Scientists identify rare evolutionary intermediates to understand the origin of eukaryotes

A new study by Yale scientists provides a key insight into a landmark event in the early evolution of life on Earth: the origin of the cell nucleus and complex cells called eukaryotes.

While simple prokaryotic bacteria were formed during the first billion years of Earth, the origin of eurkaryotes, the first cells with nuclei, took much longer. Dating from 1.7 to 2.7 billion years ago, an ancient prokaryote has been transformed for the first time with a compartment, the nucleus, designed to further protect its genetic material from malaria. environment (such as damage caused by UV rays). From this ancient event, relatively simple organisms, such as bacteria, were transformed into more sophisticated organisms that eventually gave birth to all modern animals, plants and fungi.

The details of this key event have remained inaccessible for many years as no transitional fossils have been discovered so far.

In a study conducted by Dr. Sergey Melnikov, of the Dieter Söll Laboratory of the Department of Molecular Biophysics and Biochemistry of Yale University, eventually found these missing fossils. To do this, they did not rely on the discovery of clay or rocks, but instead looked deep into the living living cells, called Archaea – the organisms believed to most closely resemble the old intermediates between the more complex bacteria and cells that we now call eukaryotic cells. .

These transition shapes look nothing like the traditional fossils we think of, such as dinosaur bones deposited in the soil or insects trapped in amber. Known as ribosomal proteins, these particular transition forms are about 100 million times smaller than our body. Melnikov and his colleagues discovered that ribosomal proteins can be used as living "molecular fossils", whose ancient origins and structure may be the key to understanding the origin of the cell nucleus.

"Simple life forms, such as bacteria, are similar to a studio: they have a single interior space that is not subdivided into separate rooms or compartments, but more complex organisms such as fungi and animals. and plants, consist of cells that are separated into several compartments, "explained Melnikov. "These microscopic compartments are connected to each other by" doors "and" doors. "To pass these doors and doors, molecules that inhabit living cells must carry special identification badges, some of which are called nuclear localization, or NLS. "

Seeking to better understand when NLS patterns might have emerged in ribosomal proteins, the Yale team evaluated their conservation among the ribosomal proteins of the three domains of life.

To date, NLS motifs have been characterized in ten ribosomal proteins of several eukaryotic species. They compared all the NLS motifs found in eukaryotic ribosomal proteins (from 482 species) and tried to find a match in bacteria (2,951 species) and Archaea (402 species).

Surprisingly, they discovered that four proteins – uL3, uL15, uL18 and uS12 – had NLS-like motifs not only in Eukarya, but also in Archaea. "Contrary to our expectations, we found that NLS-type patterns are preserved in all archaic branches, including the oldest superphylum, called DPANN," Melnikov said.

But since the Archaea do not have a nucleus, the logical question that arises is why do they have these identifiers? And what was the original biological function of these identifiers in non-compartmentalized cells? "

"If you think of an equivalent of our discovery in the macroscopic world, it looks like the discoveries made over the last century of bird-like dinosaurs, such as Caudipteryx zoui," said Melnikov. "These ancient, flightless birds have shown that it took millions of years for dinosaurs to develop wings, yet it is striking that during the first few million years their wings were not good enough to withstand the leak. "

Similarly, the study by Melnikov and colleagues suggests that although NLSs were not initially designed to allow cellular molecules to pass through microscopic gates or between cellular compartments, they could have emerged to perform a function. similar biological: helping molecules to make their own biological partners.

As Melnikov explains: "Our analysis shows that in complex cells, the same IDs allowing proteins to pass through the microscopic gates are also used to recognize the biological partners of these proteins." In complex cells, IDs fulfill two similar concepts, but in the Archaea these identifiers play only one of these functions: they help proteins recognize their biological partners and distinguish them from thousands of other molecules that float in a cell. "

But what has led to the evolution of these IDs among cellular proteins?

As Melnikov explains, "When life began to appear on the surface of our planet, the oldest life forms were probably composed of a very limited number of molecules, so it was relatively easy to these molecules find a specific partner among all other molecules However, as cells grew in size and complexity, it was possible, even likely, that the old rules of specific interactions between cellular molecules needed to be redefined, and that is how IDs have been introduced into the structure of protein cells – to help these proteins more easily identify their molecular partners in the complex environment of a complex cell.We return to the analogy with bird-like dinosaurs, our study illustrates the remarkable similarity between how evolution evolves in the macroscopic world and the one where evolution is evolving. in the world that Darwin has never seen – the microscopic world of invisible molecules that inhabit living cells. "