell membrane fusion as a nonstructural protein. When the virus enters the host cell, the p14 protein is encoded, synthesized, and delivered to the plasma membrane via the endoplasmic reticulum-Golgi transport system. During this process, the polybasic motif (PBM) at the proximal membrane terminal of the p14 cytosolic endodomain interacts with Rab11 on the Golgi. This interaction places p14 into vesicles enclosed by the AP-1 adaptor, transporting it to the plasma membrane and causing membrane fusion. In this study, we used the surface plasmon resonance principle to confirm that p141-69 had a substantial affinity for Rab11 at the membrane, and we also proved at the cellular level that Rab11 directly increased p14-induced syncytium formation and improved membrane fusion efficiency. We also found preliminary evidence that p141-69 could act as a fusion peptide to trigger liposome-cell fusion.

Unleashing the power of nucleic acid therapeutics through efficient cytosolic delivery

The approval of siRNA-based therapy for liver disease in 2018 and the subsequent success of mRNA-based SARS-CoV-2 vaccines have inaugurated a new era in nucleic acid-based therapeutics. These breakthroughs underscore the transformative potential of nucleic acid-based therapeutics, which modulate gene function, correct genetic defects, or disrupt pathological molecular processes. Such advances represent a paradigm shift in modern medicine. Despite their immense promise, the clinical realization of nucleic acid-based therapies is fundamentally constrained by endosomal entrapment, a critical barrier that significantly limits therapeutic efficacy. Overcoming this obstacle is imperative to fully unlock the potential of these therapies. Designing effective strategies to facilitate the escape of nucleic acids from endosomes-or bypassing endosomal pathways altogether-remains a central challenge in the field. In this review, we provide a comprehensive and critical analysis of current approaches aimed at enhancing endosomal escape or circumventing endosomal entrapment. By highlighting both the successes and limitations of these strategies, we aim to offer valuable insights to inform the development of more efficient and clinically viable nucleic acid delivery systems, advancing the future of molecular medicine.

Safe and effective in vivo delivery of DNA and RNA using proteolipid vehicles

Genetic medicines show promise for treating various diseases, yet clinical success has been limited by tolerability, scalability, and immunogenicity issues of current delivery platforms. To overcome these, we developed a proteolipid vehicle (PLV) by combining features from viral and non-viral approaches. PLVs incorporate fusion-associated small transmembrane (FAST) proteins isolated from fusogenic orthoreoviruses into a well-tolerated lipid formulation, using scalable microfluidic mixing. Screening a FAST protein library, we identified a chimeric FAST protein with enhanced membrane fusion activity that improved gene expression from an optimized lipid formulation. Systemically administered FAST-PLVs showed broad biodistribution and effective mRNA and DNA delivery in mouse and non-human primate models. FAST-PLVs show low immunogenicity and maintain activity upon repeat dosing. Systemic administration of follistatin DNA gene therapy with FAST-PLVs raised circulating follistatin levels and significantly increased muscle mass and grip strength. These results demonstrate the promising potential of FAST-PLVs for redosable gene therapies and genetic medicines.

Fusogenic vesicular stomatitis virus combined with natural killer T cell immunotherapy controls metastatic breast cancer

BACKGROUND

Metastatic breast cancer is a leading cause of cancer death in woman. Current treatment options are often associated with adverse side effects and poor outcomes, demonstrating the need for effective new treatments. Immunotherapies can provide durable outcomes in many cancers; however, limited success has been achieved in metastatic triple negative breast cancer. We tested whether combining different immunotherapies can target metastatic triple negative breast cancer in pre-clinical models.

METHODS

Using primary and metastatic 4T1 triple negative mammary carcinoma models, we examined the therapeutic effects of oncolytic vesicular stomatitis virus (VSVΔM51) engineered to express reovirus-derived fusion associated small transmembrane proteins p14 (VSV-p14) or p15 (VSV-p15). These viruses were delivered alone or in combination with natural killer T (NKT) cell activation therapy mediated by adoptive transfer of α-galactosylceramide-loaded dendritic cells.

RESULTS

Treatment of primary 4T1 tumors with VSV-p14 or VSV-p15 alone increased immunogenic tumor cell death, attenuated tumor growth, and enhanced immune cell infiltration and activation compared to control oncolytic virus (VSV-GFP) treatments and untreated mice. When combined with NKT cell activation therapy, oncolytic VSV-p14 and VSV-p15 reduced metastatic lung burden to undetectable levels in all mice and generated immune memory as evidenced by enhanced in vitro recall responses (tumor killing and cytokine production) and impaired tumor growth upon rechallenge.

CONCLUSION

Combining NKT cell immunotherapy with enhanced oncolytic virotherapy increased anti-tumor immune targeting of lung metastasis and presents a promising treatment strategy for metastatic breast cancer.

Entry of Enveloped Viruses into Host Cells: Membrane Fusion

Viruses are intracellular parasites that hijack the cellular machinery for their own replication. Therefore, an obligatory step in the virus life cycle is the delivery of the viral genome inside the cell. Enveloped viruses (i.e., viruses with a lipid envelope) use a two-step procedure to release their genetic material into the cell: (1) they first bind to specific surface receptors of the target cell membrane and then (2) they fuse the viral and cell membranes. This last step may occur at the cell surface or after internalization of the virus particle by endocytosis or by some other route (e.g., macropinocytosis). Remarkably, the virus-cell membrane fusion process goes essentially along the same intermediate steps than other membrane fusions that occur, for instance, in vesicular fusion at the nerve synapsis or cell-cell fusion in yeast mating. Specialized viral proteins, fusogens, promote virus-cell membrane fusion. The viral fusogens experience drastic structural rearrangements during fusion, releasing the energy required to overcome the repulsive forces that prevent spontaneous fusion of the two membranes. This chapter provides an overview of the different types of viral fusogens and their mode of action, as they are currently known. Furthermore, it outlines novel strategies for vaccine development related to stabilized viral fusogens.

