Neuronal connectivity relies on molecular motor-based axonal transport of diverse cargoes. Yet the precise players and regulatory mechanisms orchestrating such trafficking events remain largely unknown. We here report the ATPase Fignl1 as a novel regulator of bidirectional transport during axon navigation. Using a yeast two-hybrid screen and coimmunoprecipitation assays, we showed that Fignl1 binds the kinesin Kif1bβ and the dynein/dynactin adaptor Bicaudal D-1 (Bicd1) in a molecular complex including the dynactin subunit dynactin 1. Fignl1 colocalized with Kif1bβ and showed bidirectional mobility in zebrafish axons. Notably, Kif1bβ and Fignl1 loss of function similarly altered zebrafish motor axon pathfinding and increased dynein-based transport velocity of Rab3 vesicles in these navigating axons, pinpointing Fignl1/Kif1bβ as a dynein speed limiter complex. Accordingly, disrupting dynein/dynactin activity or Bicd1/Fignl1 interaction induced motor axon pathfinding defects characteristic of Fignl1 gain or loss of function, respectively. Finally, pharmacological inhibition of dynein activity partially rescued the axon pathfinding defects of Fignl1-depleted larvae. Together, our results identify Fignl1 as a key dynein regulator required for motor circuit wiring.

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