Scientists discover a potential key missing link protein linking eukaryotes and prokaryotes

Macro-microscopic structures of OdinTubulin. Credit: Caner Akıl, Kosuke Fujishima and Robert Robinson / adapted from C. Akıl et al. Science. Adv. 2022

All modern organisms belong to two classes, eukaryotes and prokaryotes. Eukaryotes (from the Greek meaning “true nucleus”) have a cell nucleus that contains most of the cell’s genetic information and includes organisms such as humans, plants, and fungi. In prokaryotes, the contents of the cell, including its genetic material, are diffusely distributed. Eukaryotes generally have much larger genomes, which is generally thought to be related to their greater complexity. Prokaryotes, however, should not be outclassed, make up the bulk of terrestrial biomass. Generally speaking, prokaryotes seem to approach the process of life by making copies of themselves as quickly as they can, while eukaryotes survive by being highly specialized in making copies of themselves- same in a special way.

A new study by a team of scientists including researchers from the Earth-Life Science Institute (ELSI) at the Tokyo Institute of Technology, the Research Institute for Interdisciplinary Sciences (RIIS) at the University of ‘Okayama and the University of Nagoya may have identified a key intermediary in the transition from prokaryotes to eukaryotes.

Most prokaryotes have unique circular genomes and reproduce asexually, which means that when it comes time for their cells to divide, they simply have to copy their circular genomes and ensure that a circular copy ends up in each daughter cell. In contrast, eukaryotes typically have multiple linear chromosomal genomes and often reproduce sexually, which means they must ensure that an appropriate set of multiple copied chromosomes ends up in each daughter cell. This is a much more complicated process, and it is still unclear how the precise sorting required for this process stems from the simpler system used by prokaryotes.

In prokaryotes, typically, the circular chromosome attaches to the cell membrane, then as the cell grows and begins to separate into two cells, this attachment ensures that one copy will end up in each daughter cell . This process is much more complex in eukaryotes. In eukaryotes, a complex protein scaffold forms, mainly based on the protein tubulin. Tubulin forms long fibers, which help attract copied chromosomes to the poles of the dividing cell. Failures in this process, called non-disjunction, result in the presence of an unequal number of chromosomes in daughter cells, giving rise to disorders known collectively as aneuploidy. In humans, this leads to many recognizable birth defects, perhaps most notably Down syndrome.

Scientists discover a potential key missing link protein linking eukaryotes and prokaryotes

Comparison of OdinTubulin assembly and microtubule assembly/disassembly used in chromosome segregation in eukaryotic cells. Credit: Robert Robinson / adapted from C. Akıl et al. Science. Adv. 2022

The origin of the processes that allowed eukaryotic cells to precisely separate chromosomes has been a mystery for some time. Prokaryotes produce a protein similar to tubulin, but rather than helping move chromosomes around, it helps pinch off the parent’s daughter cell. This protein is known as FtsZ.

When prokaryotes and eukaryotes split is yet another evolutionary mystery, this split is generally thought to have occurred around 1-2 billion years ago, but recently a group of organisms that seem to be evolutionarily intermediate has been identified. These organisms are known as the Archaea of ​​Asgard, the name of which refers to Norse creation myths. A team of scientists, including ELSI’s Caner Akil and Kosuke Fujishima, have now identified a protein similar to eukaryotic tubulin in the genome of a species of Asgard isolated from the thermal waters of Yellowstone National Park.

“These new archaeal asgard proteins, which scientists have named OdinTubulin, in another shoutout to the Nordic pantheon, are similar to both eukaryotic tubulins and prokaryotic FtsZ proteins,” says Samson Ali, from the University of Nagoya and the University of Okayama. Linh T. Tran, Okayama University, adds that “OdinTubulin may therefore represent an evolutionary intermediate between the prokaryotic FtsZ and eukaryotic microtubule-forming tubulins.”

The research has been published in Scientists progress.

Expanding the phylogenetic diversity of Asgard’s archaea

More information:
Caner Akıl et al, Tubulin structure and dynamics of Odinarchaeota and implications for the evolution of eukaryotic microtubules, Scientists progress (2022). DOI: 10.1126/sciadv.abm2225

Provided by Tokyo Institute of Technology

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