LRRC57/RABIN is a presynaptic inhibitor of Rab GTPases in glutamatergic neurons

<jats:title>SUMMARY</jats:title> <jats:p> Synaptic vesicle cycling, if not properly constrained, can result in excessive neurotransmitter release and subsequent neural pathology. Rab GTPases orchestrate synaptic vesicle trafficking through GTP-dependent interactions with effector proteins, but the restraining mechanism of these interactions is unknown. Here we identify LRRC57 (or RABIN for RAB INhibitor), a conserved brain-enriched protein in glutamatergic synapses that binds multiple GTP-loaded synaptic Rabs and competitively blocks access to their effectors. Loss of <jats:italic>Rabin</jats:italic> increased glutamate release, expanded vesicle pools, accelerated vesicle turnover, and produced circuit hyperexcitability with epileptiform activity, which was mitigated by an antiepileptic agent that targets presynaptic function. Conversely, overexpression of the <jats:italic>Rabin</jats:italic> gene suppressed neurotransmitter release and protected against induced seizures and persistent epileptiform discharges. Together, these findings define a noncanonical decoy–effector mechanism that constrains presynaptic Rab signaling to preserve excitatory circuit stability. </jats:p>