Conserved Eukaryotic Fusogens Can Fuse Viral Envelopes to Cells

Abstract

Caenorhabditis elegans AFF-1 and EFF-1 (CeFFs) proteins are essential for developmental cell-to-cell fusion and can merge insect cells. To study the structure and function of AFF-1, we constructed Vesicular Stomatitis Virus (VSV) displaying AFF-1 on the viral envelope, substituting the native fusogen VSVG. Electron microscopy and tomography revealed that AFF-1 formed distinct supercomplexes resembling pentameric and hexameric flowers on pseudoviruses. Viruses carrying AFF-1 infected mammalian cells only when CeFFs were on the target cell surface. Furthermore, we identified Fusion Family proteins (FFs) within and beyond nematodes and divergent members from the human parasitic nematode Trichinella spiralis and the chordate Branchiostoma floridae could also fuse mammalian cells. Thus FFs comprise an ancient family of cellular fusogens that can promote fusion when expressed on a viral particle.

Auto-fusion and the shaping of neurons and tubes

“…Many cell-fusions in C. elegans require two related fusogens, EFF-1 and AFF-1 [ 18 , 19 ], which define the FF fusogen family. Based on sequence analysis, the FF fusogens are only present in nematodes, some arthropods, a ctenophore and a protist [ 38 ]. However, FF fusogens are structurally related to class II viral fusogens [ 22 , 23 ], so it is conceivable that structural homologs exist in other species.…”

Genetic basis of cell–cell fusion mechanisms

“…EFF-1 and AFF-1 are type I membrane glycoproteins that belong to a family of homotypic cell-cell f usogens (F family) essential for somatic cell fusion. F proteins can fuse heterologous insect and mammalian cells via a hemifusion intermediate [83, 86, 87]. Moreover, enveloped viruses with F proteins substituted for their native fusogens are capable of infecting mammalian cells, which provides further evidence that they are bona fide fusogens (Figure 6) [87].…”

“…F proteins have been identified in forty-seven species, comprising mostly nematodes, but also ctenophores, arthropods, one chordate, and one protist. These fusion proteins are believed to evolve rapidly and the conservation is probably structural [87]. To identify structurally related F proteins acting in sexual and asexual reproduction, muscle development, bone development, placenta formation, immunity, stem cell, and cancer cell fusion it will be necessary to identify the precise structural and functional signatures essential for cell fusion.…”

“…(c) Cells transfected with empty vector and infected with VSVΔG-AFF-1 do not result in green cells and serve as negative controls. These experimental paradigm shows that AFF-1, in contrast to VSV-G, is necessary in both membranes to mediate virus-cell fusion [87]. …”

Identification and Characterization of LFD-2, a Predicted Fringe Protein Required for Membrane Integrity during Cell Fusion in Neurospora crassa

“…The mechanisms that direct cell-cell fusion are less well understood, although genetic screens have identified candidate molecules, termed fusogens, which are membrane-incorporated proteins that facilitate membrane fusion. For example, in Caenorhabditis elegans, two fusogenic glycoproteins, EFF-1 and AFF-1, are necessary and sufficient to fuse cells (5,6), but their conservation is restricted to nematodes and related organisms (7).…”