CKAP5 enables seeding of actin bundles by exploratory microtubules.

Abstract

Neuronal growth cones are cellular structures responsible for neuronal guidance in response to the cues presented to the cell from its surrounding. The function of the growth cones is heavily dependent on the cytoskeleton. Actin filaments in the form of networks and bundles are present at the periphery of the growth cone. Microtubule bundles are located in the central domain from which individual exploratory microtubules grow out towards the cell periphery. Recently, CKAP5 (in human, or its Xenopus and Drosophila equivalent, XMAP215 and MSMP, respectively) has been reported to play a role in mediating cross-talk between microtubules and actin filaments in growth cones. However, the molecular mechanism of this process is unknown. Here, we found in a reconstituted system that CKAP5 enables seeding of prevailing actin bundles by dynamic microtubules. We observed CKAP5 binding to microtubules with higher affinity than to single actin filaments. CKAP5 bound to the microtubule lattice can recruit actin filaments, which form bundles around the microtubules at concentrations insufficient for microtubule-independent actin bundle formation. When the microtubules depolymerize the actin bundles prevail at the positions and orientations predetermined by the microtubules. This readily suggests a mechanism explaining how exploratory microtubules in filo- and lamellipodia set the positions of actin bundles, for example in neuronal growth cones.