1.N.3 The Hapless2 Male Gamete Fusion Factor (Fusexin) Family
The HAP2/GCS1 gene first appeared in the common ancestor of plants, animals, and protists, and is required in the male gamete for fusion to the female gamete in the unicellular organisms Chlamydomonas and Plasmodium. Steele and Dana 2009 identified a HAP2/GCS1 gene in the genome sequence of the sponge Amphimedon queenslandica. This finding provides a continuous evolutionary history of HAP2/GCS1 from unicellular organisms into the metazoan lineage. Divergent versions of the HAP2/GCS1 gene are also present in the genomes of some but not all arthropods. By examining the expression of the HAP2/GCS1 gene in the cnidarian Hydra, we have found the first evidence supporting the hypothesis that HAP2/GCS1 was used for male gamete fusion in the ancestor of extant metazoans and that it retains that function in modern cnidarians.
The fusion of a sperm with an egg requires gamete interaction in Arabidopsis depend on small cysteine-rich EC1 (EGG CELL 1) proteins accumulating in storage vesicles of the egg cell (Sprunck et al. 2012). Upon sperm arrival, EC1-containing vesicles are exocytosed. The sperm endomembrane system responds to exogenously applied EC1 peptides by redistributing the potential gamete fusogen HAP2/GCS1 (HAPLESS 2/GENERATIVE CELL SPECIFIC 1) to the cell surface. Fertilization studies with ec1 quintuple mutants showed that successful male-female gamete interactions are necessary to prevent multiple-sperm cell delivery (Sprunck et al. 2012). Thus, mutual gamete activation, regulated exocytosis, and sperm plasma membrane modifications govern flowering plant gamete interactions.
Loss of HAPLESS 2/GENERATIVE CELL SPECIFIC 1 (HAP2/GCS1) proteins results in gamete fusion failure in diverse organisms. Arabidopsis thaliana HAP2/GCS1 is sufficient to promote mammalian cell-cell fusion (Valansi et al. 2017). Hemifusion and complete fusion depend on HAP2/GCS1 presence in both fusing cells, and expression of HAP2 on the surface of pseudotyped vesicular stomatitis virus results in homotypic virus-cell fusion. 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). This superfamily has been named 'Fusexins': fusion proteins essential for sexual reproduction and exoplasmic merger of plasma membranes. Valansi et al. 2017 suggested a common origin and evolution of sexual reproduction, enveloped virus entry into cells, and somatic cell fusion.
HAP2/GCS1, may be an ancient gamete fusogen. Using template-based structural homology modeling, Pinello et al. 2017 showed that the ectodomain of HAP2 orthologs from Tetrahymena thermophila and other species adopt a protein fold remarkably similar to the dengue virus E glycoprotein and related class II viral fusogens. Alterations to a region in and around a predicted 'fusion loop' in T. thermophila HAP2 were found to abrogate membrane pore formation in mating cells. Consistently, a synthetic peptide corresponding to the HAP2 fusion loop was found to interact directly with model membranes. Maybe sex arose as the byproduct of selection for the horizontal transfer of a 'selfish' genetic element from cell to cell via membrane fusion (Pinello et al. 2017).