Nonenveloped Avian Reoviruses Released with Small Extracellular Vesicles Are Highly Infectious

Vesicle-encapsulated nonenveloped viruses are a recently recognized alternate form of nonenveloped viruses that can avoid immune detection and potentially increase systemic transmission. Avian orthoreoviruses (ARVs) are the leading cause of various disease conditions among birds and poultry. However, whether ARVs use cellular vesicle trafficking routes for egress and cell-to-cell transmission is still poorly understood. We demonstrated that fusogenic ARV-infected quail cells generated small (~100 nm diameter) extracellular vesicles (EVs) that contained electron-dense material when observed by transmission electron microscope. Cryo-EM tomography indicated that these vesicles did not contain ARV virions or core particles, but the EV fractions of OptiPrep gradients did contain a small percent of the ARV virions released from cells. Western blotting of detergent-treated EVs revealed that soluble virus proteins and the fusogenic p10 FAST protein were contained within the EVs. Notably, virus particles mixed with the EVs were up to 50 times more infectious than virions alone. These results suggest that EVs and perhaps fusogenic FAST-EVs could contribute to ARV virulence.

The nonstructural p17 protein of a fusogenic bat-borne reovirus regulates viral replication in virus species- and host-specific manners

Nelson Bay orthoreovirus (NBV), a member of the family Reoviridae, genus Orthoreovirus, is a bat-borne virus that causes respiratory diseases in humans. NBV encodes two unique nonstructural proteins, fusion-associated small transmembrane (FAST) protein and p17 protein, in the S1 gene segment. FAST induces cell–cell fusion between infected cells and neighboring cells and the fusogenic activity is required for efficient viral replication. However, the function of p17 in the virus cycle is not fully understood. Here, various p17 mutant viruses including p17-deficient viruses were generated by a reverse genetics system for NBV. The results demonstrated that p17 is not essential for viral replication and does not play an important role in viral pathogenesis. On the other hand, NBV p17 regulated viral replication in a bat cell line but not in other human and animal cell lines. Nuclear localization of p17 is associated with the regulation of NBV replication in bat cells. We also found that p17 dramatically enhances the cell–cell fusion activity of NBV FAST protein for efficient replication in bat cells. Furthermore, we found that a protein homologue of NBV p17 from another bat-borne orthoreovirus, but not those of avian orthoreovirus or baboon orthoreovirus, also supported efficient viral replication in bat cells using a p17-deficient virus-based complementation approach. These results provide critical insights into the functioning of the unique replication machinery of bat-borne viruses in their natural hosts.

Bat-borne viruses including the severe acute respiratory syndrome coronavirus and Nipah virus generally cause highly pathogenic diseases in humans but not in their bat reservoirs. Nelson Bay orthoreovirus (NBV), a bat-borne virus associated with acute respiratory tract infections in humans, possesses two unique nonstructural proteins, FAST and p17. FAST enhances viral replication through its cell–cell fusion activity, while the function of p17 in the viral life cycle is poorly understood. In this study, we show that p17 is non-essential for viral replication in several human and animal cell lines and does not play a critical role in pathogenesis in vivo. However, p17 localizes to the nucleus and regulates viral replication specifically in cells derived from bats by enhancing the cell–cell fusion activity of FAST in a host-specific manner. Furthermore, the expression of NBV p17 or an NBV p17 homologue from another bat-borne orthoreovirus enhanced the replication of an NBV mutant deficient in p17 in bat cells, suggesting that the function of p17 is virus species-specific. These findings will contribute to our understanding of how the replication of viruses is regulated in their natural reservoirs.

Evolutionarily related small viral fusogens hijack distinct but modular actin nucleation pathways to drive cell-cell fusion

Fusion-associated small transmembrane (FAST) proteins are a diverse family of nonstructural viral proteins. Once expressed on the plasma membrane of infected cells, they drive fusion with neighboring cells, increasing viral spread and pathogenicity. Unlike viral fusogens with tall ectodomains that pull two membranes together through conformational changes, FAST proteins have short fusogenic ectodomains that cannot bridge the intermembrane gap between neighboring cells. One orthoreovirus FAST protein, p14, has been shown to hijack the actin cytoskeleton to drive cell-cell fusion, but the actin adaptor-binding motif identified in p14 is not found in any other FAST protein. Here, we report that an evolutionarily divergent FAST protein, p22 from aquareovirus, also hijacks the actin cytoskeleton but does so through different adaptor proteins, Intersectin-1 and Cdc42, that trigger N-WASP-mediated branched actin assembly. We show that despite using different pathways, the cytoplasmic tail of p22 can replace that of p14 to create a potent chimeric fusogen, suggesting they are modular and play similar functional roles. When we directly couple p22 with the parallel filament nucleator formin instead of the branched actin nucleation promoting factor N-WASP, its ability to drive fusion is maintained, suggesting that localized mechanical pressure on the plasma membrane coupled to a membrane-disruptive ectodomain is sufficient to drive cell-cell fusion. This work points to a common biophysical strategy used by FAST proteins to push rather than pull membranes together to drive fusion, one that may be harnessed by other short fusogens responsible for physiological cell-cell fusion.

