November 16, 2018

• Life may have begun with cells made wholly from simple proteins

  

  Amino acids build collagen – and they could have built proto-cells on early EarthLAGUNA DESIGN/SCIENCE PHOTO LIBRARY

  By Michael Marshall of New Scientist

  Did life begin in a world of proteins? It’s a minority view among origin-of-life researchers but it just got a boost. Researchers have built model cells out of nothing but simple proteins, and those cells can host some of the crucial processes of life.

  The small compartments within living cells are normally made from lipids, but in 2014, Stefan Schiller of the Albert Ludwig University of Freiburg in Germany and his colleagues made them using proteins instead. “So we asked the question if these ‘organelles’ also represent a plausible prebiotic protocell model,” he says.

  Proteins are built from long chains of amino acids. Schiller’s team made simple chains just five amino acids long. There are hundreds of naturally occurring amino acids, but the researchers used only seven kinds in their experiment to keep the approach simple and more likely to have occurred spontaneously on early Earth.

  The chains readily clumped together into spherical containers, which the team describe as “protocells”. This happened in pure water and in water with substances dissolved in it or mixed with alcohol.

  The protocells survived temperatures up to 100 °C, as well as being mixed with strong acids and alkalis. That implies they could endure “conditions imagined to be present on the early Earth”, says Schiller. The young planet was bombarded with meteorites and may have had a lot of active volcanoes.

  Protein protocells provide

  The team has also found that the protocells have a number of life-like properties. They can house large molecules over periods of weeks, just as living cells must play host to DNA and other substances. This included phospholipids, which most modern cells are made of. They also found that two protocells can fuse together to form one.

  What’s more, the protocells could host two crucial processes. An enzyme that helps DNA molecules to grow longer worked within them, as did the “translation” machinery that builds proteins from amino acids. In both cases, the mechanisms are highly evolved and cannot have been present in the first living organism, but the experiment is evidence that the protein-based protocells are compatible with the underlying processes.

  “One can argue that proteins are sufficient to allow for both membrane formation and enclosure of a reaction space, and performing catalytic reactions for self-sustaining purposes and potential division,” says Schiller.

  Other researchers have made protocells from lipids such as fatty acids, which are arguably more like modern cells. These protocells also display life-like properties, such as dividing to form “daughter” cells and hosting genetic material. But it is unclear whether any of these artificial protocells could have formed or survived on the early Earth.

  Journal reference: bioRxiv, DOI: 10.1101/463356

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