Doublecortin Recognizes the 13-Protofilament Microtubule Cooperatively and Tracks Microtubule Ends

SummaryNeurons, like all cells, face the problem that tubulin forms microtubules with too many or too few protofilaments (pfs). Cells overcome this heterogeneity with the γ-tubulin ring complex, which provides a nucleation template for 13-pf microtubules. Doublecortin (DCX), a protein that stabilizes microtubules in developing neurons, also nucleates 13-pf microtubules in vitro. Using fluorescence microscopy assays, we show that the binding of DCX to microtubules is optimized for the lateral curvature of the 13-pf lattice. This sensitivity depends on a cooperative interaction wherein DCX molecules decrease the dissociation rate of their neighbors. Mutations in DCX found in patients with subcortical band heterotopia weaken these cooperative interactions. Using assays with dynamic microtubules, we discovered that DCX binds to polymerization intermediates at growing microtubule ends. These results support a mechanism for stabilizing 13-pf microtubules that allows DCX to template new 13-pf microtubules through associations with the sides of the microtubule lattice.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (148 K)Download as PowerPoint slideHighlights► DCX “measures” microtubule thickness by a cooperative mechanism ► DCX tracks microtubule ends like the canonical end-tracking protein EB1 ► Single-molecule experiments define the biochemical basis of double-cortex syndrome ► DCX-GFP is a fluorescence-based marker for microtubule structure