Phase partitioning rules Rab domain formation, growth and identity

Diverse cellular processes are regulated by the formation of specific membrane domains displaying specific lipid and protein compositions. Liquid-liquid phase separation (LLPS) recently emerged as one possible mechanism for their biogenesis, but the examples remain scarce and the impact of LLPS properties on this process is not well established. Rab GTPases are present on all intracellular membranes and play a crucial role in membrane identity, trafficking and compartmentalization. Domain formation is thought to play a central role in Rab functions. Here we show that several Rab partners with common structural characteristics present LLPS properties, some being able to co-condensate or on the contrary presenting immiscible properties. These properties limit the recruitment of Rabs to the membrane sub-domains defined by the condensates of their specific partners. We further show that those LLPS properties control the recruitment of Rab5 to Rabaptin5 condensates ensuring Rab5 functions in regulating endocytic vesicle tethering and fusion. We propose a universal mechanism for Rab domain formation, delimitation, growth and docking based on LLPS properties of Rab partners.