Fine-tuning of signalling pathways is more complex than expected

The protein structure of GFAT-1 was elucidated by protein crystallization.


Most important tasks in our body's cells are performed by thousands of proteins. As we age, however, these proteins increasingly clump together, raising the risk of serious pathologies such as Alzheimer's disease and Parkinson's disease. Since 2014, the research group of Martin Denzel at the Max Planck Institute for Biology of Ageing has been studying the protein GFAT1 in the roundworm Caenorhabditis elegans and in mammalian cells. GFAT1 controls the breakdown of clumped proteins in cells and thereby influences health and lifespan in the roundworm. Now, further investigating GFAT1, researchers stumbled upon a new level of protein regulation.

In 2014, research group leader Martin Denzel discovered the protein GFAT1 as a central regulator of a signalling pathway that coordinates the metabolism of sugars, amino acids, fatty acids and DNA building blocks and thus directly influences the life span of roundworms. Crucial to this so-called hexosamine pathway is its regulation: a metabolic product is generated via the signalling pathway, which itself binds back to GFAT1 and thus limits its own production. This is also referred to as a feedback loop. If this regulation is suspended, it leads to an increased activity of the hexosamine pathway. As a result, there are fewer protein clumps in the worms and they live longer.

A novel mutation in GFAT1 increases hexosamine pathway activity

In a new study, researchers around Martin Denzel describe a mutation in the protein GFAT1 - which also provides increased activity of the hexosamine pathway. "When you talk about mutations, you automatically think of them reducing the function or activity of a protein. However, there are also mutations that increase the activity of a protein or a specific pathway associated with it. The mutation we have now discovered in GFAT1 is such a case", explains Martin Denzel. "Our experiments showed that the mutation in GFAT1 restricts the feedback of the hexosamine pathway. Normally, high levels of a metabolite produced in the hexosamine pathway reduce GFAT1 activity. This was now no longer the case", adds Sabine Ruegenberg, first author of the study.

Regulation of GFAT1 is one level more complex than thought

But this discovery was only half of the puzzle surrounding the newly discovered GFAT1 mutation. Numerous proteins are slightly modified in our cells. Small chemical changes are made that can lower or increase a protein's activity. "GFAT1 is chemically modified at several sites. The new mutation we discovered is located at exactly one such site and prevents the chemical modification of GFAT1 at this position", says Sabine Ruegenberg. "What is new about our discovery is that this chemical modification of GFAT1 controls the feedback of the hexosamine pathway. In a first level, the hexosamine pathway is regulated by a produced metabolite, which in turn restricts the activity of GFAT1 and thus limits its own abundance. In a second level, this feedback effect is inactivated via a chemical modification to GFAT1. This is novel!" The researchers' findings show that even at the cellular level, trust is good, control is better!

"In complex organisms, a lot of data has to be integrated and metabolism has to be adjusted accordingly. The activity of individual metabolic pathways must therefore be precisely balanced", says research group leader Martin Denzel. "Fine-tuning an already existing regulatory mechanism, in our case the feedback effect of the hexosamine pathway, is an elegant way to achieve this goal. The discovery of this mechanism opens up a completely new and remarkable insight into protein regulation."

Original study:
Sabine Ruegenberg, Felix A. M. C. Mayr, Ilian Atanassov, Ulrich Baumann, Martin S. Denzel.
Protein kinase A controls the hexosamine pathway by tuning the feedback inhibition of GFAT-1.
Nature Communications, 2021
Published online: 12.04.2021, DOI: 10.1038/s41467-021-22320-y

Learn more about the research of the group of Martin Denzel.