Institut Jacques Monod Seminars – Joël Lemière

Invited by the Romet-Lemonne / Jégou Lab, Joël Lemière (Assistant Researcher in the Department of Cell and Tissue Biology at UCSF, San Francisco, USA) will present an Institut Jacques Monod Seminars on the theme:

Osmotic control of nuclear size and the N/C ratio in fission yeast

Abstract:

The size of the nucleus scales robustly with cell size so that the nuclear-to-cell volume ratio (N/C ratio) is typically maintained as a constant in many cell types. Previous studies show that the N/C ratio is not determined merely by the amount of DNA but is influenced by nuclear envelope transport and mechanics. We recently developed a quantitative model for nuclear size control based upon a balance of colloid osmotic pressures in the nucleoplasm and cytoplasm. This model posits that the N/C ratio is determined by the numbers of macromolecules in the nucleoplasm and cytoplasm as well as the nuclear envelope membrane tension.

In the fission yeast S. pombe, the nucleus behaves as an ideal osmometer whose volume is primarily dictated by osmotic forces, with no detectable contribution of membrane tension. Thus, we predict that the key determinants of nuclear size in fission yeast are simply the numbers of macromolecules in the nucleoplasm and cytoplasm. Here, to test this model, we quantified the effects of massive overexpression (~10^7molecules) of exogenous proteins targeted to either the nucleus or the cytoplasm. In both cases, the resulting N/C ratio quantitatively aligned with our model’s prediction. We have thus developed a tunable system for altering the N/C ratio by overexpression of a single exogenous protein. These data provide critical quantitative support for the osmotic model for nuclear size control.