A Canadian perspective on severe acute respiratory syndrome coronavirus 2 infection and treatment: how prevalent underlying inflammatory disease contributes to pathogenesis

The coronavirus disease 2019 (COVID-19), a serious respiratory illness caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has emerged as a global pandemic. Canada reported its first case of COVID-19 on the 25th January 2020. By March 2020, the virus had spread within Canadian communities reaching the most frail and vulnerable elderly population in long-term care facilities. The majority of cases were reported in the provinces of Quebec, Ontario, Alberta, and British Columbia, and the highest mortality was seen among individuals aged 65 years or older. Canada has the highest prevalence and incidence rates of several chronic inflammatory diseases, such as multiple sclerosis, inflammatory bowel disease, and Parkinson's disease. Many elderly Canadians also live with comorbid medical illnesses, such as hypertension, diabetes, cardiovascular disease, and chronic lung disease, and are more likely to suffer from severe COVID-19 with a poor prognosis. It is becoming increasingly evident that underlying inflammatory disease contributes to the pathogenesis of SARS-CoV-2. Here, we review the mechanisms behind SARS-CoV-2 infection, and the host inflammatory responses that lead to resolution or progression to severe COVID-19 disease. Furthermore, we discuss the landscape of COVID-19 therapeutics that are currently in development in Canada.

A viral fusogen hijacks the actin cytoskeleton to drive cell-cell fusion

Cell-cell fusion, which is essential for tissue development and used by some viruses to form pathological syncytia, is typically driven by fusogenic membrane proteins with tall (>10 nm) ectodomains that undergo conformational changes to bring apposing membranes in close contact prior to fusion. Here we report that a viral fusogen with a short (<2 nm) ectodomain, the reptilian orthoreovirus p14, accomplishes the same task by hijacking the actin cytoskeleton. We show that phosphorylation of the cytoplasmic domain of p14 triggers N-WASP-mediated assembly of a branched actin network. Using p14 mutants, we demonstrate that fusion is abrogated when binding of an adaptor protein is prevented and that direct coupling of the fusogenic ectodomain to branched actin assembly is sufficient to drive cell-cell fusion. This work reveals how the actin cytoskeleton can be harnessed to overcome energetic barriers to cell-cell fusion.

Intracellular Vesicle Fusion Requires a Membrane-Destabilizing Peptide Located at the Juxtamembrane Region of the v-SNARE

Intracellular vesicle fusion is mediated by soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) and Sec1/Munc18 (SM) proteins. It is generally accepted that membrane fusion occurs when the vesicle and target membranes are brought into close proximity by SNAREs and SM proteins. In this work, we demonstrate that, for fusion to occur, membrane bilayers must be destabilized by a conserved membrane-embedded motif located at the juxtamembrane region of the vesicle-anchored v-SNARE. Comprised of basic and hydrophobic residues, the juxtamembrane motif perturbs the lipid bilayer structure and promotes SNARE-SM-mediated membrane fusion. The juxtamembrane motif can be functionally substituted with an unrelated membrane-disrupting peptide in the membrane fusion reaction. These findings establish the juxtamembrane motif of the v-SNARE as a membrane-destabilizing peptide. Requirement of membrane-destabilizing peptides is likely a common feature of biological membrane fusion.

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.

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.

Efficient reovirus- and measles virus-mediated pore expansion during syncytium formation is dependent on annexin A1 and intracellular calcium

Orthoreovirus fusion-associated small transmembrane (FAST) proteins are dedicated cell-cell fusogens responsible for multinucleated syncytium formation and are virulence determinants of the fusogenic reoviruses. While numerous studies on the FAST proteins and enveloped-virus fusogens have delineated steps involved in membrane fusion and pore formation, little is known about the mechanics of pore expansion needed for syncytiogenesis. We now report that RNA interference (RNAi) knockdown of annexin A1 (AX1) expression dramatically reduced both reptilian reovirus p14 and measles virus F and H protein-mediated pore expansion during syncytiogenesis but had no effect on pore formation. A similar effect was obtained by chelating intracellular calcium, which dramatically decreased syncytiogenesis in the absence of detectable effects on p14-induced pore formation. Coimmunoprecipitation revealed calcium-dependent interaction between AX1 and p14 or measles virus F and H proteins, and fluorescence resonance energy transfer (FRET) demonstrated calcium-dependent p14-AX1 interactions in cellulo. Furthermore, antibody inhibition of extracellular AX1 had no effect on p14-induced syncytium formation but did impair cell-cell fusion mediated by the endogenous muscle cell fusion machinery in C2C12 mouse myoblasts. AX1 can therefore exert diverse, fusogen-specific effects on cell-cell fusion, functioning as an extracellular mediator of differentiation-dependent membrane fusion or as an intracellular promoter of postfusion pore expansion and syncytium formation following virus-mediated cell-cell fusion.

IMPORTANCE

Numerous enveloped viruses and nonenveloped fusogenic orthoreoviruses encode membrane fusion proteins that induce syncytium formation, which has been linked to viral pathogenicity. Considerable insights into the mechanisms of membrane fusion have been obtained, but processes that drive postfusion expansion of fusion pores to generate syncytia are poorly understood. This study identifies intracellular calcium and annexin A1 (AX1) as key factors required for efficient pore expansion during syncytium formation mediated by the reptilian reovirus p14 and measles virus F and H fusion protein complexes. Involvement of intracellular AX1 in syncytiogenesis directly correlates with a requirement for intracellular calcium in p14-AX1 interactions and pore expansion but not membrane fusion and pore formation. This is the first demonstration that intracellular AX1 is involved in pore expansion, which suggests that the AX1 pathway may be a common host cell response needed to resolve virus-induced cell-cell fusion pores.

