Oligomerization of fusogenic peptides promotes membrane fusion by enhancing membrane destabilization

Intracellular protein delivery: New insights into the therapeutic applications and emerging technologies

The inability to cross the plasma membranes traditionally limited the therapeutic use of recombinant proteins. However, in the last two decades, novel technologies made delivering proteins inside the cells possible. This allowed researchers to unlock intracellular targets, once considered 'undruggable', bringing a new research area to emerge. Protein transfection systems display a large potential in a plethora of applications. However, their modality of action is often unclear, and cytotoxic effects are elevated, whereas experimental conditions to increase transfection efficacy and cell viability still need to be identified. Furthermore, technical complexity often limits in vivo experimentation, while challenging industrial and clinical translation. This review highlights the applications of protein transfection technologies, and then critically discuss the current methodologies and their limitations. Physical membrane perforation systems are compared to systems exploiting cellular endocytosis. Research evidence of the existence of either extracellular vesicles (EVs) or cell-penetrating peptides (CPPs)- based systems, that circumvent the endosomal systems is critically analysed. Commercial systems, novel solid-phase reverse protein transfection systems, and engineered living intracellular bacteria-based mechanisms are finally described. This review ultimately aims at finding new methodologies and possible applications of protein transfection systems, while helping the development of an evidence-based research approach.

Self-assembled short peptides: Recent advances and strategies for potential pharmaceutical applications

Self-assembled short peptides have intrigued scientists due to the convenience of synthesis, good biocompatibility, low toxicity, inherent biodegradability and fast response to change in the physiological environment. Therefore, it is necessary to present a comprehensive summary of the recent advances in the last decade regarding the construction, route of administration and application of self-assembled short peptides based on the knowledge on their unique and specific ability of self-assembly. Herein, we firstly explored the molecular mechanisms of self-assembly of short peptides, such as non-modified amino acids, as well as Fmoc-modified, N-functionalized, and C-functionalized peptides. Next, cell penetration, fusion, and peptide targeting in peptide-based drug delivery were characterized. Then, the common administration routes and the potential pharmaceutical applications (drug delivery, antibacterial activity, stabilizers, imaging agents, and applications in bioengineering) of peptide drugs were respectively summarized. Last but not least, some general conclusions and future perspectives in the relevant fields were briefly listed. Although with certain challenges, great opportunities are offered by self-assembled short peptides to the fascinating area of drug development.

Peptide-based delivery of therapeutics in cancer treatment

Current delivery strategies for cancer therapeutics commonly cause significant systemic side effects due to required high doses of therapeutic, inefficient cellular uptake of drug, and poor cell selectivity. Peptide-based delivery systems have shown the ability to alleviate these issues and can significantly enhance therapeutic loading, delivery, and cancer targetability. Peptide systems can be tailor-made for specific cancer applications. This review describes three peptide classes, targeting, cell penetrating, and fusogenic peptides, as stand-alone nanoparticle systems, conjugations to nanoparticle systems, or as the therapeutic modality. Peptide nanoparticle design, characteristics, and applications are discussed as well as peptide applications in the clinical space.

Membrane-Bound Configuration and Lipid Perturbing Effects of Hemagglutinin Subunit 2 N-Terminus Investigated by Computer Simulations

Hemagglutinin (HA) mediated fusion of influenza virus envelope with host lipid membrane is a critical step warrantying virus entry to the cell. Despite tremendous advances in structural biology methods, the knowledge concerning the details of HA2 subunit insertion into the target membrane and its subsequent bilayer perturbing effect is still rather limited. Herein, based on a set of molecular dynamics simulations, we investigate the structure and interaction with lipid membrane of the N-terminal HA2 region comprising a trimer of fusion peptides (HAfps) tethered by flexible linkers to a fragment of coiled-coil stem structure. We find that, prior to insertion into the membrane, HAfps within the trimers do not sample space individually but rather associate into a compact hydrophobic aggregate. Once within the membrane, they fold into tight helical hairpins, which remain at the lipid-water interface. However, they can also assume stable, membrane-spanning configurations of significantly increased membrane-perturbing potential. In this latter case, HAfps trimers centre around the well-hydrated transmembrane channel-forming distinct, symmetric assemblies, whose wedge-like shape may play a role in promoting membrane curvature. We also demonstrate that, following HAfps insertion, the coiled-coil stem spontaneously tilts to almost membrane-parallel orientation, reflecting experimentally observed configuration adopted in the course of membrane fusion by complete HA2 units at the rim of membrane contact zones.

Membranotropic and biological activities of the membrane fusion peptides from SARS-CoV spike glycoprotein: The importance of the complete internal fusion peptide domain

Fusion peptides (FP) are prominent hydrophobic segments of viral fusion proteins that play critical roles in viral entry. FPs interact with and insert into the host lipid membranes, triggering conformational changes in the viral protein that leads to the viral-cell fusion. Multiple membrane-active domains from the severe acute respiratory syndrome (SARS) coronavirus (CoV) spike protein have been reported to act as the functional fusion peptide such as the peptide sequence located between the S1/S2 and S2' cleavage sites (FP1), the S2'-adjacent fusion peptide domain (FP2), and the internal FP sequence (cIFP). Using a combined biophysical approach, we demonstrated that the α-helical coiled-coil-forming internal cIFP displayed the highest membrane fusion and permeabilizing activities along with membrane ordering effect in phosphatidylcholine (PC)/phosphatidylglycerol (PG) unilamellar vesicles compared to the other two N-proximal fusion peptide counterparts. While the FP1 sequence displayed intermediate membranotropic activities, the well-conserved FP2 peptide was substantially less effective in promoting fusion, leakage, and membrane ordering in PC/PG model membranes. Furthermore, Ca2+ did not enhance the FP2-induced lipid mixing activity in PC/phosphatidylserine/cholesterol lipid membranes, despite its strong erythrocyte membrane perturbation. Nonetheless, we found that the three putative SARS-CoV membrane-active fusion peptide sequences here studied altered the physical properties of model and erythrocyte membranes to different extents. The importance of the distinct membranotropic and biological activities of all SARS-CoV fusion peptide domains and the pronounced effect of the internal fusion peptide sequence to the whole spike-mediated membrane fusion process are discussed.

Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse amino acid sequences with the ability to cross cellular membranes. CPPs can deliver several bioactive cargos, including proteins, peptides, nucleic acids and chemotherapeutics, into cells. Ever since their discovery, synthetic and natural CPPs have been utilized in therapeutics delivery, gene editing and cell imaging in fundamental research and clinical experiments. Over the years, CPPs have gained significant attention due to their low cytotoxicity and high transduction efficacy. In the last decade, multiple investigations demonstrated the potential of CPPs as carriers for the delivery of therapeutics to treat various types of cancer. Besides their remarkable efficacy owing to fast and efficient delivery, a crucial benefit of CPP-based cancer treatments is delivering anticancer agents selectively, rather than mediating toxicities toward normal tissues. To obtain a higher therapeutic index and to improve cell and tissue selectivity, CPP-cargo constructions can also be complexed with other agents such as nanocarriers and liposomes to obtain encouraging outcomes. This review summarizes various types of CPPs conjugated to anticancer cargos. Furthermore, we present a brief history of CPP utilization as delivery systems for anticancer agents in the last decade and evaluate several reports on the applications of CPPs in basic research and preclinical studies.

Computational Insight Into the Mechanism of SARS-CoV-2 Membrane Fusion

Membrane fusion, a key step in the early stages of virus propagation, allows the release of the viral genome in the host cell cytoplasm. The process is initiated by fusion peptides that are small, hydrophobic components of viral membrane-embedded glycoproteins and are typically conserved within virus families. Here, we attempted to identify the correct fusion peptide region in the Spike protein of SARS-CoV-2 by all-atom molecular dynamics simulations of dual membrane systems with varied oligomeric units of putative candidate peptides. Of all of the systems tested, only a trimeric unit of a 40-amino-acid region (residues 816-855 of SARS-CoV-2 Spike) was effective in triggering the initial stages of membrane fusion, within 200 ns of simulation time. Association of this trimeric unit with dual membranes resulted in the migration of lipids from the upper leaflet of the lower bilayer toward the lower leaflet of the upper bilayer to create a structural unit reminiscent of a fusion bridge. We submit that residues 816-855 of Spike represent the bona fide fusion peptide of SARS-CoV-2 and that computational methods represent an effective way to identify fusion peptides in viral glycoproteins.

Membranotropic peptides mediating viral entry

The means used by enveloped viruses to bypass cellular membranes are well characterized; however, the mechanisms used by non-enveloped viruses to deliver their genome inside the cell remain unresolved and poorly defined. The discovery of short, membrane interacting, amphipathic or hydrophobic sequences (known as membranotropic peptides) in both enveloped and non-enveloped viruses suggests that these small peptides are strongly involved in breaching the host membrane and in the delivery of the viral genome into the host cell. Thus, in spite of noticeable differences in entry, this short stretches of membranotropic peptides are probably associated with similar entry-related events. This review will uncover the intrinsic features of viral membranotropic peptides involved in viral entry of both naked viruses and the ones encircled with a biological membrane with the objective to better elucidate their different functional properties and possible applications in the biomedical field.

Lytic Inactivation of Human Immunodeficiency Virus by Dual Engagement of gp120 and gp41 Domains in the Virus Env Protein Trimer

We recently reported the discovery of a recombinant chimera, denoted DAVEI (dual-acting virucidal entry inhibitor), which is able to selectively cause specific and potent lytic inactivation of both pseudotyped and fully infectious human immunodeficiency virus (HIV-1) virions. The chimera is composed of the lectin cyanovirin-N (CVN) fused to the 20-residue membrane-proximal external region (MPER) of HIV-1 gp41. Because the Env gp120-binding CVN domain on its own is not lytic, we sought here to determine how the MPER_(DAVEI) domain is able to endow the chimera with virolytic activity. We used a protein engineering strategy to identify molecular determinants of MPER_(DAVEI) that are important for function. Recombinant mutagenesis and truncation demonstrated that the MPER_(DAVEI) domain could be significantly minimized without loss of function. The dependence of lysis on specific MPER sequences of DAVEI, determination of minimal linker length, and competition by a simplified MPER surrogate peptide suggested that the MPER domain of DAVEI interacts with the Env spike trimer, likely with the gp41 region. This conclusion was further supported by observations from binding of the biotinylated MPER surrogate peptide to Env protein expressed on cells, monoclonal antibody competition, a direct binding enzyme-linked immunosorbent assay on viruses with varying numbers of trimeric spikes on their surfaces, and comparison of maximal interdomain spacing in DAVEI to that in high-resolution structures of Env. The finding that MPER_(DAVEI) in CVN-MPER linker sequences can be minimized without loss of virolytic function provides an improved experimental path for constructing size-minimized DAVEI chimeras and molecular tools for determining how simultaneous engagement of gp120 and gp41 by these chimeras can disrupt the metastable virus Env spike.

Mapping out the intricate relationship of the HIV envelope protein and the membrane environment

The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Fusing simulation and experiment: The effect of mutations on the structure and activity of the influenza fusion peptide

During the infection process, the influenza fusion peptide (FP) inserts into the host membrane, playing a crucial role in the fusion process between the viral and host membranes. In this work we used a combination of simulation and experimental techniques to analyse the molecular details of this process, which are largely unknown. Although the FP structure has been obtained by NMR in detergent micelles, there is no atomic structure information in membranes. To answer this question, we performed bias-exchange metadynamics (BE-META) simulations, which showed that the lowest energy states of the membrane-inserted FP correspond to helical-hairpin conformations similar to that observed in micelles. BE-META simulations of the G1V, W14A, G12A/G13A and G4A/G8A/G16A/G20A mutants revealed that all the mutations affect the peptide’s free energy landscape. A FRET-based analysis showed that all the mutants had a reduced fusogenic activity relative to the WT, in particular the mutants G12A/G13A and G4A/G8A/G16A/G20A. According to our results, one of the major causes of the lower activity of these mutants is their lower membrane affinity, which results in a lower concentration of peptide in the bilayer. These findings contribute to a better understanding of the influenza fusion process and open new routes for future studies.

