Stochastic activation and bistability in a Rab GTPase regulatory network

Significance Small GTPases of the Rab family play a crucial role in the intracellular organization of eukaryotic cells. These molecular switches transition between active guanosine triphosphate- and inactive guanosine diphosphate-bound states in concert to regulate membrane trafficking in space and time. Despite its ubiquity, the collective network dynamics of the Rab activity switch are unknown. Here, we use in vitro reconstitution and mathematical modeling to answer fundamental long-standing questions of small GTPase regulation. We discover that the minimal Rab5 regulatory system operates as an intrinsically bistable and stochastic switch, which can precisely tune the collective GTPase state transitions in the cytoplasm. Furthermore, the indispensable positive feedback of Rab5 activation drives emergent spatial activity patterns, a general small GTPase system behavior.