Shank3 oligomerization governs material properties of the postsynaptic density condensate and synaptic plasticity.

Cells contain numerous types of membraneless organelles or biological condensates formed via phase separation. Cellular biological condensates have broad material properties ranging from Newtonian fluids to elastic solids. How the material property of a biological condensate is regulated for cellular functions is poorly understood. Here, we discovered that, like native postsynaptic densities (PSDs), the reconstituted PSD condensate forms a soft glass material without signs of irreversible amyloid structure formation. Such glass-like PSD condensate formation is based on percolation of the PSD protein network via specific and multivalent interactions among scaffold proteins. Disruption of Shank3 SAM domain-mediated oligomerization, one type of SHANK3 mutation observed in Phelan-McDermid syndrome patients, softened the PSD condensate by weakening its network percolation, impaired synaptic transmission and plasticity, and caused autistic-like behavior in mice. Thus, our study suggests that the material properties of the PSD condensate are critical for learning and memory mediated by neuronal synapses.