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COLLAPSIN *RF 1*, a member of the newly recognized semaphorin family *RF 2-4*, contributes to axonal pathfinding during neural development by inhibiting growth cone extension [1-5]. The mechanism of collapsin action is poorly understood. Here we use a Xenopus laevis oocyte expression system to identify molecules involved in collapsin signalling, because several experiments have raised the possibility that heterotrimeric GTP-binding proteins might participate in these events [6-9]. A collapsin response mediator protein of relative molecular mass (Mr) 62K (CRMP-62) required for collapsin-induced inward currents in X. laevis oocytes is isolated. CRMP-62 shares homology with UNC-33, a nematode neuronal protein required for appropriately directed axonal extension [10-12]. CRMP-62 is localized exclusively in the developing chick nervous system. Introduction of anti-CRMP-62 antibodies into dorsal root ganglion neurons blocks collapsin-induced growth cone collapse. CRMP-62 appears to be an intracellular component of a signalling cascade initiated by an unidentified transmembrane collapsin-binding protein.

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