Common chemical label shields proteins from degradation

Researchers at the University of Bergen (UiB) have discovered that proteins use a common chemical label to protect them from degradation, which affects motility and aging.

Proteins are key to all processes in our cells and understanding their functions and regulation is paramount.

For many years, we have known that almost all human proteins are modified by a specific chemical group, but its functional effect remains unchanged.”

Thomas Arnesen, Professor, Department of Biomedicine, University of Bergen

She explains:

“One of the most common protein modifications in human cells is N-terminal acetylation, which is the addition of a small chemical group (acetyl) to the early end (N-terminus) of a protein. The modification is initiated by a group of N-terminal acetyltransferases (NATs). Although the enzyme is “ubiquitous” in human cells, the functional role of this modification remains enigmatic, Arnesen explained.

He is an investigator in a new study that reveals that a key function of this protein modification is to protect the protein from degradation and is essential for normal longevity and motility.

CRISPR-Cas9 technology sheds new light on N-terminal acetylation

To address this question, molecular biologist and researcher Sylvia Verland spent two years at the Donnelly Center for Cellular and Biomolecular Research at the University of Toronto, Canada, supported by a FRIPRO mobility grant from the Research Council of Norway.

Here, he used the established CRISPR-Cas9 technology and robust screening platform available in an optimal scientific environment to determine the functional roles of human NAT enzymes. Verland focused on NatC, a major human NAT enzyme, and genome-wide screening of human NatC KO cells revealed many human genes likely to be involved in N-terminal acetylation.

“This research would not have seen the light of day without the inspiring scientific environment at the Donnelly Center combined with financial support from the Marie Skłodowska-Curie Actions,” said Verland.

Back in the Arnesen lab at UIB, Verland explored the molecular implications of his genetic findings with the help of PhD student Ini Kjos and other lab members. Biochemical, cell biology and proteomics experiments have demonstrated that N-terminal acetylation acts as a shield to protect many proteins from protein degradation. Proteins lacking N-terminal acetylation were recognized by the cellular degradation machinery (Figure 1).

“N-terminal acetylation has the ability to dictate the lifespan of a protein and affects our cells in many ways,” Verland said. “This is true for humans, and it’s also true for fruit flies, which are a very useful model for studying these protein changes,” he continued.

N-terminal acetylation may influence aging

In parallel, a research group of Rui Martinho, an investigator at the University of Aveiro in Portugal, was working on the biological effects of NatC-mediated N-terminal acetylation using a fruit fly model (Drosophila).

Postdoctoral researcher Rui Silva and colleagues studied flies lacking N-terminal acetylation. The two teams decided to combine their efforts and coordinated their experiments over the past two years. Flies lacking NatC were viable, but these flies had reduced longevity and reduced motility with age (Figure 2). These effects can be partially reversed by expressing a conserved protein between flies and humans that is found to be a key target of NatC protection.

Decoding the NatC puzzle

In conclusion, using an unbiased and global genetic screen combined with cellular phenotyping, the team discovered a general function for N-terminal acetylation to protect proteins from degradation in human cells.

Molecular investigations have defined the cellular factors (ubiquitin ligases) responsible for the degradation of a major class of human proteins when lacking N-terminal acetylation. A NatC-mediated protective role of specific proteins is evident in both human cells and fruit flies. The impact of these pathways on longevity and mobility in older individuals underscores the important role of protein N-terminal acetylation.

“This work uncovers some secrets and shows how N-terminal acetylation shapes individual protein fates,” concludes Thomas Arnesen.

The Norwegian part of this work was supported by research grants from the Research Council of Norway, the Norwegian Health Authorities of Western Norway, the Norwegian Cancer Society and the European Research Council (ERC).