TY - JOUR TI - Physical observables to determine the nature of membrane-less cellular sub-compartments AU - Heltberg, Mathias L AU - Miné-Hattab, Judith AU - Taddei, Angela AU - Walczak, Aleksandra M AU - Mora, Thierry A2 - Seminara, Agnese A2 - Faraldo-Gómez, José D A2 - Ronceray, Pierre VL - 10 PY - 2021 DA - 2021/10/22 SP - e69181 C1 - eLife 2021;10:e69181 DO - 10.7554/eLife.69181 UR - https://doi.org/10.7554/eLife.69181 AB - The spatial organization of complex biochemical reactions is essential for the regulation of cellular processes. Membrane-less structures called foci containing high concentrations of specific proteins have been reported in a variety of contexts, but the mechanism of their formation is not fully understood. Several competing mechanisms exist that are difficult to distinguish empirically, including liquid-liquid phase separation, and the trapping of molecules by multiple binding sites. Here, we propose a theoretical framework and outline observables to differentiate between these scenarios from single molecule tracking experiments. In the binding site model, we derive relations between the distribution of proteins, their diffusion properties, and their radial displacement. We predict that protein search times can be reduced for targets inside a liquid droplet, but not in an aggregate of slowly moving binding sites. We use our results to reject the multiple binding site model for Rad52 foci, and find a picture consistent with a liquid-liquid phase separation. These results are applicable to future experiments and suggest different biological roles for liquid droplet and binding site foci. KW - membrane-less sub-compartments KW - liquid droplet KW - polymer binding model KW - liquid-liquid phase separation KW - cellular foci JF - eLife SN - 2050-084X PB - eLife Sciences Publications, Ltd ER -