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.

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.

Entry of enveloped viruses into host cells: membrane fusion

Viruses are intracellular parasites that hijack the cellular machinery for their own replication. Therefore, an obligatory step in the virus life cycle is the delivery of the viral genome inside the cell. Enveloped viruses (i.e., viruses with a lipid envelope) use a two-step procedure to release their genetic material into the cell: (i) they first bind to specific surface receptors of the target cell membrane and then, (ii) they fuse the viral and cell membranes. This last step may occur at the cell surface or after internalization of the virus particle by endocytosis or by some other route (e.g., macropinocytosis). Remarkably, the virus-cell membrane fusion process goes essentially along the same intermediate steps as other membrane fusions that occur for instance in vesicular fusion at the nerve synapsis or cell-cell fusion in yeast mating. Specialized viral proteins, fusogens, promote virus-cell membrane fusion. The viral fusogens experience drastic structural rearrangements during fusion, liberating the energy required to overcome the repulsive forces that prevent spontaneous fusion of the two membranes. This chapter describes the different types of viral fusogens and their mode of action, as are currently known.

Biophysical and functional assays for viral membrane fusion peptides

Membrane fusion is a protein catalyzed biophysical reaction that involves the simultaneous intermixing of two phospholipid bilayers and of the aqueous compartments bound by their respective bilayers. In the case of enveloped virus fusogens, short hydrophobic or amphipathic fusion peptides that are components of the larger fusion complex are essential for the membrane merger event. The process of cell-cell membrane fusion and syncytium formation induced by the nonenveloped fusogenic orthoreoviruses is driven by the Fusion-Associated Small Transmembrane (FAST) proteins, which are similarly dependent on the action of fusion peptides. In this article, we describe some simple methods for the biophysical characterization of viral membrane fusion peptides. Liposomes serve as an ideal model system for characterizing peptide-membrane interactions because their size, shape and composition can be readily manipulated. We present details of fluorescence assays used to elucidate the kinetics of membrane fusion as well as complimentary assays used to characterize peptide-induced liposome binding and aggregation.

Delivery of intracellular-acting biologics in pro-apoptotic therapies

The recent elucidation of molecular regulators of apoptosis and their roles in cellular oncogenesis has motivated the development of biomacromolecular anticancer therapeutics that can activate intracellular apoptotic signaling pathways. Pharmaceutical scientists have employed a variety of classes of biologics toward this goal, including antisense oligodeoxynucleotides, small interfering RNA, proteins, antibodies, and peptides. However, stability in the in vivo environment, tumor-specific biodistribution, cell internalization, and localization to the intracellular microenvironment where the targeted molecule is localized pose significant challenges that limit the ability to directly apply intracellular-acting, pro-apoptotic biologics for therapeutic use. Thus, approaches to improve the pharmaceutical properties of therapeutic biomacromolecules are of great significance and have included chemically modifying the bioactive molecule itself or formulation with auxiliary compounds. Recently, promising advances in delivery of pro-apoptotic biomacromolecular agents have been made using tools such as peptide "stapling", cell penetrating peptides, fusogenic peptides, liposomes, nanoparticles, smart polymers, and synergistic combinations of these components. This review will discuss the molecular mediators of cellular apoptosis, the respective mechanisms by which these mediators are dysregulated in cellular oncogenesis, the history and development of both nucleic-acid and amino-acid based drugs, and techniques to achieve intracellular delivery of these biologics. Finally, recent applications where pro-apoptotic functionality has been achieved through delivery of intracellular-acting biomacromolecular drugs will be highlighted.

Membrane perturbation elicits an IRF3-dependent, interferon-independent antiviral response

We previously found that enveloped virus binding and penetration are necessary to initiate an interferon-independent, IRF3-mediated antiviral response. To investigate whether membrane perturbations that accompany membrane fusion-dependent enveloped-virus entry are necessary and sufficient for antiviral-state induction, we utilized a reovirus fusion-associated small transmembrane (FAST) protein. Membrane disturbances during FAST protein-mediated fusion, in the absence of additional innate immune response triggers, are sufficient to elicit interferon-stimulated gene induction and establishment of an antiviral state. Using sensors of membrane disruption to activate an IRF3-dependent, interferon-independent antiviral state may provide cells with a rapid, broad-spectrum innate immune response to enveloped-virus infections.

Role of temperature-independent lipoplex-cell membrane interactions in the efficiency boost of multicomponent lipoplexes

Multicomponent lipoplexes have recently emerged as especially promising transfection candidates, as they are from 10 to 100 times more efficient than binary complexes usually employed for gene delivery purposes. Previously, we investigated a number of chemical-physical properties of DNA-lipid complexes that were proposed to affect transfection efficiency (TE) of lipoplexes, such as nanoscale structure, size, surface potential, DNA-protection ability and DNA release from complexes upon interaction with cellular lipids. Although some minor differences between multicomponent and binary lipoplexes were found, they did not correlate clearly with efficiency. Instead, here we show that a marked difference between the cell internalization mechanism of binary and multicomponent lipoplexes does exist. Multicomponent lipoplexes significantly transfect cells at 4 °C, when endocytosis does not take place suggesting that they can enter cells via a temperature-independent mechanism. Confocal fluorescence microscopy experiments showed the existence of a correlation between endosomal escape and TE. Multicomponent lipoplexes exhibited a distinctive ability of endosomal escape and release DNA into the nucleus, whereas, poorly efficient binary lipoplexes exhibited minor, if any, endosomal rupture ability and remained confined in perinuclear late endosomes. Stopped-flow mixing measurements showed that the fusion rates of multicomponent cationic liposomes with anionic vesicles, used as model systems of cell membranes, were definitely shorter than those of binary liposomes. As either lipoplex uptake and endosomal escape involve fusion between lipoplex and cellular membranes, we suggest that a mechanism of lipoplex-cellular membrane interaction, driven by lipid mixing between cationic and anionic cellular lipids, does explain the TE boost of multicomponent lipoplexes.

