IQSEC2/BRAG1 may modulate postsynaptic density assembly through Ca²⁺-induced phase separation

IQSEC2, a high-confidence neurodevelopmental disorder risk gene product, is essential for neuronal development and synaptic plasticity. Previous studies established that IQSEC2 dynamically regulates synaptic signaling via Ca²⁺-dependent release of autoinhibition. In this study, using in vivo mouse models and in vitro biochemistry approaches, we discover that IQSEC2 orchestrates postsynaptic density assembly and dynamics via Ca²⁺-triggered phase separation. Mechanistically, Ca²⁺-induced conformational opening leads to phase separation–mediated condensation of IQSEC2 at synapses, a process that requires the N-terminal multimerization domain and intrinsically disordered regions of IQSEC2. We identified a single-point mutation, F367A, in IQSEC2, which exhibits constitutive activity by structurally mimicking the Ca²⁺-activated state of the WT protein. Mice carrying the Iqsec2_F367A mutation have elevated basal synaptic transmission and impaired activity-dependent plasticity assayed in hippocampal neurons and spatial learning deficits. Thus, IQSEC2 can bidirectionally modulate synaptic strengths via Ca²⁺-dependent phase separation, and dysregulation of phase separation may be a contributing factor in IQSEC2-related neurodevelopmental disorders.