Identification of the Fusion Peptide-Containing Region in Betacoronavirus Spike Glycoproteins

The fusion peptides (FP) play an essential role in fusion of viral envelope with cellular membranes. The location and properties of the FPs in the spike (S) glycoproteins of different coronaviruses (CoV) have not yet been determined. Through amino acid sequence analysis of S proteins of representative CoVs, we identified a common region as a possible FP (pFP) that shares the characteristics of FPs of class I viral fusion proteins, including high Ala/Gly content, intermediate hydrophobicity, and few charged residues. To test the hypothesis that this region contains the CoV FP, we systemically mutated every residue in the pFP of Middle East respiratory syndrome betacoronavirus (MERS-CoV) and found that 11 of the 22 residues in the pFP (from G953 to L964, except for A956) were essential for S protein-mediated cell-cell fusion and virus entry. The synthetic MERS-CoV pFP core peptide (955IAGVGWTAGL964) induced extensive fusion of liposome membranes, while mutant peptide failed to induce any lipid mixing. We also selectively mutated residues in pFPs of two other β-CoVs, severe acute respiratory syndrome coronavirus (SARS-CoV) and mouse hepatitis virus (MHV). Although the amino acid sequences of these two pFPs differed significantly from that of MERS-CoV and each other, most of the pFP mutants of SARS-CoV and MHV also failed to mediate membrane fusion, suggesting that these pFPs are also the functional FPs. Thus, the FPs of 3 different lineages of β-CoVs are conserved in location within the S glycoproteins and in their functions, although their amino acid sequences have diverged significantly during CoV evolution.

IMPORTANCE

Within the class I viral fusion proteins of many enveloped viruses, the FP is the critical mediator of fusion of the viral envelope with host cell membranes leading to virus infection. FPs from within a virus family, like influenza viruses or human immunodeficiency viruses (HIV), tend to share high amino acid sequence identity. In this study, we determined the location and amino acid sequences of the FPs of S glycoproteins of 3 β-CoVs, MERS-CoV, SARS-CoV, and MHV, and demonstrated that they were essential for mediating cell-cell fusion and virus entry. Interestingly, in marked contrast to the FPs of influenza and HIV, the primary amino acid sequences of the FPs of β-CoVs in 3 different lineages differed significantly. Thus, during evolution the FPs of β-CoVs have diverged significantly in their primary sequences while maintaining the same essential biological functions. Our findings identify a potential new target for development of drugs against CoVs.

Self-assembly molecular dynamics simulations shed light into the interaction of the influenza fusion Peptide with a membrane bilayer

Influenza virus is one of the most devastating human pathogens. In order to infect host cells, this virus fuses its membrane with the host membrane in a process mediated by the glycoprotein hemagglutinin. During fusion, the N-terminal region of hemagglutinin, which is known as the fusion peptide (FP), inserts into the host membrane, promoting lipid mixing between the viral and host membranes. Therefore, this peptide plays a key role in the fusion process, but the exact mechanism by which it promotes lipid mixing is still unclear. To shed light into this matter, we performed molecular dynamics (MD) simulations of the influenza FP in different environments (water, dodecylphosphocholine (DPC) micelles, and a dimyristoylphosphatidylcholine (DMPC) membrane). While in pure water the peptide lost its initial secondary structure, in simulations performed in the presence of DPC micelles it remained stable, in agreement with previous experimental observations. In simulations performed in the presence of a preassembled DMPC bilayer, the peptide became unstructured and was unable to insert into the membrane as a result of technical limitations of the method used. To overcome this problem, we used a self-assembly strategy, assembling the membrane together with the peptide. These simulations revealed that the peptide can adopt a membrane-spanning conformation, which had not been predicted by previous MD simulation studies. The peptide insertion had a strong effect on the membrane, lowering the bilayer thickness, disordering nearby lipids, and promoting lipid tail protrusion. These results contribute to a better understanding of the role of the FP in the fusion process.

Bovine serum albumin with glycated carboxyl groups shows membrane-perturbing activities

The aim of the present study aimed to investigate whether glycated bovine serum albumin (BSA) showed novel activities on the lipid-water interface. Mannosylated BSA (Man-BSA) was prepared by modification of the carboxyl groups with p-aminophenyl α-d-mannopyranoside. In contrast to BSA, Man-BSA notably induced membrane permeability of egg yolk phosphatidylcholine (EYPC)/egg yolk sphingomyelin (EYSM)/cholesterol (Chol) and EYPC/EYSM vesicles. Noticeably, Man-BSA induced the fusion of EYPC/EYSM/Chol vesicles, but not of EYPC/EYSM vesicles. Although BSA and Man-BSA showed similar binding affinity for lipid vesicles, the lipid-bound conformation of Man-BSA was distinct from that of BSA. Moreover, Man-BSA adopted distinct structure upon binding with the EYPC/EYSM/Chol and EYPC/EYSM vesicles. Man-BSA could induce the fusion of EYPC/EYSM/Chol vesicles with K562 and MCF-7 cells, while Man-BSA greatly induced the leakage of Chol-depleted K562 and MCF-7 cells. The modified BSA prepared by conjugating carboxyl groups with p-aminophenyl α-d-glucopyranoside also showed membrane-perturbing activities. Collectively, our data indicate that conjugation of carboxyl groups with monosaccharide generates functional BSA with membrane-perturbing activities on the lipid-water interface.

gH625: a milestone in understanding the many roles of membranotropic peptides

Here, we review the current knowledge about viral derived membranotropic peptides, and we discuss how they may be used for many therapeutic applications. While they have been initially discovered in viral fusion proteins and have been involved in the mechanism of viral entry, it is now clear that their features and their mode of interaction with membrane bilayers can be exploited to design viral inhibitors as well as to favor delivery of cargos across the cell membrane and across the blood–brain barrier. The peptide gH625 has been extensively used for all these purposes and provides a significant contribution to the field. We describe the roles of this sequence in order to close the gap between the many functions that are now emerging for membranotropic peptides.

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Membranotropic peptides and their therapeutic applications

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Membrane fusion, viral inhibition, drug delivery

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gH625, a peptide derived from Herpes simplex virus type I: a case study

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gH625 in vitro and in vivo delivery across the blood–brain barrier

The three lives of viral fusion peptides

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The presence of a fusion peptide (FP) is a hallmark of viral fusion glycoproteins.

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Structure–function relationships underlying FP conservation remain greatly unknown.

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FPs establish interactions satisfying their folding within pre-fusion glycoproteins.

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Upon fusion activation FPs insert into and restructure target membranes.

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FPs can finally combine with transmembrane domains to form integral membrane bundles.

