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Fine motor coordination depends on the precise synaptic connection between individual motor neurons and muscles. Recent studies have revealed the roles of extracellular signals such as Wnt, Netrin, and Semaphorin in synapse specificity. Little is known about their intracellular mechanisms in synapse patterning.In C. elegans, DA class motor neurons form en passant synapses along their axon on the dorsal nerve cord. Each DA neuron innervates a unique and tiled segment of muscle field by restricting its synapse to a distinct subaxonal domain - a phenomenon we term synaptic tiling. SEMAs/Semaphorins and their receptor PLX-1/Plexin were previously shown to be critical for the tiled synaptic innervation pattern between two neighboring neurons DA8 and DA9. Recently, structural and biochemical studies have predicted that mammalian Plexin acts as a GTPase activating protein (GAP) for Rap small GTPases.In this study, among three rap genes in the C. elegans genome, rap-2 is found to be required for synaptic tiling and functions through cycling between GTP- and GDP-bound forms. The genetic study has illustrated that rap-2 acts downstream of plx-1 to regulate synaptic tiling, supporting that PLX-1 acts as a RapGAP to regulate the spatial activity of RAP-2. MIG-15 is identified as an effector of RAP-2 in synaptic tiling. mig-15 mutants display severe synaptic tiling defects due to the increased synapse number of DA8 and DA9.iiiMIG-15 overexpression experiments demonstrated that MIG-15 controls both the length of synaptic domain and the number of synapses, while Plexin and RAP-2 define the length of the synaptic domain. PLX-1 overexpression experiments indicated that PLX-1 specifies synapse distribution via RAP-2 small GTPase and MIG-15 kinase. Overall, this study identified two novel components of Plexin signaling in the spatial regulation of synaptic pattern formation.