Arabidopsis HAP2/GCS1 is a gamete fusion protein homologous to somatic and viral fusogens

Cell–cell fusion is inherent to sexual reproduction. Loss of HAPLESS 2/GENERATIVE CELL SPECIFIC 1 (HAP2/GCS1) proteins results in gamete fusion failure in diverse organisms, but their exact role is unclear. In this study, we show that Arabidopsis thaliana HAP2/GCS1 is sufficient to promote mammalian cell–cell fusion. Hemifusion and complete fusion depend on HAP2/GCS1 presence in both fusing cells. Furthermore, expression of HAP2 on the surface of pseudotyped vesicular stomatitis virus results in homotypic virus–cell fusion. We demonstrate that the Caenorhabditis elegans Epithelial Fusion Failure 1 (EFF-1) somatic cell fusogen can replace HAP2/GCS1 in one of the fusing membranes, indicating that HAP2/GCS1 and EFF-1 share a similar fusion mechanism. Structural modeling of the HAP2/GCS1 protein family predicts that they are homologous to EFF-1 and viral class II fusion proteins (e.g., Zika virus). We name this superfamily Fusexins: fusion proteins essential for sexual reproduction and exoplasmic merger of plasma membranes. We suggest a common origin and evolution of sexual reproduction, enveloped virus entry into cells, and somatic cell fusion.

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Arabidopsis HAP2/GCS1 is a gamete fusion protein homologous to somatic and viral fusogens

10.1101/097568 ◽

2016 ◽

Cited By ~ 1

Author(s):

Clari Valansi ◽

David Moi ◽

Evgenia Leikina ◽

Elena Matveev ◽

Martín Graña ◽

...

Keyword(s):

Sexual Reproduction ◽

Somatic Cell ◽

Cell Fusion ◽

Fusion Proteins ◽

Mammalian Cells ◽

Generative Cell ◽

Target Cells ◽

Complete Fusion ◽

Gamete Fusion ◽

Cell Cell

AbstractCell-cell fusion is inherent to any form of sexual reproduction. Loss of HAPLESS 2/GENERATIVE CELL SPECIFIC 1 (HAP2/GCS1) proteins results in gamete fusion failure in different organisms but their exact role is unclear. Here we show that Arabidopsis HAP2/GCS1 expression in mammalian cells is sufficient to promote cell-cell fusion. Hemifusion and complete fusion depend on HAP2/GCS1 presence in both fusing cells. Furthermore, expression of HAP2 on the surface of pseudotyped vesicular stomatitis virus and on the target cells results in HAP2-dependent virus-cell fusion. This bilateral requirement can be bypassed by replacing the plant gene with C. elegans EFF-1 somatic cell fusogen in one of the fusing cells or the virus, indicating that HAP2/GCS1 and EFF-1 share a similar fusion mechanism. Structural modeling of the HAP2/GCS1 protein family predicts that they are homologous to EFF-1 and class II fusion proteins from enveloped viruses (e.g. dengue and Zika viruses). We name this superfamily FUSEXINS: FUSion proteins essential for sexual reproduction and EXoplasmic merger of plasma membranes. Thus, Fusexins unify the origin and evolution of sexual reproduction, enveloped virus entry into cells and somatic cell fusion.

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Fusexins, HAP2/GCS1 and Evolution of Gamete Fusion

Frontiers in Cell and Developmental Biology ◽

10.3389/fcell.2021.824024 ◽

2022 ◽

Vol 9 ◽

Author(s):

Nicolas G. Brukman ◽

Xiaohui Li ◽

Benjamin Podbilewicz

Keyword(s):

