Doublecortin recognizes the longitudinal curvature of the microtubule end and lattice

Microtubule ends have distinct biochemical and structural features from those of the lattice. Several proteins that control microtubule behavior can distinguish the end of a microtubule from the lattice. The end-binding protein EB1, for example, recognizes the nucleotide state of microtubule ends, which are enriched in GTP-tubulin. EB1 shares its binding site with Doublecortin (DCX), a protein expressed in developing neurons. We showed recently that DCX binds with highest affinity to microtubule ends.

Here we show that DCX recognizes microtubule ends by a novel mechanism based on lattice curvature. Using single-molecule microscopy, we show that DCX "comets" do not elongate at faster microtubule growth rates and DCX does not recognize two out of three GTP analogs. We demonstrate that DCX binds with higher affinity to curved microtubule lattices than to straight ones. We find that curvature recognition is a property of single DCX molecules. Straightening of protofilaments (pfs) at microtubule ends with paclitaxel significantly attenuates end-recognition by DCX, but not EB1. Mutations in DCX found in patients with double cortex syndrome disrupted curvature recognition.

We propose a model in which DCX recognizes microtubule ends through specific interactions with their structure. We conclude that microtubule ends have two distinct features that proteins can recognize independently, namely a structural feature related to curvature and nucleotide state.