In a yeast cell, the nucleus takes up about six percent of the volume, like a small ball in a room. A hundred years ago, biologists noticed that bigger cells have bigger nuclei, but why this is so remained a mystery. Some talked about protein motors, others about envelope tension, still others about shape. A new study tests a simple physical idea: the nucleus inflates like a pump, driven by the pressure of invisible particles.
Scientists took yeast and made them produce a glowing protein either inside the nucleus or outside it. When the protein accumulated in the nucleus, the nucleus swelled sevenfold, and its share of the cell grew from six to nineteen percent. When the protein stayed in the cytoplasm, the nucleus shrank and deformed. So it is not the mass of protein that matters, but the number of particles pushing the envelope from inside.
To separate pressure from simple displacement, the researchers replaced large protein complexes in frog egg extract with small molecules of the same mass. The nuclei became smaller, even though the total weight of protein did not change. This confirmed that size is dictated by osmotic pressure, not by occupied space. The model estimated the number of active particles in the cell at roughly seven to twenty million, which matches data on protein counts.
This explains why nuclei change size as cells age, and with them genome activity changes too. The osmotic pump, once considered mere background, turned out to be a regulator of cell life. If we learn to control this pressure, we could influence cell fate, from stem cells to cancer cells.
The concentration of osmotically active macromolecules in the cell is only about 0.24–0.68 mM, hundreds of times lower than the concentration of small molecules, yet it is enough to determine nuclear size.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.