The ARHGAP32 isoform PX-RICS is specifically targeted to inhibitory synapses by binding to gephyrin.
Precise regulation of excitatory-inhibitory balance is critical for neural circuit function, and its disruption underlies neurodevelopmental disorders such as autism spectrum disorder (ASD) and epilepsy. PX-RICS, a major ARHGAP32 splice variant enriched at inhibitory synapses, has been linked to cognitive dysfunctions; however, the molecular basis of its synaptic targeting and function remains unknown. Here, we identify gephyrin as the primary synaptic anchor for PX-RICS and determine the 2.2 Å crystal structure of their complex. Our structural analysis reveals that the N-terminal gephyrin-binding region (GBR) engages gephyrin E-domain through conserved hydrophobic interactions, explaining the isoform-specific targeting of PX-RICS (but not RICS) to inhibitory synapses. This binding interface overlaps with the neurotransmitter receptor binding site on gephyrin, suggesting a competitive yet dynamic interaction landscape among these inhibitory synaptic proteins. Arhgap32ΔGBR mice exhibit key features of ARHGAP32-related disorders, including impaired social novelty recognition and increased seizure susceptibility, indicating that gephyrin-mediated anchoring is critical for PX-RICS to function in inhibitory synapses.