The fusion of egg and sperm to create a new individual has fascinated science for more than 150 years. How this process came about is just one of the questions scientists ask. Now, a group of researchers scattered around the world, including Montevideo, has determined that the protein promoting sperm-egg fusion —a trait defining sexual reproduction in plants and animals— could have arisen in Archaea —prokaryotes similar to bacteria— around 3 billion years ago, long before Eukaryotes and their sexual lifestyle appeared on Earth.
Image: Héctor Romero, Martín Graña, Mauricio Langleib.
The work was published last week in Nature Communications and illustrates how nature often acts as a “sloppy engineer/artisan,” said Martín Graña, scientist at IP Montevideo Bioinformatics Unit and among the co-corresponding authors of this interdisciplinary work.
“Indeed, evolution operates with the material available at a given moment. In biology, many morphological and molecular innovations have arisen by combining and/or reusing existing parts; in a similar way as when playing with LEGOs, we discover shapes and/or functions, initially not intended for the set of pieces in the box”, added Graña. Amongst the South American scientists who took part in the research are: PhD student Mauricio Langleib (IPMon/UdelaR), Héctor Romero (Faculty of Science, Udelar). Pablo Aguilar, from the Institute of Physiology, Molecular Biology and Neurosciences (IFIBYNE-CONICET) of Buenos Aires, was also involved in this investigation; he was a former G5 PI at IP Montevideo, from 2008 to 2013.
In sexual reproduction, a new individual is built from the combination of the two genetic halves of it parents. This universal process has existed since the dawn of eukaryotic life, that is organisms whose cells have a defined nucleus, from amoebas to bears, for example.
But fusion, unlike cell division that happens all the time to make more cells, only happens on special occasions. This occurs during sex (at fertilization or mating) and also in the building of muscles and bones in vertebrates, or the human placenta. Uncontrolled cell fusion would be lethal to any organism; just imagine brain cells fusioning with those of the mouth or hands. To avoid this lack of control, plants and animals use special proteins called fusogens, which manage when and where fusion occurs.
In action, fusogens are just a few proteins that do a monumental job. There are perhaps a few tens of proteins that fuse cells, which are 10.000 times larger. This might explain why the identification of cell fusogens has been difficult for science. In fact, we still do not know the sexual fusogen of vertebrates, including humans. What is known is that in plants, invertebrate animals and protozoa,the protein that fuses gametes is called HAP2.
A few years ago, these investigators found that HAP2 has the same three-dimensional shape as the proteins used by some enveloped viruses (for example Zika, Dengue, Rubella), to get into the organism they infect. They baptized this expanded family of fusogens: fusexins. The surprising double “affiliation” of fusexins –namely, eukaryotes and viruses– created a dilemma regarding their origin: were they a eukaryotic invention co-opted by viruses, or conversely, a viral innovation captured by a primitive eukaryotic cell, with which it discovered a magic trick to invent sexual reproduction?
One more step
In a new twist, the international team showed that creatures as old as archaea contain a protein homologous to HAP2, which they named Fusexin1. Its fusogenic activity was demonstrated in baby hamster kidney cells that, by producing Fusexin1, began to fuse with each other. This implies that it could be the ancestral molecule: the mother of all “fusexins”, both viral and sexual, says Graña.
Moreover, Fusexin1 genes are embedded in pieces of DNA that are mobile. That strongly suggests that they form part of genome elements that can be shared between cells. It is well known that both bacteria and archaea practice ancient forms of sex where discrete pieces of DNA are transferred. “Finding a protein that fuses gametes in plants and other eukaryotes presumably involved in archaeal sex events is extremely exciting” says Mauricio Langleib.“This mechanism could be the precursor of eukaryotic sex as we currently know it”, the student explains.
This multidisciplinary work involved research groups —in addition to the ones already mentioned— from the Karolinska Institute (Sweden), Technion – Israel Institute of Technology, University of Lausanne (Switzerland) and the European Synchrotron Radiation Laboratory (France), as well as DeepMind, a Google company dedicated to machine learning that has shaken the area of artificial intelligence with historic achievements (AlphaGo; AlphaFold). For the study, the researchers combined bioinformatics, computational evolutionary biology, protein domain prediction with AlphaFold, X-ray crystallography, as well as functional studies showing that the ancient protein Fusexin1 functions as a fusogen.
In the near future, the next steps will be to better understand the role of Fusexin1 in archaea and to establish the evolutionary history connecting the ancient Fusexin1 with the more recent eukaryotic HAP2. Likewise, the archaeal fusexins and fusion proteins already discovered could help to understand how cells evolved from apparently simple forms to the complex life forms that reproduce sexually today.