Membrane fusion protein

[3] Eukaryotic genomes contain several gene families, of host and viral origin, which encode products involved in driving membrane fusion.

While adult somatic cells do not typically undergo membrane fusion under normal conditions, gametes and embryonic cells follow developmental pathways to non-spontaneously drive membrane fusion, such as in placental formation, syncytiotrophoblast formation, and neurodevelopment.

HAP2 is a fusexin (similar to viral class II) found in diverse eukaryotes including Toxoplasma, vascular plants, and fruit flies.

[6] Its origin is unclear, as the broader grouping of fusexins could be older than the viral class II with the discovery of archaeal homologs.

[7] Enveloped viruses readily overcome the thermodynamic barrier of merging two plasma membranes by storing kinetic energy in fusion (F) proteins.

They are found in reoviruses, which are non-enveloped viruses and are specialized for cell-cell rather than virus-cell fusion, forming syncytia.

They might encourage fusion by inducing membrane curvature using a variety of hydrophobic motifs and modified residues.

Based on where the root is placed, a number of different hypotheses regarding the history of these families – their horizontal transfer and vertical inheritance – can be generated.

[7] Older comparisons excluding archaeal sequences would strongly favor an interpretation where HAP2/GCS1 is acquired from a virus,[6] but the grouping of Fsx1 with HAP2/GCS1 has allowed the possibility of a much more ancient source.