Salt-dependent rheology and surface tension of protein condensates using optical traps

TL;DR

This study introduces a new optical trap-based method to analyze how salt concentration affects the rheology and surface tension of protein condensates, revealing the importance of electrostatic interactions in their material properties.

Contribution

The paper presents a novel optical trap technique to measure the frequency-dependent rheology and surface tension of protein condensates as a function of salt concentration.

Findings

Salt concentration influences the elastic modulus, viscosity, and surface tension of protein condensates.

PGL-3 droplets are mainly viscous but also show elastic behavior.

Electrostatic interactions play a key role in the material properties of protein condensates.

Abstract

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.