The end of a chromosome is like a parking spot at the entrance to a building: it has to be occupied, or the cell will take it for damage and start repairing something that was never broken. In yeast, the role of parking attendant goes to the protein Cdc13. It recognizes the telomere, the terminal stretch of the chromosome, and latches onto it. But just how it does this stayed a mystery for a long time: does it arrive as a pair right away, like two friends in one car, or does it come alone, take the spot, and wait for a partner?
To watch this process, the researchers attached two glowing labels to the telomeric DNA that pass energy to each other when they are close together. When the protein lands on the DNA, the labels move apart and the glow changes. This made it possible to tell three states apart: an empty spot, one protein on the DNA, and two proteins. It turned out that first a single Cdc13 lands on the telomere, and only then does a second one join it.
Tests with mutants confirmed the picture. A protein that cannot pair up with a partner stays on its own. And a protein that cannot grab the DNA does not land at all. Direct counting of molecules showed that a short stretch of telomere accommodates at most two proteins, while a longer one holds up to four. What is more, the pair of proteins withstands a rise in salt concentration, whereas the lone one gets washed off.
This two-step docking explains how a cell tells a genuine chromosome end from an accidental break. A single protein quickly checks whether it has landed in the right place, and if not, it leaves. If the spot is right, a second protein arrives and locks the complex in place. It works much like a security patrol: first one officer checks out the location, then calls a partner so the two of them can stand guard together.
Now that it is clear that two proteins form the stable complex, the next step is to see its structure at atomic resolution. That will help explain why it takes exactly two to protect the telomere firmly, and how other proteins working nearby influence the process. But even now it is clear that parking at the end of a chromosome is trickier than it seemed: first one, then the second.
The dissociation constant of the Cdc13 complex with telomeric DNA is about 1 nanomolar, which means that at a protein concentration of just one molecule per billion water molecules, half the DNA is already bound. For comparison, the typical dissociation constant of many enzymes with their substrate lies in the micromolar range, that is, a thousand times weaker.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.