Biochemical controls: nucleolus formation (Introduction)

by David Turell , Tuesday, August 15, 2023, 17:10 (256 days ago) @ David Turell

Controlled in part by one molecule:

https://phys.org/news/2023-08-nucleolus-evolved.html

"MIT biologists have now discovered that a single scaffolding protein is responsible for the formation of one of these condensates, which forms within a cell organelle called the nucleolus. Without this protein, known as TCOF1, this condensate cannot form.

"The findings could help to explain a major evolutionary shift, which took place around 300 million years ago, in how the nucleolus is organized. Until that point, the nucleolus, whose role is to help build ribosomes, was divided into two compartments. However, in amniotes (which include reptiles, birds, and mammals), the nucleolus developed a condensate that acts as a third compartment. Biologists do not yet fully understand why this shift happened.

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"Now that the researchers know how this condensate, known as the fibrillar center, forms, they may be able to more easily study its function in cells. The findings also offer insight into how other condensates may have originally evolved in cells, the researchers say.

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"'Almost every cellular process that is essential for the functioning of the cell has been associated somehow with condensate formation and activity," Calo says. "However, it's not very well sorted out how these condensates form."

"In a 2022 study, Calo and his colleagues identified a protein region that seemed to be involved in forming condensates. In that study, the researchers used computational methods to identify and compare stretches of proteins known as low-complexity regions (LCRs), from many different species. LCRs are sequences of a single amino acid repeated many times, with a few other amino acids sprinkled in.

"That work also revealed that a nucleolar protein known as TCOF1 contains many glutamate-rich LCRs that can help scaffold biomolecular assemblies.

"In the new study, the researchers found that whenever TCOF1 is expressed in cells, condensates form. These condensates always include proteins usually found within a particular condensate known as the fibrillar center (FC) of the nucleolus. The FC is known to be involved in the production of ribosomal RNA, a key component of ribosomes, the cell complex responsible for building all cellular proteins.

"However, despite its importance in assembling ribosomes, the fibrillar center appeared only around 300 million years ago; single-celled organisms, invertebrates, and the earliest vertebrates (fish) do not have it.

The new study suggests that TCOF1 was essential for this transition from a "bipartite" to "tripartite" nucleolus.

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"The researchers also found that the essential region of TCOF1 that helps it form scaffolds is the glutamate-rich low-complexity regions. These LCRs appear to interact with other glutamate-rich regions of neighboring TCOF1 molecules, helping the proteins assemble into a scaffold that can then attract other proteins and biomolecules that help form the fibrillar center.

"'What's really exciting about this work is that it gives us a molecular handle to control a condensate, introduce it into a species that doesn't have it, and also get rid of it in a species that does have it. That could really help us unlock the structure-to-function relationship and figure out what is the role of the third compartment," Jaberi-Lashkari says.

"Based on the findings of this study, the researchers hypothesize that cellular condensates that emerged earlier in evolutionary history may have originally been scaffolded by a single protein, as TCOF1 scaffolds the fibrillar center, but gradually evolved to become more complex.

"'Our hypothesis, which is supported by the data in the paper, is that these condensates might originate from one scaffold protein that behaves as a single component, and over time, they become multicomponent," Calo says."

Comment: bit by bit the entire complexity of the single cell is being unraveled. Designs like this are not results of chance mutations.