Vesicle dynamics in synapsin-induced condensates by passive X-ray microrheology.

The collective dynamics of subcellular biological processes is often difficult to assess experimentally due to the challenges associated with spatial and temporal resolution, labeling, or multiple scattering. X-ray photon correlation spectroscopy is, in principle, well suited to probe collective dynamics by quantifying dispersion relations in complex fluids in general and biomolecular systems in particular. However, the low scattering signal and the sensitivity to radiation damage set stringent limits to many applications. Probing the dynamics of vesicles in protein-induced condensates is a case in point. Here, we use lipid vesicles with a hard silica core, called colloid-supported lipid bilayers, as labeled vesicles for enhanced X-ray contrast. We then probe structure and dynamics in solutions of vesicles and synapsin, a protein known for its property of inducing liquid-liquid phase separation and forming condensates that recruit vesicles, organizing them into clusters in presynaptic nerve terminals. The dynamics in these systems is found to exhibit evidence for both liquid-like and network-like phases. Our results reveal distinct effective-diffusion constants at varying protein concentrations. At the same time the stretched exponential decay of the correlation functions provides clear evidence for nondiffusive behavior within the condensates.