RSY-1 is a local inhibitor of presynaptic assembly in C. elegans.

As fundamental units of neuronal communication, chemical synapses are composed of presynaptic and postsynaptic specializations that form at specific locations with defined shape and size. Synaptic assembly must be tightly regulated to prevent overgrowth of the synapse size and number, but the molecular mechanisms that inhibit synapse assembly are poorly understood. We identified regulator of synaptogenesis-1 (RSY-1) as an evolutionarily conserved molecule that locally antagonized presynaptic assembly. The loss of RSY-1 in Caenorhabditis elegans led to formation of extra synapses and recruitment of excessive synaptic material to presynaptic sites. RSY-1 directly interacted with and negatively regulated SYD-2/liprin-alpha, a master assembly molecule that recruits numerous synaptic components to presynaptic sites. RSY-1 also bound and regulated SYD-1, a synaptic protein required for proper functioning of SYD-2. Thus, local inhibitory mechanisms govern synapse formation.