Fusion peptides comprise conserved hydrophobic domains absolutely required for the fusogenic activity of glycoproteins from divergent virus families. After 30 years of intensive research efforts, the structures and functions underlying their high degree of sequence conservation are not fully elucidated. The long-hydrophobic viral fusion peptide (VFP) sequences are structurally constrained to access three successive states after biogenesis. Firstly, the VFP sequence must fulfill the set of native interactions required for (meta) stable folding within the globular ectodomains of glycoprotein complexes. Secondly, at the onset of the fusion process, they get transferred into the target cell membrane and adopt specific conformations therein. According to commonly accepted mechanistic models, membrane-bound states of the VFP might promote the lipid bilayer remodeling required for virus-cell membrane merger. Finally, at least in some instances, several VFPs co-assemble with transmembrane anchors into membrane integral helical bundles, following a locking movement hypothetically coupled to fusion-pore expansion. Here we review different aspects of the three major states of the VFPs, including the functional assistance by other membrane-transferring glycoprotein regions, and discuss briefly their potential as targets for clinical intervention.

Design, construction, and characterization of high-performance membrane fusion devices with target-selectivity

Membrane fusion proteins such as the hemagglutinin glycoprotein have target recognition and fusion accelerative domains, where some synergistically working elements are essential for target-selective and highly effective native membrane fusion systems. In this work, novel membrane fusion devices bearing such domains were designed and constructed. We selected a phenylboronic acid derivative as a recognition domain for a sugar-like target and a transmembrane-peptide (Leu-Ala sequence) domain interacting with the target membrane, forming a stable hydrophobic α-helix and accelerating the fusion process. Artificial membrane fusion behavior between the synthetic devices in which pilot and target liposomes were incorporated was characterized by lipid-mixing and inner-leaflet lipid-mixing assays. Consequently, the devices bearing both the recognition and transmembrane domains brought about a remarkable increase in the initial rate for the membrane fusion compared with the devices containing the recognition domain alone. In addition, a weakly acidic pH-responsive device was also constructed by replacing three Leu residues in the transmembrane-peptide domain by Glu residues. The presence of Glu residues made the acidic pH-dependent hydrophobic α-helix formation possible as expected. The target-selective liposome-liposome fusion was accelerated in a weakly acidic pH range when the Glu-substituted device was incorporated in pilot liposomes. The use of this pH-responsive device seems to be a potential strategy for novel applications in a liposome-based delivery system.

Antimicrobial and cell-penetrating peptides induce lipid vesicle fusion by folding and aggregation

According to their distinct biological functions, membrane-active peptides are generally classified as antimicrobial (AMP), cell-penetrating (CPP), or fusion peptides (FP). The former two classes are known to have some structural and physicochemical similarities, but fusogenic peptides tend to have rather different features and sequences. Nevertheless, we found that many CPPs and some AMPs exhibit a pronounced fusogenic activity, as measured by a lipid mixing assay with vesicles composed of typical eukaryotic lipids. Compared to the HIV fusion peptide (FP23) as a representative standard, all designer-made peptides showed much higher lipid-mixing activities (MSI-103, MAP, transportan, penetratin, Pep1). Native sequences, on the other hand, were less fusogenic (magainin 2, PGLa, gramicidin S), and pre-aggregated ones were inactive (alamethicin, SAP). The peptide structures were characterized by circular dichroism before and after interacting with the lipid vesicles. A striking correlation between the extent of conformational change and the respective fusion activities was found for the series of peptides investigated here. At the same time, the CD data show that lipid mixing can be triggered by any type of conformation acquired upon binding, whether α-helical, β-stranded, or other. These observations suggest that lipid vesicle fusion can simply be driven by the energy released upon membrane binding, peptide folding, and possibly further aggregation. This comparative study of AMPs, CPPs, and FPs emphasizes the multifunctional aspects of membrane-active peptides, and it suggests that the origin of a peptide (native sequence or designer-made) may be more relevant to define its functional range than any given name.

Localization of antimicrobial peptides on polymerized liposomes leading to their enhanced efficacy against Pseudomonas aeruginosa

Antimicrobial peptide IG-25 (a truncated version of LL-37 of the cathelicidin family) tethering an azido-capped poly(ethylene glycol) chain at the N-terminus was site-specifically attached to alkynyl-terminated polymerized liposomes using copper catalyzed "click" reaction, leading to an 18 fold enhancement in efficacy against Pseudomonas aeruginosa when compared to LL-37 without any increase in cytotoxicity to human corneal epithelial cells.

Insights into the mechanism of HIV-1 envelope induced membrane fusion as revealed by its inhibitory peptides

HIV-1 fusion with its target cells is mediated by the glycoprotein 41 (gp41) transmembrane subunit of the viral envelope glycoprotein (ENV). The current models propose that gp41 undergoes several conformational changes between the apposing viral and cell membranes to facilitate fusion. In this review we focus on the progress that has been made in revealing the dynamic role of the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) regions within gp41 to the fusion process. The involvement of these regions in the formation of the gp41 pre-hairpin and hairpin conformations during an ongoing fusion event was mainly discovered by their derived inhibitory peptides. For example, the core structure within the hairpin conformation in a dynamic fusion event is suggested to be larger than its high resolution structure and its minimal boundaries were determined in situ. Also, inhibitory peptides helped reveal the dual contribution of the NHR to the fusion process. Finally, we will also discuss several developments in peptide design that has led to a deeper understanding of the mechanism of viral membrane fusion.

Major antiparallel and minor parallel β sheet populations detected in the membrane-associated human immunodeficiency virus fusion peptide

The HIV gp41 protein catalyzes fusion between viral and host cell membranes, and its apolar N-terminal region or "fusion peptide" binds to the host cell membrane and plays a key role in fusion. "HFP" is a construct containing the fusion peptide sequence, induces membrane vesicle fusion, and is an important fusion model system. Earlier solid-state nuclear magnetic resonance (SSNMR) studies showed that when HFP is associated with membranes with ∼30 mol % cholesterol, the first 16 residues have predominant β strand secondary structure and a fraction of the strands form antiparallel β sheet structure with residue 16→1/1→16 or 17→1/1→17 registries for adjacent strands. In some contrast, other SSNMR and infrared studies have been interpreted to support a large fraction of an approximately in-register parallel registry of adjacent strands. However, the samples had extensive isotopic labeling, and other structural models were also consistent with the data. This SSNMR study uses sparse labeling schemes that reduce ambiguity in the determination of the fraction of HFP molecules with parallel β registry. Quantitative analysis of the data shows that the parallel fraction is at most 0.15 with a much greater fraction of antiparallel 16→1/1→16 and 17→1/1→17 registries. These data strongly support a model of HFP-induced vesicle fusion caused by antiparallel rather than parallel registries and provide insight into the arrangement of gp41 molecules during HIV-host cell fusion. This study is an example of quantitative determination of a complex structural distribution by SSNMR, including experimentally validated inclusion of natural abundance contributions to the SSNMR data.

