The OP18/stathmin phospho-gradient. (A), The structure of alpha/beta tubulin subunits bound OP/18 stathmin. (B), Structure of the FRET sensor COPY. Cyan fluorescent protein (CFP) is bound to the N-terminus, and yellow fluorescent protein (YFP) is bound to the C-terminus of OP/18 stathmin. COPY adopts a rigid structure when bound to tubulin preventing FRET between CFP and YFP. Phosphorylation of COPY releases tubulin allowing interaction of CFP with YFP to produce FRET emissions. (C), A gradient of FRET emissions surrounding mitotic chromatin in HeLa cells is indicative of a gradient of phosphorylated OP/18 stathmin. (D), OP/18 stathmin could act as a local activator and long-range inhibitor of Aurora B kinase activation through effects on microtubule stability. Aurora B is activated by microtubules . Phosphorylation of tubulin-bound OP18/stathmin increases free tubulin, inhibits its ability to induce microtubule catastrophe (promoting microtubule stability/polymerization), and increases OP18/stathmin diffusion by a factor of 2. Phosphorylated stathmin can then diffuse to the periphery where it is de-phosphorylated resulting in tubulin binding/sequestration and promotion of microtubule catastrophe. The gradient of Aurora B activity is shown in pink. (E), Computer generated Turing pattern based on the difference in diffusion coefficients of free and tubulin-bound OP18/stathmin .