CKAP5 enables formation of persistent actin bundles templated by dynamically instable microtubules.

Cytoskeletal rearrangements and crosstalk between microtubules and actin filaments are vital for living organisms. Recently, an abundantly present microtubule polymerase, CKAP5 (XMAP215 homolog), has been reported to play a role in mediating crosstalk between microtubules and actin filaments in the neuronal growth cones. However, the molecular mechanism of this process is unknown. Here, we demonstrate, in a reconstituted system, that CKAP5 enables the formation of persistent actin bundles templated by dynamically instable microtubules. We explain the templating by the difference in CKAP5 binding to microtubules and actin filaments. Binding to the microtubule lattice with higher affinity, CKAP5 enables the formation of actin bundles exclusively on the microtubule lattice, at CKAP5 concentrations insufficient to support any actin bundling in the absence of microtubules. Strikingly, when the microtubules depolymerize, actin bundles prevail at the positions predetermined by the microtubules. We propose that the local abundance of available CKAP5-binding sites in actin bundles allows the retention of CKAP5, resulting in persisting actin bundles. In line with our observations, we found that reducing CKAP5 levels in vivo results in a decrease in actin-microtubule co-localization in growth cones and specifically decreases actin intensity at microtubule plus ends. This readily suggests a mechanism explaining how exploratory microtubules set the positions of actin bundles, for example, in cytoskeleton-rich neuronal growth cones.