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.

Direct simulation of protein-mediated vesicle fusion: lung surfactant protein B

We simulated spontaneous fusion of small unilamellar vesicles mediated by lung surfactant protein B (SP-B) using the MARTINI force field. An SP-B monomer triggers fusion events by anchoring two vesicles and facilitating the formation of a lipid bridge between the proximal leaflets. Once a lipid bridge is formed, fusion proceeds via a previously described stalk - hemifusion diaphragm - pore-opening pathway. In the absence of protein, fusion of vesicles was not observed in either unbiased simulations or upon application of a restraining potential to maintain the vesicles in close proximity. The shape of SP-B appears to enable it to bind to two vesicles at once, forcing their proximity, and to facilitate the initial transfer of lipids to form a high-energy hemifusion intermediate. Our results may provide insight into more general mechanisms of protein-mediated membrane fusion, and a possible role of SP-B in the secretory pathway and transfer of lung surfactant to the gas exchange interface.

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.

Membrane fusion triggers rapid degradation of two gamete-specific, fusion-essential proteins in a membrane block to polygamy in Chlamydomonas

The plasma membranes of gametes are specialized for fusion, yet, once fusion occurs, in many organisms the new zygote becomes incapable of further membrane fusion reactions. The molecular mechanisms that underlie this loss of fusion capacity (block to polygamy) remain unknown. During fertilization in the green alga Chlamydomonas, the plus gamete-specific membrane protein FUS1 is required for adhesion between the apically localized sites on the plasma membranes of plus and minus gametes that are specialized for fusion, and the minus-specific membrane protein HAP2 is essential for completion of the membrane fusion reaction. HAP2 (GCS1) family members are also required for fertilization in Arabidopsis, and for the membrane fusion reaction in the malaria organism Plasmodium berghei. Here, we tested whether Chlamydomonas gamete fusion triggers alterations in FUS1 and HAP2 and renders the plasma membranes of the cells incapable of subsequent fusion. We find that, even though the fusogenic sites support multi-cell adhesions, triploid zygotes are rare, indicating a fusion-triggered block to the membrane fusion reaction. Consistent with the extinction of fusogenic capacity, both FUS1 and HAP2 are degraded upon fusion. The rapid, fusion-triggered cleavage of HAP2 in zygotes is distinct from degradation occurring during constitutive turnover in gametes. Thus, gamete fusion triggers specific degradation of fusion-essential proteins and renders the zygote incapable of fusion. Our results provide the first molecular explanation for a membrane block to polygamy in any organism.

Multifaceted sequence-dependent and -independent roles for reovirus FAST protein cytoplasmic tails in fusion pore formation and syncytiogenesis

Fusogenic reoviruses utilize the FAST proteins, a novel family of nonstructural viral membrane fusion proteins, to induce cell-cell fusion and syncytium formation. Unlike the paradigmatic enveloped virus fusion proteins, the FAST proteins position the majority of their mass within and internal to the membrane in which they reside, resulting in extended C-terminal cytoplasmic tails (CTs). Using tail truncations, we demonstrate that the last 8 residues of the 36-residue CT of the avian reovirus p10 FAST protein and the last 20 residues of the 68-residue CT of the reptilian reovirus p14 FAST protein enhance, but are not required for, pore expansion and syncytium formation. Further truncations indicate that the membrane-distal 12 residues of the p10 and 47 residues of the p14 CTs are essential for pore formation and that a residual tail of 21 to 24 residues that includes a conserved, membrane-proximal polybasic region present in all FAST proteins is insufficient to maintain FAST protein fusion activity. Unexpectedly, a reextension of the tail-truncated, nonfusogenic p10 and p14 constructs with scrambled versions of the deleted sequences restored pore formation and syncytiogenesis, while reextensions with heterologous sequences partially restored pore formation but failed to rescue syncytiogenesis. The membrane-distal regions of the FAST protein CTs therefore exert multiple effects on the membrane fusion reaction, serving in both sequence-dependent and sequence-independent manners as positive effectors of pore formation, pore expansion, and syncytiogenesis.

Intracellular delivery of bovine lactoferricin's antimicrobial core (RRWQWR) kills T-leukemia cells

Bovine lactoferricin (LfcinB) is a cationic antimicrobial peptide with potent cytotoxic activity against cancer cells. The antimicrobial activity of LfcinB resides in its RRWQWR amino acid sequence (referred to here as LfcinB6); however, the anticancer activity of LfcinB6 is not known. Here, we show that free LfcinB6 did not kill T-leukemia or breast cancer cells but LfcinB6 was strongly cytotoxic when delivered to the cytosolic compartment by fusogenic liposomes. LfcinB6 bound weakly to isolated mitochondria but, unlike LfcinB, did not permeabilize mitochondria or cause cytochrome c to be released. Cathepsin B and caspase activity were important for cytotoxicity caused by intracellular LfcinB6 whereas reactive oxygen species were not involved. The mechanism of LfcinB6-induced cytotoxicity is therefore different from that of LfcinB. We suggest that LfcinB6, in combination with a fusogenic liposome delivery system that selectively targets malignant cells, has potential as a novel anticancer agent.