Architecture of a nascent viral fusion pore

Enveloped viruses use specialized protein machinery to fuse the viral membrane with that of the host cell during cell invasion. In influenza virus, hundreds of copies of the haemagglutinin (HA) fusion glycoprotein project from the virus surface. Despite intensive study of HA and its fusion activity, the protein's modus operandi in manipulating viral and target membranes to catalyse their fusion is poorly understood. Here, the three-dimensional architecture of influenza virus-liposome complexes at pH 5.5 was investigated by electron cryo-tomography. Tomographic reconstructions show that early stages of membrane remodeling take place in a target membrane-centric manner, progressing from punctate dimples, to the formation of a pinched liposomal funnel that may impinge on the apparently unperturbed viral envelope. The results suggest that the M1 matrix layer serves as an endoskeleton for the virus and a foundation for HA during membrane fusion. Fluorescence spectroscopy monitoring fusion between liposomes and virions shows that leakage of liposome contents takes place more rapidly than lipid mixing at pH 5.5. The relation of 'leaky' fusion to the observed prefusion structures is discussed.

Central domain of DivIB caps the C-terminal regions of the FtsL/DivIC coiled-coil rod

DivIB(FtsQ), FtsL, and DivIC(FtsB) are enigmatic membrane proteins that are central to the process of bacterial cell division. DivIB(FtsQ) is dispensable in specific conditions in some species, and appears to be absent in other bacterial species. The presence of FtsL and DivIC(FtsB) appears to be conserved despite very low sequence conservation. The three proteins form a complex at the division site, FtsL and DivIC(FtsB) being associated through their extracellular coiled-coil region. We report here structural investigations by NMR, small-angle neutron and x-ray scattering, and interaction studies by surface plasmon resonance, of the complex of DivIB, FtsL, and DivIC from Streptococcus pneumoniae, using soluble truncated forms of the proteins. We found that one side of the "bean"-shaped central beta-domain of DivIB interacts with the C-terminal regions of the dimer of FtsL and DivIC. This finding is corroborated by sequence comparisons across bacterial genomes. Indeed, DivIB is absent from species with shorter FtsL and DivIC proteins that have an extracellular domain consisting only of the coiled-coil segment without C-terminal conserved regions (Campylobacterales). We propose that the main role of the interaction of DivIB with FtsL and DivIC is to help the formation, or to stabilize, the coiled-coil of the latter proteins. The coiled-coil of FtsL and DivIC, itself or with transmembrane regions, could be free to interact with other partners.

Fusion peptide from influenza hemagglutinin increases membrane surface order: an electron-spin resonance study

A spin-labeling study of interactions of a fusion peptide from the hemagglutinin of the influenza virus, wt20, and a fusion-inactive mutant DeltaG1 with dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoyl-phosphatdylcholine bilayers was performed. We found that upon binding of wt20, the ordering of headgroups and the ordering of acyl chains near the headgroup increased significantly, in a manner consistent with a cooperative phenomenon. However, changes in the order at the end of the acyl chains were negligible. The ordering effect of wt20 on the headgroup was much stronger at pH 5 than at pH 7. No effect of DeltaG1 binding on the order of bilayers was evident. We also found that 1-palmitoyl-2-hydroxyl phosphatidylcholine, a membrane-fusion inhibitor, decreased the ordering of DMPC headgroups, whereas arachidonic acid, a membrane-fusion promoter, increased the ordering of DMPC headgroups. These results suggest that increases in headgroup ordering may be important for membrane fusion. We propose that upon binding of wt20, which is known to affect only the outer leaflet of the bilayer, this outer leaflet becomes more ordered, and thus more solid-like. Then the coupling between the hardened outer leaflet and the softer inner leaflet generates bending stresses in the bilayer, which tend to increase the negative curvature of the bilayer. We suggest that the increased ordering in the headgroup region enhances dipolar interactions and lowers electrostatic energy, which may provide an energy source for membrane fusion. Possible roles of bending stresses in promoting membrane fusion are discussed.

Complex lipid requirements for SNARE- and SNARE chaperone-dependent membrane fusion

Membrane fusion without lysis has been reconstituted with purified yeast vacuolar SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), the SNARE chaperones Sec17p/Sec18p and the multifunctional HOPS complex, which includes a subunit of the SNARE-interactive Sec1-Munc18 family, and vacuolar lipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidic acid (PA), cardiolipin (CL), ergosterol (ERG), diacylglycerol (DAG), and phosphatidylinositol 3-phosphate (PI3P). We now report that many of these lipids are required for rapid and efficient fusion of the reconstituted SNARE proteoliposomes in the presence of SNARE chaperones. Omission of either PE, PA, or PI3P from the complete set of lipids strongly reduces fusion, and PC, PE, PA, and PI3P constitute a minimal set of lipids for fusion. PA could neither be replaced by other lipids with small headgroups such as DAG or ERG nor by the acidic lipids PS or PI. PA is needed for full association of HOPS and Sec18p with proteoliposomes having a minimal set of lipids. Strikingly, PA and PE are as essential for SNARE complex assembly as for fusion, suggesting that these lipids facilitate functional interactions among SNAREs and SNARE chaperones.

Investigation of SNARE-Mediated Membrane Fusion Mechanism Using Atomic Force Microscopy

Membrane fusion is driven by specialized proteins that reduce the free energy penalty for the fusion process. In neurons and secretory cells, soluble N-ethylmaleimide-sensitive factor-attachment protein (SNAP) receptors (SNAREs) mediate vesicle fusion with the plasma membrane during vesicular content release. Although, SNAREs have been widely accepted as the minimal machinery for membrane fusion, the specific mechanism for SNARE-mediated membrane fusion remains an active area of research. Here, we summarize recent findings based on force measurements acquired in a novel experimental system that uses atomic force microscope (AFM) force spectroscopy to investigate the mechanism(s) of membrane fusion and the role of SNAREs in facilitating membrane hemifusion during SNARE-mediated fusion. In this system, protein-free and SNARE-reconstituted lipid bilayers are formed on opposite (trans) substrates and the forces required to induce membrane hemifusion and fusion or to unbind single v-/t-SNARE complexes are measured. The obtained results provide evidence for a mechanism by which the pulling force generated by interacting trans-SNAREs provides critical proximity between the membranes and destabilizes the bilayers at fusion sites by broadening the hemifusion energy barrier and consequently making the membranes more prone to fusion.