Cell Fusion ◽

Generative Cell ◽

Class Ii ◽

Flowering Plants ◽

Evolutionary Relationships ◽

Gamete Fusion ◽

Somatic Fusion ◽

Viral Glycoproteins ◽

Significant Divergence ◽

Cell Cell

Gamete fusion is the climax of fertilization in all sexually reproductive organisms, from unicellular fungi to humans. Similarly to other cell-cell fusion events, gamete fusion is mediated by specialized proteins, named fusogens, that overcome the energetic barriers during this process. In recent years, HAPLESS 2/GENERATIVE CELL-SPECIFIC 1 (HAP2/GCS1) was identified as the fusogen mediating sperm-egg fusion in flowering plants and protists, being both essential and sufficient for the membrane merger in some species. The identification of HAP2/GCS1 in invertebrates, opens the possibility that a similar fusogen may be used in vertebrate fertilization. HAP2/GCS1 proteins share a similar structure with two distinct families of exoplasmic fusogens: the somatic Fusion Family (FF) proteins discovered in nematodes, and class II viral glycoproteins (e.g., rubella and dengue viruses). Altogether, these fusogens form the Fusexin superfamily. While some attributes are shared among fusexins, for example the overall structure and the possibility of assembly into trimers, some other characteristics seem to be specific, such as the presence or not of hydrophobic loops or helices at the distal tip of the protein. Intriguingly, HAP2/GCS1 or other fusexins have neither been identified in vertebrates nor in fungi, raising the question of whether these genes were lost during evolution and were replaced by other fusion machinery or a significant divergence makes their identification difficult. Here, we discuss the biology of HAP2/GCS1, its involvement in gamete fusion and the structural, mechanistic and evolutionary relationships with other fusexins.

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Virus-Mediated Cell-Cell Fusion

International Journal of Molecular Sciences ◽

10.3390/ijms21249644 ◽

2020 ◽

Vol 21 (24) ◽

pp. 9644

Author(s):

Héloïse Leroy ◽

Mingyu Han ◽

Marie Woottum ◽

Lucie Bracq ◽

Jérôme Bouchet ◽

...

Keyword(s):

Cell Fusion ◽

Fusion Proteins ◽

Target Cells ◽

Special Focus ◽

Human Pathogens ◽

General Process ◽

Tissue Organization ◽

Donor Cells ◽

Infected Cells ◽

Cell Cell

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

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Ultrastructural plasma membrane asymmetries in tension and curvature promote yeast cell fusion

10.1101/2021.03.18.435973 ◽

2021 ◽

Author(s):

Olivia Muriel ◽

Laetitia Michon ◽

Wanda Kukulski ◽

Sophie G Martin

Keyword(s):

Cell Wall ◽

Plasma Membrane ◽

Sexual Reproduction ◽

Membrane Fusion ◽

Cell Fusion ◽

Turgor Pressure ◽

Wall Material ◽

M Cells ◽

P Cells ◽

Cell Cell

Cell-cell fusion is central to the process of fertilization for sexual reproduction. This necessitates the remodeling of peri-cellular matrix or cell wall material and the merging of plasma membranes. In walled fission yeast S. pombe, the fusion of P and M cells during sexual reproduction relies on the fusion focus, an actin structure that concentrates glucanase-containing secretory vesicles for local cell wall digestion necessary for membrane fusion. Here, we present a correlative light and electron microscopy (CLEM) quantitative study of a large dataset of 3D tomograms of the fusion site, which revealed the ultrastructure of the fusion focus as an actin-containing, vesicle-dense structure excluding other organelles. Unexpectedly, the data revealed asymmetries between the two gametes: M-cells exhibit a taut and convex plasma membrane that progressively protrudes into P-cells, which exhibit a more slack, wavy plasma membrane. These asymmetries are relaxed upon plasma membrane fusion, with observations of ramified pores that may result from multiple initiations or inhomogeneous expansion. We show that P-cells have a higher exo- to endocytosis ratio than M-cells, and that local reduction in exocytosis abrogates membrane waviness and compromises cell fusion significantly more in P- than M-cells. Reciprocally, reduction of turgor pressure specifically in M-cells prevents their protrusions into P-cells and delays cell fusion. Thus, asymmetric membrane conformations, which result from differential turgor pressure and exocytosis/endocytosis ratios between mating types, favor cell-cell fusion.

