Emergent mechanics of a networked multivalent protein condensate.

Multivalent proteins can form membraneless condensates in cells by liquid-liquid phase separation, and significant efforts have been made to study their biochemical properties. Here, we demonstrate the emergent mechanics of a functional multivalent condensate reconstituted with six postsynaptic density proteins, using atomic-force-microscopy-based mesoscale rheology and quantitative fluorescence measurements. The measured relaxation modulus and protein mobility reveal that the majority (80%) of the proteins in the condensate are mobile and diffuse through a dynamically cross-linked network made of the remaining (20%) non-mobile scaffold proteins. This percolating structure gives rise to a two-mode mechanical relaxation with an initial exponential decay followed by a long-time power-law decay, which differs significantly from simple Maxwell fluids. The power-law rheology with an exponent α ≃ 0.5 is a hallmark of weak bonds' binding/unbinding dynamics in the multivalent protein network. The concurrent molecular and mechanical profiling thus provides a reliable readout for characterizing the mechanical state of protein condensates and investigating their physiological functions and associations with diseases.