“I’m a professional pin-in-a-haystack researcher,” geneticist Thijn Brummelkamp replies when asked why he excels at finding proteins and genes that other people haven’t found, despite the fact that some have managed to remain elusive for so long. long as forty years. His research group at the Netherlands Cancer Institute has once again succeeded in tracking down one of these ‘mystery genes’ – the gene that ensures the creation of the final form of the protein actin, a main component of our skeleton. cellular. These findings were published today in La science.
Cell biologists are very interested in actin because actin – a protein of which we produce more than 100 kilograms during our lifetime – is a main component of the cellular skeleton and one of the most abundant molecules in a cell. Large amounts can be found in every type of cell and it has many functions: it gives shape to the cell and makes it firmer, it plays an important role in cell division, it can propel cells forward and gives strength to our muscles. People with defective actin proteins often suffer from muscle diseases. Much is known about the function of actin, but how is the final form of this important protein made and what gene is behind it? “We didn’t know,” says Brummelkamp, whose mission is to discover the function of our genes.
Genetics in human haploid cells
Brummelkamp has developed a number of unique methods for this purpose during his career, which allowed him to be the first to inactivate genes on a large scale for his genetic research on human cells twenty years ago. “You can’t run into people like fruit flies and see what happens. Since 2009, Brummelkamp and his team have been using haploid cells — cells containing only one copy of each gene instead of two (one from your father and one from your mother). Although this combination of two genes is the basis of our entire existence, it also creates unwanted noise when performing a genetic experiment because mutations usually occur in only one version of a gene (that of your father , for example) and not in the other.
Versatile method for genetics in human cells
In collaboration with other researchers, Brummelkamp uses this versatile method to find the genetic causes of particular conditions. He has already shown how the Ebola virus and a number of other viruses, as well as certain forms of chemotherapy, manage to enter a cell. He also studied why cancer cells resist certain types of therapy and discovered a protein present in cancer cells that acts as a brake on the immune system. This time, he went in search of a gene that matures actin and, therefore, the skeleton of the cell.
In search of scissors
Before a protein is completely “finished” — or mature, as the researchers describe it in Science — and can fully perform its function in the cell, it usually must first be stripped of a specific amino acid. This amino acid is then cut from a protein by a pair of molecular scissors. This is also what happens with actin. We knew from which side of the actin the relevant amino acid is cut. However, no one has been able to find the enzyme that acts like scissors in this process.
Peter Haahr, postdoc in Brummelkamp’s group, worked on the following experiment: he first caused random mutations (errors) in random haploid cells. Next, he selected cells containing the immature actin by adding a fluorescently labeled antibody to his cells that matched the exact location where the amino acid was cut. In a third and final step, he studied which gene mutated after this process.
They called it ‘ACTMAP’
Then came the “eureka” moment: Haahr had traced the molecular scissors that cut the essential amino acid actin. These scissors turned out to be controlled by a gene with a previously unknown function; one with which no researcher had ever worked. This means that the researchers were able to name the gene themselves and settled on ACTMAP (ACTin MAturation Protease).
To test whether a lack of ACTMAP causes problems in living things, they turned off the gene in mice. They observed that actin in the cellular skeleton of these mice remained unfinished, as expected. They were surprised to find that the mice remained alive, but suffered from muscle weakness. The researchers conducted this research with scientists from the VU Amsterdam.
More scissors found in the cell skeleton
ACTMAP is not the first mysterious gene discovered by Brummelkamp that plays a role in the function of our cellular skeleton. Using the same method, his group has in recent years been able to detect three unknown molecular scissors that cut an amino acid from tubulin, the other main component of the cell skeleton. These scissors allow tubulin to properly perform its dynamic functions inside the cell. The last scissors (MATCAP) were discovered and described in La science This year. Thanks to this previous work on the cellular skeleton, Brummelkamp succeeded in arriving at actin.
Mission: map the 23,000 genes
“Unfortunately, our new discovery on actin does not tell us how to cure certain muscle conditions,” explains Thijn Brummelkamp. “But we provided fundamental new insights into the cell skeleton that may be useful to others later. Plus, Brummelkamp, whose mission is to one day be able to map the function of all of our 23,000 genes, can tick another new gene off his mammoth list. After all, we don’t know what half of our genes do, which means we can’t intervene if something goes wrong.