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Reptilian Reovirus Utilizes a Small Type III Protein with an External Myristylated Amino Terminus To Mediate Cell-Cell Fusion

Journal of Virology ◽

10.1128/jvi.78.8.4342-4351.2004 ◽

2004 ◽

Vol 78 (8) ◽

pp. 4342-4351 ◽

Cited By ~ 66

Author(s):

Jennifer A. Corcoran ◽

Roy Duncan

Keyword(s):

Fusion Protein ◽

Membrane Fusion ◽

Cell Fusion ◽

Fusion Proteins ◽

Signal Sequence ◽

Topological Analysis ◽

Protein Family ◽

Sequence Comparisons ◽

Type Iii ◽

Cell Cell

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

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Cell Fusion and Syncytium Formation in Betaherpesvirus Infection

Viruses ◽

10.3390/v13101973 ◽

2021 ◽

Vol 13 (10) ◽

pp. 1973

Author(s):

Jiajia Tang ◽

Giada Frascaroli ◽

Xuan Zhou ◽

Jan Knickmann ◽

Wolfram Brune

Keyword(s):

Cell Fusion ◽

Neutralizing Antibodies ◽

Plasma Membranes ◽

Cancer Metastasis ◽

Syncytium Formation ◽

Tissue Growth ◽

Viral Envelope ◽

Cell Cell

Cell–cell fusion is a fundamental and complex process that occurs during reproduction, organ and tissue growth, cancer metastasis, immune response, and infection. All enveloped viruses express one or more proteins that drive the fusion of the viral envelope with cellular membranes. The same proteins can mediate the fusion of the plasma membranes of adjacent cells, leading to the formation of multinucleated syncytia. While cell–cell fusion triggered by alpha- and gammaherpesviruses is well-studied, much less is known about the fusogenic potential of betaherpesviruses such as human cytomegalovirus (HCMV) and human herpesviruses 6 and 7 (HHV-6 and HHV-7). These are slow-growing viruses that are highly prevalent in the human population and associated with several diseases, particularly in individuals with an immature or impaired immune system such as fetuses and transplant recipients. While HHV-6 and HHV-7 are strictly lymphotropic, HCMV infects a very broad range of cell types including epithelial, endothelial, mesenchymal, and myeloid cells. Syncytia have been observed occasionally for all three betaherpesviruses, both during in vitro and in vivo infection. Since cell–cell fusion may allow efficient spread to neighboring cells without exposure to neutralizing antibodies and other host immune factors, viral-induced syncytia may be important for viral dissemination, long-term persistence, and pathogenicity. In this review, we provide an overview of the viral and cellular factors and mechanisms identified so far in the process of cell–cell fusion induced by betaherpesviruses and discuss the possible consequences for cellular dysfunction and pathogenesis.

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Characterization of cell–cell fusion mediated by herpes simplex virus 2 glycoproteins gB, gD, gH and gL in transfected cells

Journal of General Virology ◽

10.1099/0022-1317-81-8-2017 ◽

2000 ◽

Vol 81 (8) ◽

pp. 2017-2027 ◽

Cited By ~ 111

Author(s):

Martin I. Muggeridge

Keyword(s):

Herpes Simplex ◽

Cell Fusion ◽

Plasma Membranes ◽

Syncytium Formation ◽

Giant Cells ◽

Minimal Set ◽

Herpes Simplex Viruses ◽

Transfected Cells ◽

Simplex Virus ◽

Cell Cell

The mechanisms by which herpes simplex viruses (HSV) mediate fusion between their envelope and the plasma membrane during entry into cells, and between the plasma membranes of adjacent infected and uninfected cells to form multinucleated giant cells, are poorly understood. Four viral glycoproteins (gB, gD, gH and gL) are required for virus–cell fusion, whereas these plus several others are required for cell–cell fusion (syncytium formation). A better understanding would be aided by the availability of a model system, whereby fusion could be induced with a minimal set of proteins, in the absence of infection. A suitable system has now been developed for HSV-2, using transfected COS7, 293 or HEp-2 cells. Insofar as the minimal set of HSV-2 proteins required to cause cell–cell fusion in this system is gB, gD, gH and gL, it would appear to resemble virus–cell fusion rather than syncytium formation. However, the ability of a mutation in gB to enhance the fusion of both transfected cells and infected cells, while having no effect on virus–cell fusion, points to the opposite conclusion. The differential effects of a panel of anti-HSV antibodies, and of the fusion-inhibitor cyclosporin A, confirm that the fusion of transfected cells shares some properties with virus–cell fusion and others with syncytium formation. It may therefore prove useful for determining how these processes differ, and for testing the hypothesis that some viral proteins prevent membrane fusion until the appropriate point in the virus life-cycle, with other proteins then overcoming this block.

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