Besides compartments wrapped in membranes, cells have droplets with no membrane at all. They form when certain proteins and RNAs stick loosely together and separate from the fluid around them, a process called phase separation (Banani et al. 2017). The first described were granules in worm embryos that flowed and fused like liquid (Brangwynne et al. 2009). Strong evidence: Several independent studies agree.
Droplets gather the molecules for one job in one place, in yeast, plants and animals alike. A seed uses a protein that forms droplets when water arrives to decide whether to germinate (Dorone et al. 2021). Their physical state matters: in some human diseases of the nervous system, droplets that should stay liquid harden into clumps (Patel et al. 2015). I studied this with the prion protein and with RNA droplets in fly embryos.
Liquid droplets. Liquid droplets are round, merge when they touch and swap molecules with their surroundings. This lets a cell collect the molecules for one job in one place, and release them again.
In my research: Droplet or clump? Phase separation of the prion protein →
Sources
- Brangwynne CP, et al. (2009) Germline P granules are liquid droplets that localize by controlled dissolution/condensation. Science 324:1729–1732. doi:10.1126/science.1172046
- Banani SF, Lee HO, Hyman AA, Rosen MK (2017) Biomolecular condensates: organizers of cellular biochemistry. Nature Reviews Molecular Cell Biology 18:285–298. doi:10.1038/nrm.2017.7
- Dorone Y, et al. (2021) A prion-like protein regulator of seed germination undergoes hydration-dependent phase separation. Cell 184:4284–4298. doi:10.1016/j.cell.2021.06.009
- Patel A, et al. (2015) A liquid-to-solid phase transition of the ALS protein FUS accelerated by disease mutation. Cell 162:1066–1077. doi:10.1016/j.cell.2015.07.047