Evolution of CPEB4 Dynamics Across its Liquid-Liquid Phase Separation Transition.

Knowledge about the structural and dynamic properties of proteins that form membrane-less organelles in cells via liquid-liquid phase separation (LLPS) is required for understanding the process at a molecular level. We used spin labeling and electron paramagnetic resonance (EPR) spectroscopy to investigate the dynamic properties (rotational diffusion) of the low complexity N-terminal domain of cytoplasmic polyadenylation element binding-4 protein (CPEB4) across its LLPS transition, which takes place with increasing temperature. We report the coexistence of three spin labeled CPEB4 (CPEB4*) populations with distinct dynamic properties representing different conformational spaces, both before and within the LLPS state. Monomeric CPEB4* exhibiting fast motion defines population and shows low abundance prior to and following LLPS. Populations and are part of CPEB4* assemblies where corresponds to loose conformations with intermediate range motions and population represents compact conformations with strongly attenuated motions. As the temperature increased the population of component increased reversibly at the expense of component , indicating the existence of an ⇌ equilibrium. We correlated the macroscopic LLPS properties with the ⇌ exchange process upon varying temperature and CPEB4* and salt concentrations. We hypothesized that weak transient intermolecular interactions facilitated by component lead to LLPS, with the small assemblies integrated within the droplets. The LLPS transition, however, was not associated with a clear discontinuity in the correlation times and populations of the three components. Importantly, CPEB4 exhibits LLPS properties where droplet formation occurs from a preformed microscopic assembly rather than the monomeric protein molecules.