Class I and class II viral fusion protein structures reveal similar principles in membrane fusion (Review)

Recent crystal structures of Flavivirus and Alphavirus fusion proteins (class II) confirm two major principles of protein machineries that mediate the merger of two opposing lipid bilayers. First, the fusion protein can bridge both membranes tethered by two membrane anchors. Second, refolding or domain rearrangement steps lead to the positioning of both anchors into close proximity at the same end of an elongated structure. Although these two steps are in principle sufficient to pull two opposing membranes together and initiate membrane fusion, accumulating evidence suggests that the process requires the concerted action of a number of fusion proteins at and outside the contact sites. This review will focus on the structures of viral class I and class II fusion proteins and their similarities in facilitating membrane fusion.

Pulling force generated by interacting SNAREs facilitates membrane hemifusion

In biological systems, membrane fusion is mediated by specialized proteins. Although soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptors (SNAREs) provide the minimal molecular machinery required to drive membrane fusion, the precise mechanism for SNARE-mediated fusion remains to be established. Here, we used atomic force microscope (AFM) spectroscopy to determine whether the pulling force generated by interacting SNAREs is directly coupled to membrane fusion. The mechanical strength of the SNARE binding interaction was determined by single molecule force measurements. It was revealed that the forced unbinding of the SNARE complex formed between opposing (trans) bilayers involves two activation barriers; where the steep inner barrier governs the transition from the bound to an intermediate state and the outer barrier governs the transition between the intermediate and the unbound state. Moreover, truncation of either SNAP-25 or VAMP 2 reduced the slope of the inner barrier significantly and, consequently, reduced the pulling strength of the SNARE complex; thus, suggesting that the inner barrier determines the binding strength of the SNARE complex. In parallel, AFM compression force measurements revealed that truncated SNAREs were less efficient than native SNAREs in facilitating hemifusion of the apposed bilayers. Together, these findings reveal a mechanism by which a pulling force generated by interacting trans-SNAREs reduces the slope of the hemifusion barrier and, subsequently, facilitates hemifusion and makes the membranes more prone to fusion.

Reconstituted membrane fusion requires regulatory lipids, SNAREs and synergistic SNARE chaperones

The homotypic fusion of yeast vacuoles, each with 3Q- and 1R-SNARE, requires SNARE chaperones (Sec17p/Sec18p and HOPS) and regulatory lipids (sterol, diacylglycerol and phosphoinositides). Pairs of liposomes of phosphatidylcholine/phosphatidylserine, bearing three vacuolar Q-SNAREs on one and the R-SNARE on the other, undergo slow lipid mixing, but this is unaffected by HOPS and inhibited by Sec17p/Sec18p. To study these essential fusion components, we reconstituted proteoliposomes of a more physiological composition, bearing vacuolar lipids and all four vacuolar SNAREs. Their fusion requires Sec17p/Sec18p and HOPS, and each regulatory lipid is important for rapid fusion. Although SNAREs can cause both fusion and lysis, fusion of these proteoliposomes with Sec17p/Sec18p and HOPS is not accompanied by lysis. Sec17p/Sec18p, which disassemble SNARE complexes, and HOPS, which promotes and proofreads SNARE assembly, act synergistically to form fusion-competent SNARE complexes, and this synergy requires phosphoinositides. This is the first chemically defined model of the physiological interactions of these conserved fusion catalysts.

The dynamic envelope of a fusion class II virus. E3 domain of glycoprotein E2 precursor in Semliki Forest virus provides a unique contact with the fusion protein E1

In alphaviruses, here represented by Semliki Forest virus, infection requires an acid-responsive spike configuration to facilitate membrane fusion. The creation of this relies on the chaperone function of glycoprotein E2 precursor (p62) and its maturation cleavage into the small external E3 and the membrane-anchored E2 glycoproteins. To reveal how the E3 domain of p62 exerts its control of spike functions, we determine the structure of a p62 cleavage-impaired mutant virus particle (SQL) by electron cryomicroscopy. A comparison with the earlier solved wild type virus structure reveals that the E3 domain of p62(SQL) forms a bulky side protrusion in the spike head region. This establishes a gripper over part of domain II of the fusion protein, with a cotter-like connection downward to a hydrophobic cluster in its central beta-sheet. This finding reevaluates the role of the precursor from being only a provider of a shield over the fusion loop to a structural playmate in formation of the fusogenic architecture.

Excess vacuolar SNAREs drive lysis and Rab bypass fusion

Although concentrated soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) drive liposome fusion and lysis, the fusion of intracellular membranes also requires Rab GTPases, Rab effectors, SM proteins, and specific regulatory lipids and is accompanied by little or no lysis. To rationalize these findings, we generated yeast strains that overexpress all four vacuolar SNAREs (4SNARE(++)). Although vacuoles with physiological levels of Rab, Rab effector/SM complex, and SNAREs support rapid fusion without Rab- and SNARE-dependent lysis, vacuoles from 4SNARE(++) strains show extensive lysis and a reduced need for the Rab Ypt7p or regulatory lipids for fusion. SNARE overexpression and the addition of pure homotypic fusion and vacuole protein sorting complex (HOPS), which bears the vacuolar SM protein, enables ypt7Delta vacuoles to fuse, allowing direct comparison of Rab-dependent and Rab-independent fusion. Because 3- to 40-fold more of each of the five components that form the SNARE/HOPS fusion complex are required for vacuoles from ypt7Delta strains to fuse at the same rate as vacuoles from wild-type strains, the apparent forward rate constant of 4SNARE/HOPS complex assembly is enhanced many thousand-fold by Ypt7p. Rabs function in normal membrane fusion by concentrating SNAREs, other proteins (e.g., SM), and key lipids at a fusion site and activating them for fusion without lysis.

