Molecular Transport across Phase Boundaries*

Cells can achieve compartmentalization of biochemical processes via organelles by the selective admission of biomolecules. Organelles are enclosed by a membrane or, in the case of biomolecular condensates, by the condensate-bulk interface. 

While transport across membranes has been studied for decades, how biomolecular condensates regulate transport across their interface is less clear. This severely hampers our understanding of dynamical condensate function. A main challenge in capturing interfacial effects is the need to accurately measure several physical properties, such as diffusion and partition coefficients to meaningfully constrain our physical models. Here, we use a combination of quantitative (live) imaging techniques and theory to quantify the transport of molecules across the condensate-bulk interface in PGL-3 and FUS model condensates and compare to several theoretical models. We find that, under many conditions, the droplet interface can be explained within the experimental error bounds by taking into account the partition coefficient and different diffusivities inside and outside of the condensate. I will comment on further interfacial transport effects and discuss consequences for how cells can regulate transport into condensates given these constraints. It will be exciting to generalize our findings to more complicated (in vivo) systems and I will discuss work in progress in this direction.