Inside a living cell, there are tiny liquid droplets where proteins gather together, like ingredients in a drop of vinaigrette. Some proteins form the droplet's scaffold, while others, clients, don't create the droplet themselves but happily jump in and change its composition. Scientists took three proteins that normally work at sites where cells attach to surfaces: two scaffold proteins (FAK and Cas) and one client (paxillin).

It turned out that both a chemical tag on FAK (phosphorylation) and the addition of paxillin lead to the same result: FAK inside the droplet becomes more abundant, while Cas becomes less so. To understand why, the researchers built a simple mathematical model, akin to a recipe for mixing liquids. The model showed that the client acts like an invisible hand, retuning interactions between scaffold proteins even if the scaffolds themselves haven't changed.

Then the scientists tested the model's predictions in real droplets, adding paxillin in varying amounts. It turned out that the mere presence of the client can push the droplet to a critical point — a state where it becomes especially sensitive to small changes. This reveals a simple way for the cell to control the composition of its liquid organelles without complex rearrangements: just call in the right client.