HAP2/GCS1 is essential for gamete fusion in plants, invertebrates, and protists. Valansi et al. demonstrate that a plant HAP2 is an authentic fusion protein that can fuse animal cells.

Cell-cell fusion is inherent to sexual reproduction. Loss of HAP LESS 2/GEN ERA TIVE CELL SPE CIF IC 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.

AFF-1-coated virus infects skin cells and rarely muscle cells in C. elegans• Epithelial cell-specific infection requires eff-1 activity • Pseudoviral infection depends on the amount of eff-1 in target cells • Ectopic EFF-1 in muscle cells induces their fusion and viral infectionAbstract Viral-based vectors are widely used for fundamental and translational research; however, production of a tissue-specific targeting vector is often a challenging step. Here, we develop a cell type-specific delivery system, based on vesicular stomatitis virus lacking the viral fusion protein (fusogen) and pseudotyped with AFF-1, a C. elegans cell-to-cell bilateral fusogen from the fusexin family. Following injection of AFF-1-coated virus into the C. elegans pseudocoelom that functions as a circulatory system, we observe specific infection of different cells that express EFF-1 or AFF-1 fusogens. EFF-1-expressing epidermal cells are preferentially infected, whereas mononucleated body wall muscles (BWMs) that do not express any fusogen are rarely infected. The epidermal cells infection is abolished when the host cells do not express EFF-1, while ectopic expression of EFF-1 on BWMs induces their abnormal cell-cell fusion, produces paralyzed-dumpy animals and enhances muscle infection by AFF-1-coated viruses. Thus, our data uncover that AFF-1-coated pseudotyped viruses can be retargeted to desired cells in a controlled manner and serve as a cell-specific delivery system in nematodes with potential applications in regenerative processes, therapies such as dendritic reconnection following injuries, and studies of diverse cellular and viral fusogens.VSV-G is a class III viral fusogen whereas the Caenorhabditis elegans EFF-1 and AFF-1, which fuse somatic cells during development, are structural homologs of class II viral proteins and gamete HAP2(GCS1) fusogens from the fusexin family [14][15][16][17][18][19][20][21][22]. EFF-1 and AFF-1 also participate in maintaining neuronal architecture and neuronal reconnection following injury [23][24][25][26][27]. Studied viral glycoproteins, including VSV-G, use a unilateral fusion mechanism that depends on the expression of receptors only on the target cells [28,29]. In contrast, EFF-1 and AFF-1 are bilateral fusogens-their presence is required on the membranes of both apposing cells to mediate fusion [6,16,18,30,31]. These two fusogens can act in either a homotypic or a heterotypic manner [6,16,18,21] and mediate heterotypic cell fusion of Sf9 insect and Baby Hamster Kidney (BHK) cells [6,16]. Finally, VSV viruses containing a GFP substituting the VSV-G coding sequence (VSV∆G) [32,33] that are coated with AFF-1 (VSV∆G-AFF-1) specifically infect AFF-1 or EFF-1 expressing BHK cells [6]. Thus, in contrast to the pseudotyped virus coated with the native, unilateral G glycoprotein (VSVΔG-G), infection by VSVΔG-AFF-1 requires fusogen expression on the host cell membrane. To date, however, VSVΔG coated with AFF-1, EFF-1 or any other non-viral fusogen have not been tested for infection in a living organism ...