Charged residues are involved in membrane fusion mediated by a hydrophilic peptide located in vesicular stomatitis virus G protein

Membrane fusion is an essential step of the internalization process of the enveloped animal viruses. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion at the acidic environment of the endosomal compartment. In a previous work, we identified a specific sequence in VSV G protein, comprising the residues 145 to 164, directly involved in membrane interaction and fusion. Unlike fusion peptides from other viruses, this sequence is very hydrophilic, containing six charged residues, but it was as efficient as the virus in catalyzing membrane fusion at pH 6.0. Using a carboxyl-modifying agent, dicyclohexylcarbodiimide (DCCD), and several synthetic mutant peptides, we demonstrated that the negative charges of peptide acidic residues, especially Asp153 and Glu158, participate in the formation of a hydrophobic domain at pH 6.0, which is necessary to the peptide-induced membrane fusion. The formation of the hydrophobic region and the membrane fusion itself were dependent on peptide concentration in a higher than linear fashion, suggesting the involvement of peptide oligomerization. His148 was also necessary to hydrophobicity and fusion, suggesting that peptide oligomerization occurs through intermolecular electrostatic interactions between the positively-charged His and a negatively-charged acidic residue of two peptide molecules. Oligomerization of hydrophilic peptides creates a hydrophobic region that is essential for the interaction with the membrane that results in fusion.

Configuration of influenza hemagglutinin fusion peptide monomers and oligomers in membranes

The 20 N-terminal residues of the HA2 subunit of influenza hemagglutinin (HA), known as the fusion peptide, play a crucial role in membrane fusion. Molecular dynamics simulations with implicit solvation are employed here to study the structure and orientation of the fusion peptide in membranes. As a monomer the alpha-helical peptide adopts a shallow, slightly tilted orientation along the lipid tail-head group interface. The average angle of the peptide with respect to membrane plane is 12.4 degrees . We find that the kinked structure proposed on the basis of NMR data is not stable in our model because of the high energy cost related to the membrane insertion of polar groups. Because hemagglutinin-mediated membrane fusion is promoted by low pH, we examined the effect of protonation of the Glu and Asp residues. The configurations of the protonated peptides were slightly deeper in the membrane but at similar angles. Finally, because HA is a trimer, we modeled helical fusion peptide trimers. We find that oligomerization affects the insertion depth of the peptide and its orientation with respect to the membrane: a trimer exhibits equally favorable configurations in which some or all of the helices in the bundle insert obliquely deep into the membrane.

Conformational flexibility and strand arrangements of the membrane-associated HIV fusion peptide trimer probed by solid-state NMR spectroscopy

The human immunodeficiency virus (HIV) fusion peptide (HFP) is the N-terminal apolar region of the HIV gp41 fusion protein and interacts with target cell membranes and promotes membrane fusion. The free peptide catalyzes vesicle fusion at least to the lipid mixing stage and serves as a useful model fusion system. For gp41 constructs which lack the HFP, high-resolution structures show trimeric protein and suggest that at least three HFPs interact with the membrane with their C-termini in close proximity. In addition, previous studies have demonstrated that HFPs which are cross-linked at their C-termini to form trimers (HFPtr) catalyze fusion at a rate which is 15-40 times greater than that of non-cross-linked HFP. In the present study, the structure of membrane-associated HFPtr was probed with solid-state nuclear magnetic resonance (NMR) methods. Chemical shift and intramolecular (13)CO-(15)N distance measurements show that the conformation of the Leu-7 to Phe-11 region of HFPtr has predominant helical conformation in membranes without cholesterol and beta strand conformation in membranes containing approximately 30 mol % cholesterol. Interstrand (13)CO-(13)CO and (13)CO-(15)N distance measurements were not consistent with an in-register parallel strand arrangement but were consistent with either (1) parallel arrangement with adjacent strands two residues out-of-register or (2) antiparallel arrangement with adjacent strand crossing between Phe-8 and Leu-9. Arrangement 1 could support the rapid fusion rate of HFPtr because of placement of the apolar N-terminal regions of all strands on the same side of the oligomer while arrangement 2 could support the assembly of multiple fusion protein trimers.

Direct visualization of membrane leakage induced by the antibiotic peptides: maculatin, citropin, and aurein

Membrane lysis caused by antibiotic peptides is often rationalized by means of two different models: the so-called carpet model and the pore-forming model. We report here on the lytic activity of antibiotic peptides from Australian tree frogs, maculatin 1.1, citropin 1.1, and aurein 1.2, on POPC or POPC/POPG model membranes. Leakage experiments using fluorescence spectroscopy indicated that the peptide/lipid mol ratio necessary to induce 50% of probe leakage was smaller for maculatin compared with aurein or citropin, regardless of lipid membrane composition. To gain further insight into the lytic mechanism of these peptides we performed single vesicle experiments using confocal fluorescence microscopy. In these experiments, the time course of leakage for different molecular weight (water soluble) fluorescent markers incorporated inside of single giant unilamellar vesicles is observed after peptide exposure. We conclude that maculatin and its related peptides demonstrate a pore-forming mechanism (differential leakage of small fluorescent probe compared with high molecular weight markers). Conversely, citropin and aurein provoke a total membrane destabilization with vesicle burst without sequential probe leakage, an effect that can be assigned to a carpeting mechanism of lytic action. Additionally, to study the relevance of the proline residue on the membrane-action properties of maculatin, the same experimental approach was used for maculatin-Ala and maculatin-Gly (Pro-15 was replaced by Ala or Gly, respectively). Although a similar peptide/lipid mol ratio was necessary to induce 50% of leakage for POPC membranes, the lytic activity of maculatin-Ala and maculatin-Gly decreased in POPC/POPG (1:1 mol) membranes compared with that observed for the naturally occurring maculatin sequence. As observed for maculatin, the lytic action of Maculatin-Ala and maculatin-Gly is in keeping with the formation of pore-like structures at the membrane independently of lipid composition.

Opinion: Cell entry machines: a common theme in nature?

Molecular machines orchestrate the translocation and entry of pathogens through host cell membranes, in addition to the uptake and release of molecules during endocytosis and exocytosis. Viral cell entry requires a family of glycoproteins, and the structural organization and function of these viral glycoproteins are similar to the SNARE proteins, which are known to be involved in intracellular vesicle fusion, endocytosis and exocytosis. Here, we propose that a family of bacterial membrane proteins that are responsible for cell-mediated adherence and entry resembles the structural architecture of both viral fusion proteins and eukaryotic SNAREs and might therefore share similar, but distinct, mechanisms of cell membrane translocation. Furthermore, we propose that the recurrence of these molecular machines across species indicates that these architectural motifs were evolutionarily selected because they provided the best solution to ensure the survival of pathogens within a particular environment.

