  An increasing number of proteins with intrinsically disordered domains have
been shown to phase separate in buffer to form liquid-like phases. These
protein condensates serve as simple models for the investigation of the more
complex membrane-less organelles in cells. To understand the function of such
proteins in cells, the material properties of the condensates they form are
important. However, these material properties are not well understood. Here, we
develop a novel method based on optical traps to study the frequency-dependent
rheology and the surface tension of PGL-3 condensates as a function of salt
concentration. We find that PGL-3 droplets are predominantly viscous but also
exhibit elastic properties. As the salt concentration is reduced, their elastic
modulus, viscosity and surface tension increase. Our findings show that salt
concentration has a strong influence on the rheology and dynamics of protein
condensates suggesting an important role of electrostatic interactions for
their material properties.