Physico-chemical characteristics of lipoplexes influence cell uptake mechanisms and transfection efficacy

Background

Formulation of DNA/cationic lipid complexes (lipoplexes) designed for nucleic acid delivery mostly results in positively charged particles which are thought to enter cells by endocytosis. We recently developed a lipoplex formulation called Neutraplex that allows preparation of both cationic and anionic stable complexes with similar lipid content and ultrastructure.

Methodology/Principal Findings

To assess whether the global net charge could influence cell uptake and activity of the transported oligonucleotides (ON), we prepared lipoplexes with positive and negative charges and compared: (i) their physicochemical properties by zeta potential analysis and dynamic light scattering, (ii) their cell uptake by fluorescence microscopy and flow cytometry, and (iii) the biological activity of the transported ON using a splicing correction assay. We show that positively or negatively charged lipoplexes enter cells cells using both temperature-dependent and -independent uptake mechanisms. Specifically, positively charged lipoplexes predominantly use a temperature-dependent transport when cells are incubated OptiMEM medium. Anionic lipoplexes favour an energy-independent transport and show higher ON activity than cationic lipoplexes in presence of serum. However, lipoplexes with high positive global net charge and OptiMEM medium give the highest uptake and ON activity levels.

Conclusions

These findings suggest that, in addition to endocytosis, lipoplexes may enter cell via a temperature-independent mechanism, which could be mediated by lipid mixing. Such characteristics might arise from the specific lipoplex ultrastructure and should be taken into consideration when developing lipoplexes designed for in vivo or ex vivo nucleic acid transfer.

Enhanced fusion pore expansion mediated by the trans-acting Endodomain of the reovirus FAST proteins

The reovirus fusion-associated small transmembrane (FAST) proteins are virus-encoded membrane fusion proteins that function as dedicated cell–cell fusogens. The topology of these small, single-pass membrane proteins orients the majority of the protein on the distal side of the membrane (i.e., inside the cell). We now show that ectopic expression of the endodomains of the p10, p14, and p15 FAST proteins enhances syncytiogenesis induced by the full-length FAST proteins, both homotypically and heterotypically. Results further indicate that the 68-residue cytoplasmic endodomain of the p14 FAST protein (1) is endogenously generated from full-length p14 protein expressed in virus-infected or transfected cells; (2) enhances syncytiogenesis subsequent to stable pore formation; (3) increases the syncytiogenic activity of heterologous fusion proteins, including the differentiation-dependent fusion of murine myoblasts; (4) exerts its enhancing activity from the cytosol, independent of direct interactions with either the fusogen or the membranes being fused; and (5) contains several regions with protein–protein interaction motifs that influence enhancing activity. We propose that the unique evolution of the FAST proteins as virus-encoded cellular fusogens has allowed them to generate a trans-acting, soluble endodomain peptide to harness a cellular pathway or process involved in the poorly understood process that facilitates the transition from microfusion pores to macrofusion and syncytiogenesis.

The reovirus FAST proteins are the only known examples of nonenveloped virus membrane fusion proteins. Functioning as virus-encoded cellular fusogens, they mediate cell–cell membrane fusion and syncytium formation rather than virus–cell fusion. The FAST proteins are also the smallest protein fusogens and assume an unusual membrane topology, positioning the majority of their mass within or internal to the membrane in which they reside. We have been interested in reconciling the donor membrane-biased structural features of the FAST proteins with their ability to orchestrate the multi-step cell–cell membrane fusion process that leads to syncytium formation. We now show that the FAST proteins generate a soluble endodomain fragment that functions in trans from the cytosol, enhancing the capacity of diverse viral and cellular fusogens to drive the conversion of fusion pores into syncytia. The FAST proteins may therefore function in a similar manner as membrane receptors whose signalling activity requires regulated intramembrane proteolysis to generate a soluble signalling peptide. The endodomain signalling peptide of the FAST proteins provides a novel approach to identify cellular effectors involved in the fusion pore expansion stage of biological cell–cell membrane fusion.

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.

A lipid-protein hybrid model for tight junction

The epithelial tight junction (TJ) was first described ultrastructurally as a fusion of the outer lipid leaflets of the adjoining cell membrane bilayers (hemifusion). The discovery of an increasing number of integral TJ and TJ-associated proteins has eclipsed the original lipid-based model with the wide acceptance of a protein-centric model for the TJ. In this review, we stress the importance of lipids in TJ structure and function. A lipid-protein hybrid model accommodates a large body of information supporting the lipidic characteristics of the TJ, harmonizes with the accumulating evidence supporting the TJ as an assembly of lipid rafts, and focuses on an important, but relatively unexplored, field of lipid-protein interactions in the morphology, physiology, and pathophysiology of the TJ.

Membrane lysis during biological membrane fusion: collateral damage by misregulated fusion machines

In the canonical model of membrane fusion, the integrity of the fusing membranes is never compromised, preserving the identity of fusing compartments. However, recent molecular simulations provided evidence for a pathway to fusion in which holes in the membrane evolve into a fusion pore. Additionally, two biological membrane fusion models-yeast cell mating and in vitro vacuole fusion-have shown that modifying the composition or altering the relative expression levels of membrane fusion complexes can result in membrane lysis. The convergence of these findings showing membrane integrity loss during biological membrane fusion suggests new mechanistic models for membrane fusion and the role of membrane fusion complexes.

