From spirals to flagellar beating: How pivot-like defects control semiflexible filament dynamics in motility assays

Abstract

We demonstrate that internal pivot-like defects, arising from rigor mutant motor proteins that bind without stepping, fundamentally reshape the dynamics of semiflexible filaments in two-dimensional motility assays. Using large-scale numerical simulations, we show that such internal pivots establish a previously unrecognized boundary condition, intermediate between free and clamped filaments, that decisively governs filament behavior. Strikingly, by tuning the pivot position, motor activity, and processivity, filaments undergo sharp transitions from tightly wound spiral states to extended, flagella-like beating. Spiral formation is stabilized by a balance between motor-driven forces and bending rigidity, with intermediate stiffness yielding the most robust spirals. Unlike generic active polymer models, our framework isolates the distinct role of rigor-bound motor proteins, revealing how they function as internal control elements governing the transition between spiral and flagellar dynamics. This minimal yet physically grounded model yields experimentally testable predictions and reveals how localized defects can act as key regulators of cytoskeletal organization and dynamics.