A high affinity RIM binding protein Aplip1 interaction prevents the formation of ectopic axonal active zones

Synaptic vesicles SVs fuse at active zones AZs covered by a protein scaffold, at Drosophila synapses comprised of ELKS family member Bruchpilot BRP and RIM binding protein RBP . We here demonstrate axonal co transport of BRP and RBP using intravital live imaging, with both proteins co accumulating in axonal aggregates of several transport mutants. RBP, via its C terminal Src homology 3 SH3 domains, binds Aplip1 JIP1, a transport adaptor involved in kinesin dependent SV transport. We show in atomic detail that RBP C terminal SH3 domains bind a proline rich PxxP motif of Aplip1 JIP1 with submicromolar affinity. Pointmutating this PxxP motif provoked formation of ectopic AZ like structures at axonal membranes. Direct interactions between AZ proteins and transport adaptors seem to provide complex avidity and shield synaptic interaction surfaces of pre assembled scaffold protein transport complexes, thus, favouring physiological synaptic AZ assembly over premature assembly at axonal membrane