Archaeal origins of gamete fusion
Sexual reproduction consists of genome reduction by meiosis and subsequent gamete fusion. Fusexins, are a superfamily of structurally conserved proteins (fusogens) that mediate three types of exoplasmic membrane fusion events: eukaryotic gamete fusion, somatic cell-cell fusion, and entry of enveloped viruses to host cells. Here, we unveil a fourth family of fusexins in genomes of Archaea and metagenomes from different environments. We provide structural, functional and evolutionary evidence indicating that these archaeal proteins are cellular fusogens: First, the crystal structure of a trimeric archaeal Fusexin1 reveals high similarity to eukaryotic fusexins and in particular to gamete GCS1/HAP2 fusogens from plants and protists. Second, ectopically expressed Fusexin1 can fuse mammalian cells, and this process involves the conserved fusion loop and the additional globular domain. Third, genomic analyses indicate that archaeal fusexins are carried by integrated mobile genetic elements, suggesting potential roles in cell fusion and gene exchange in archaea, as well as different scenarios for the evolutionary history of fusexins. We will also discuss work in progress to
