Dissecting Rate-Limiting Processes in Biomolecular Condensate Exchange Dynamics

An increasing number of biomolecules have been shown to phase-separate into biomolecular condensates — membraneless subcellular compartments capable of regulating distinct biochemical processes within living cells. The speed with which they exchange components with the cellular environment can influence how fast biochemical reactions occur inside condensates and how fast condensates respond to environmental changes, thereby directly impacting condensate function. While Fluores-cence Recovery After Photobleaching (FRAP) experiments are routinely performed to measure this exchange timescale, it remains a challenge to distinguish the various physical processes limiting fluorescence recovery and identify each associated timescale. Here, we present a reaction-diffusion model for condensate exchange dynamics and show that such exchange can differ significantly from that of conventional liquid droplets due to the presence of a percolated molecular network, which gives rise to different mobility species in the dense phase. In this model, exchange can be limited by diffusion of either the high- or low-mobility species in the dense phase, diffusion in the dilute phase, or the attachment/detachment of molecules to/from the network at the surface or throughout the bulk of the condensate. Through a combination of analytic derivations and numerical simulations in each of these limits, we quantify the contributions of these distinct physical processes to the overall exchange timescale. Demonstrated on a biosynthetic DNA nanostar system, our model offers insight into the predominant physical mechanisms driving condensate material exchange and provides an experimentally testable scaling relationship between the exchange timescale and condensate size. Interestingly, we observe a newly predicted regime in which the exchange timescale scales nonquadratically with condensate size.