A virus-encoded cell-cell fusion machine dependent on surrogate adhesins

The reovirus fusion-associated small transmembrane (FAST) proteins function as virus-encoded cellular fusogens, mediating efficient cell–cell rather than virus–cell membrane fusion. With ectodomains of only ∼20–40 residues, it is unclear how such diminutive viral fusion proteins mediate the initial stages (i.e. membrane contact and close membrane apposition) of the fusion reaction that precede actual membrane merger. We now show that the FAST proteins lack specific receptor-binding activity, and in their natural biological context of promoting cell–cell fusion, rely on cadherins to promote close membrane apposition. The FAST proteins, however, are not specifically reliant on cadherin engagement to mediate membrane apposition as indicated by their ability to efficiently utilize other adhesins in the fusion reaction. Results further indicate that surrogate adhesion proteins that bridge membranes as close as 13 nm apart enhance FAST protein-induced cell–cell fusion, but active actin remodelling is required for maximal fusion activity. The FAST proteins are the first example of membrane fusion proteins that have specifically evolved to function as opportunistic fusogens, designed to exploit and convert naturally occurring adhesion sites into fusion sites. The capacity of surrogate, non-cognate adhesins and active actin remodelling to enhance the cell–cell fusion activity of the FAST proteins are features perfectly suited to the structural and functional evolution of these fusogens as the minimal fusion component of a virus-encoded cellular fusion machine. These results also provide a basis for reconciling the rudimentary structure of the FAST proteins with their capacity to fuse cellular membranes.

Much of our current understanding of how proteins mediate membrane fusion derives from the study of enveloped virus fusion proteins. These fusion protein complexes function autonomously to co-ordinately regulate virus–cell attachment and subsequent membrane merger. In contrast, the reovirus Fusion-Associated Small Transmembrane (FAST) proteins are the only example of virus-encoded cellular fusogens, specifically designed to mediate cell–cell rather than virus–cell membrane fusion. In view of their small size, it was unclear if, or how, the FAST proteins are responsible for promoting the membrane attachment and close apposition stages of the fusion reaction. We now show that the FAST proteins have specifically evolved to function as the fusion component in a biphasic cell–cell fusion reaction, where the membrane attachment and membrane merger stages represent two distinct, uncoupled phases. Exploiting cadherins as surrogate adhesins, the FAST proteins have retained within their rudimentary structures the minimal determinants required to convert pre-existing adherens junctions into sites of cell–cell membrane fusion. These results raise the interesting possibility that other, yet to be identified cellular fusion proteins may resemble the FAST proteins, using separate adhesins and less complex fusion proteins in a similar biphasic membrane fusion reaction.

Viral and developmental cell fusion mechanisms: conservation and divergence

Membrane fusion is a fundamental requirement in numerous developmental, physiological, and pathological processes in eukaryotes. So far, only a limited number of viral and cellular fusogens, proteins that fuse membranes, have been isolated and characterized. Despite the diversity in structures and functions of known fusogens, some common principles of action apply to all fusion reactions. These can serve as guidelines in the search for new fusogens, and may allow the formulation of a cross-species, unified theory to explain divergent and convergent evolutionary principles of membrane fusion.

Bovine lactoferricin causes apoptosis in Jurkat T-leukemia cells by sequential permeabilization of the cell membrane and targeting of mitochondria

Bovine lactoferricin (LfcinB) is a cationic antimicrobial peptide that kills Jurkat T-leukemia cells by the mitochondrial pathway of apoptosis. However, the process by which LfcinB triggers mitochondria-dependent apoptosis is not well understood. Here, we show that LfcinB-induced apoptosis in Jurkat T-leukemia cells was preceded by LfcinB binding to, and progressive permeabilization of the cell membrane. Colloidal gold electron microscopy revealed that LfcinB entered the cytoplasm of Jurkat T-leukemia cells prior to the onset of mitochondrial depolarization. LfcinB was not internalized by endocytosis because endocytosis inhibitors did not prevent LfcinB-induced cytotoxicity. Furthermore, intracellular delivery of LfcinB via fusogenic liposomes caused the death of Jurkat T-leukemia cells, as well as normal human fibroblasts. Collectively, these findings suggest that LfcinB caused damage to the cell membrane that allowed LfcinB to enter the cytoplasm of Jurkat T-leukemia cells and mediate cytotoxicity. In addition, confocal microscopy showed that intracellular LfcinB co-localized with mitochondria in Jurkat T-leukemia cells, while flow cytometry and colloidal gold electron microscopy showed that LfcinB rapidly associated with purified mitochondria. Furthermore, purified mitochondria treated with LfcinB rapidly lost transmembrane potential and released cytochrome c. We conclude that LfcinB-induced apoptosis in Jurkat T-leukemia cells resulted from cell membrane damage and the subsequent disruption of mitochondrial membranes by internalized LfcinB.

The First Family of Cell-Cell Fusion

Our understanding of the developmentally critical process of cell-cell fusion has been greatly advanced by the identification of the first family of cell-cell fusion proteins. Together, the two founding members of the FF family execute the majority of cell-cell fusion events in C. elegans.

