Structural Basis of Protein Condensation on Microtubules

Abstract Targeting protein for Xklp2 (TPX2) is a key factor that stimulates branching microtubule nucleation during cell division. Upon binding to microtubules, TPX2 forms condensates via liquid-liquid phase separation, which facilitates recruitment of microtubule nucleation factors and tubulin. We report the structure of the TPX2 C-terminal minimal active domain (TPX2α5-α7) on the microtubule lattice, determined by magic-angle-spinning NMR. We demonstrate that TPX2α5-α7 can form a co-condensate with soluble tubulin and a liquid layer on microtubules, by binding between two adjacent protofilaments and at the intersection of four tubulin heterodimers. These interactions stabilize the microtubules and promote the recruitment of tubulin. Aromatic residues form dense hydrophobic interactions that stabilize folding of TPX2α5-α7. This work informs on how the phase-separated TPX2α5-α7 behaves on microtubules and represents the first structural characterization of a protein that forms a liquid layer on a cytoskeletal filament.