Bilayer conformation of fusion peptide of influenza virus hemagglutinin: a molecular dynamics simulation study

Unraveling the conformation of membrane-bound viral fusion peptides is essential for understanding how those peptides destabilize the bilayer topology of lipids that is important for virus-cell membrane fusion. Here, molecular dynamics (MD) simulations were performed to investigate the conformation of the 20 amino acids long fusion peptide of influenza hemagglutinin of strain X31 bound to a dimyristoyl phosphatidylcholine (DMPC) bilayer. The simulations revealed that the peptide adopts a kinked conformation, in agreement with the NMR structures of a related peptide in detergent micelles. The peptide is located at the amphipathic interface between the headgroups and hydrocarbon chains of the lipid by an energetically favorable arrangement: The hydrophobic side chains of the peptides are embedded into the hydrophobic region and the hydrophilic side chains are in the headgroup region. The N-terminus of the peptide is localized close to the amphipathic interface. The molecular dynamics simulations also revealed that the peptide affects the surrounding bilayer structure. The average hydrophobic thickness of the lipid phase close to the N-terminus is reduced in comparison with the average hydrophobic thickness of a pure dimyristoyl phosphatidylcholine bilayer.

Prm1 prevents contact-dependent lysis of yeast mating pairs

Membrane fusion requires localized destabilization of two phospholipid bilayers, but unrestrained membrane destabilization could result in lysis. prm1 mutant yeast cells have a defect at the plasma membrane fusion stage of mating that typically results in the accumulation of prezygotes that have fingers of membrane-bound cytoplasm projecting from one cell of each pair into its mating partner in the direction of the osmotic gradient between the cells. However, some prm1 mating pairs fuse successfully whereas the two cells in other prm1 mating pairs simultaneously lyse. Lysis only occurs if both mating partners are prm1 mutants. Osmotic stabilization does not protect prm1 mating pairs from lysis, indicating that lysis is not caused by a cell wall defect. prm1 mating pairs without functional mitochondria still lyse, ruling out programmed cell death. No excess lysis was found after pheromone treatment of haploid prm1 cells, and lysis did not occur in mating pairs when prm1 was combined with the fus1 and fus2 mutations to block cell wall remodeling. Furthermore, short (<1 microm) cytoplasmic microfingers indicating the completion of cell wall remodeling appeared immediately before lysis. In combination, these results demonstrate that plasma membrane contact is a prerequisite for lysis. Cytoplasmic microfingers are unlikely to cause lysis since most prm1 mating pairs with microfingers do not lyse, and microfingers were also detected before fusion in some wild-type mating pairs. The lysis of prm1 mutant mating pairs suggests that the Prm1 protein stabilizes the membrane fusion event of yeast mating.

Plasticity of influenza haemagglutinin fusion peptides and their interaction with lipid bilayers

A detailed molecular dynamics study of the haemagglutinin fusion peptide (N-terminal 20 residues of the HA2 subunits) in a model bilayer has yielded useful information about the molecular interactions leading to insertion into the lipids. Simulations were performed on the native sequence, as well as a number of mutant sequences, which are either fusogenic or nonfusogenic. For the native sequence and fusogenic mutants, the N-terminal 11 residues of the fusion peptides are helical and insert with a tilt angle of approximately 30 degrees with respect to the membrane normal, in very good agreement with experimental data. The tilted insertion of the native sequence peptide leads to membrane bilayer thinning and the calculated order parameters show larger disorder of the alkyl chains. These results indicate that the lipid packing is perturbed by the fusion peptide and could be used to explain membrane fusion. For the nonfusogenic sequences investigated, it was found that most of them equilibrate parallel to the interface plane and do not adopt a tilted conformation. The presence of a charged residue at the beginning of the sequence (G1E mutant) resulted in a more difficult case, and the outcomes do not fall straightforwardly into the general picture. Sequence searches have revealed similarities of the fusion peptide of influenza haemagglutinin with peptide sequences such as segments of porin, amyloid alpha eta peptide, and a peptide from the prion sequence. These results confirm that the sequence can adopt different folds in different environments. The plasticity and the conformational dependence on the local environment could be used to better understand the function of fusion peptides.

Uncoupling of photoreceptor peripherin/rds fusogenic activity from biosynthesis, subunit assembly, and targeting: a potential mechanism for pathogenic effects

Inherited defects in the RDS gene cause a multiplicity of progressive retinal diseases in humans. The gene product, peripherin/rds (P/rds), is a member of the tetraspanin protein family required for normal vertebrate photoreceptor outer segment (OS) architecture. Although its molecular function remains uncertain, P/rds has been suggested to catalyze membrane fusion events required for the OS renewal process. This study investigates the importance of two charged residues within a predicted C-terminal helical region for protein biosynthesis, localization, and interaction with model membranes. Targeted mutagenesis was utilized to neutralize charges at Glu(321) and Lys(324) individually and in combination to generate three mutant variants. Studies were conducted on variants expressed as 1) full-length P/rds in COS-1 cells, 2) glutathione S-transferase fusion proteins in Escherichia coli, and 3) membrane-associated green fluorescent protein fusion proteins in transgenic Xenopus laevis. None of the mutations affected biosynthesis of full-length P/rds in COS-1 cells as assessed by Western blotting, sedimentation velocity, and immunofluorescence microscopy. Although all mutations reside within a recently identified localization signal, none altered the ability of this region to direct OS targeting in transgenic X. laevis retinas. In contrast, individual or simultaneous neutralization of the charged amino acids Glu(321) and Lys(324) abolished the ability of the C-terminal domain to promote model membrane fusion as assayed by lipid mixing. These results demonstrate that, although overlapping, C-terminal determinants responsible for OS targeting and fusogenicity are separable and that fusogenic activity has been uncoupled from other protein properties. The observation that subunit assembly and OS targeting can both proceed normally in the absence of fusogenic activity suggests that properly assembled and targeted yet functionally altered proteins could potentially generate pathogenic effects within the vertebrate photoreceptor.