Fig. 4.

Schematic illustrating the proposed mechanism of PF cluster formation and rupture. (A) Exemplary structure of the splaying MT tip determined by the competition between the stabilizing inter-PF interactions and the twist-bending dynamics of individual PFs as indicated. Schematic visualization of a pair of neighboring PFs (purple lines). For illustration, other PFs at the MT tip are not considered, and the MT body, to which the PFs are rigidly attached, is shown as a black thick line. Both the distance between the PFs and their lengths are drawn not to scale for clarity. Given this simplified representation, the order parameters that determine the extent of PF outward bending and splaying are expressed in terms of the PFs’ eigenmodes: twist-bending or CV₁ and the difference in tangential separation between the PFs or $\mathrm{\Delta }C{V}_2=C{V}_{2,{\text{PF}}_2}-C{V}_{2,{\text{PF}}_1}$. (B) Free energy profiles describing the bending and splaying dynamics of the double-PF system as a function of the nucleotide state, generated by resampling the results of Figs. 2A and 3C (Materials and Methods).