A role for a complex between activated G protein-coupled receptors in yeast cellular mating

Cell-cell fusion is a fundamental process that facilitates a wide variety of biological events in organisms ranging from yeast to humans. However, relatively little is actually understood with respect to fusion mechanisms. In the model organism Saccharomyces cerevisiae, mating of opposite-type cells is triggered by pheromone activation of the G protein-coupled receptors, alpha-factor receptor (Ste2p) and a-factor receptor (Ste3p), leading to mitogen-activated protein kinase signaling, growth arrest, and cellular fusion events. Herein we now provide evidence of a role for these receptors in the later cell fusion stage of mating. In vitro assays demonstrated the ability of the receptors to promote mixing of proteoliposomes containing phosphatidylserine, potentially based on a pheromone-dependent interaction between Ste2p and Ste3p that was confirmed by tandem affinity purification and cellular pull-down assays. The cellular mating activity of Ste2p was subsequently probed in vivo. Notably, a receptor-null yeast strain expressing N-terminally truncated Ste2p yielded a phenotype demonstrating wild-type signaling but arrested mating. The arrested prezygotes showed evidence of some cell wall erosion but no membrane juxtaposition at the fusion site. Further, in vitro analyses correlated this mutation with loss of the interaction between Ste2p and Ste3p and inhibition of related lipid mixing. Overall, these results support a role for a complex between activated yeast pheromone receptors in later cell fusion stages of mating, possibly mediating events at the level of cell wall digestion and membrane juxtaposition before membrane fusion.

The C. elegans developmental fusogen EFF-1 mediates homotypic fusion in heterologous cells and in vivo

During cell-cell fusion, two cells' plasma membranes merge, allowing the cytoplasms to mix and form a syncytium. Little is known about the mechanisms of cell fusion. Here, we asked whether eff-1, shown previously to be essential for fusion in Caenorhabditis elegans, acts directly in the fusion machinery. We show that expression of EFF-1 transmembrane protein drives fusion of heterologous cells into multinucleate syncytia. We obtained evidence that EFF-1-mediated fusion involves a hemifusion intermediate characterized by membrane mixing without cytoplasm mixing. Furthermore, syncytiogenesis requires EFF-1 in both fusing cells. To test whether this mechanism also applies in vivo, we conducted genetic mosaic analysis of C. elegans and found that homotypic epidermal fusion requires EFF-1 in both cells. Thus, although EFF-1-mediated fusion shares characteristics with viral and intracellular fusion, including an apparent hemifusion step, it differs from these reactions in the homotypic organization of the fusion machinery.

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.

Cationic antimicrobial peptides as novel cytotoxic agents for cancer treatment

Cancer treatment by conventional chemotherapy is hindered by toxic side effects and the frequent development of multi-drug resistance by cancer cells. Cationic antimicrobial peptides (CAPs) are a promising new class of natural-source drugs that may avoid the shortcomings of conventional chemotherapy because certain CAPs exhibit selective cytotoxicity against a broad spectrum of human cancer cells, including neoplastic cells that have acquired a multi-drug-resistant phenotype. Tumour cell killing by CAPs is usually by a cell membrane-lytic effect, although some CAPs can trigger apoptosis in cancer cells via mitochondrial membrane disruption. Furthermore, certain CAPs are potent inhibitors of blood vessel development (angiogenesis) that is associated with tumour progression. This article reviews the mechanisms by which CAPs exert anticancer activity and discusses the potential application of selected CAPs as therapeutic agents for the treatment of human cancers.

Virus membrane-fusion proteins: more than one way to make a hairpin

Enveloped animal viruses deliver their genetic contents into host cells by a fusion reaction between the virus membrane, which is derived from the host-cell membrane during virus budding, and the host-cell membrane. Studying the molecular mechanisms of virus membrane-fusion reactions is important, as they are paradigms for cellular membrane-fusion reactions and potential targets for antiviral therapies.

The fusion reactions are driven by virus membrane-fusion proteins, which undergo a major conformational change that is triggered by interactions with the target cell. Currently, two classes of virus membrane-fusion proteins are known — class I and class II. Class I proteins have been well characterized and refold to a hairpin conformation that drives membrane fusion.

The class II membrane-fusion proteins are considered in detail, using the E1 protein of the alphavirus Semliki Forest virus (SFV) and the E protein of the flavivirus tick-borne encephalitis virus (TBE) as examples.

In spite of the lack of any detectable amino-acid-sequence similarity, the ectodomains of the alphavirus (E1) and flavivirus (E) fusion proteins have remarkably similar secondary and tertiary structures. Both proteins fold co-translationally with a companion protein, p62 and prM, respectively. One important difference between the viruses is that different budding sites are used — new alphavirus virions bud from the plasma membrane, whereas flavivirus particles bud into the endoplasmic reticulum as immature virions, which are then transported via the exocytic pathway.

The structure of the E1 and E proteins is considered in detail, as are the conformational changes that occur during target-membrane insertion and fusion. Unlike class I fusion proteins, which are already in trimeric form before fusion, class II proteins are dimers that must rearrange during fusion to form a stable membrane-inserted homotrimer. However, despite the fact that class I and class II proteins have very different structures, both classes refold during fusion to give a similar overall 'hairpin' conformation.

Evidence suggests that class II trimers interact cooperatively during membrane insertion and fusion. A model for five-fold interactions at the fusion site, including the formation of a transient hemifusion intermediate, is proposed.

It is likely that class I and II fusion proteins use the same overall mechanism, suggesting that there could be a universal mechanism of membrane fusion. The possibility that there could be further classes of membrane-fusion proteins in addition to class I and class II is discussed.

Despite markedly different structures, both class I and class II viral membrane-fusion proteins adopt a hairpin conformation, inducing fusion of viral and cellular membranes. This review focuses on the class II proteins, using Semliki Forest virus and tick-borne encephalitis virus fusion proteins as examples.

Structure–function studies have defined two classes of viral membrane-fusion proteins that have radically different architectures but adopt a similar overall 'hairpin' conformation to induce fusion of the viral and cellular membranes and therefore initiate infection. In both classes, the hairpin conformation is achieved after a conformational change is triggered by interaction with the target cell. This review will focus in particular on the properties of the more recently described class II proteins.