The internal organization of cells is largely determined by the architecture and orientation of the microtubule network. Microtubules serve as polar tracks for the selective transport of specific molecular motors toward either their plus or minus ends. How both motors reciprocally move microtubules and organize the network’s arrangement and polarity is unknown. Here, we combined experiments on reconstituted systems and theory to study the interaction of microtubules with both plus- and minus-end directed motors bound to a uid membrane. Depending on motor concentrations, the system could lead either to the constant transport of microtubules or to their alignment, stacking, and immobilization in regular bands that separate motors into domains of opposite polarities. In bands, microtubules shared the same polarity and segregated the two opposing motors accordingly. These regular patterns resulted from the balance of forces produced by the two motors as they walked in opposite directions along microtubules. The system was maintained in a dynamic steady state in which the directional transport of microtubule-bound motors compensates for the random diffusion of lipid-bound motors. The size of motor domains depended on their respective concentrations. The constant ow of motors allowed the system to respond to variations in motor concentrations by moving microtubules to adapt to the new force balance. The polar sorting and linear arrangement of microtubules associated with the segregation of motors of opposite polarity are typical of cellular architectures, which these data may help to better understand.

Further, we develop a general theory for externally spinning particles in a non-Markovian bath. Without any applied force, the interplay between rotation and stochastic noise-induced local deformations leads to enhanced diffusion. Our theory also uncovers that for a spinning particle, orthogonal displacement components are correlated. These correlations are non-local in time and exhibit properties akin to the Magnus deflection. We present experimental evidence supporting these non-trivial phenomena in viscoelastic fluids.

Significance: Microtubule networks in cells can adopt different polarized architectures. Most of these architectures are thought to result from the localization of microtubule nucleators. However, microtubules can be transported by molecular motors. This process can generate radial asters. It is also thought to generate bipolar spindles, but the underlying process is less clear. These data and theory show that two types of motors of opposite directions and microtubules can self-organize into parallel alignment of polarity-sorted microtubules, acting as an active barrier that segregates the two motors on either side of the polar barrier. These results reveal a self-organization process that could be responsible for the emergence of polar or bipolar structures in cells.

 References: 1. Utzschneider et al. “Force balance of opposing diffusive motors generates polarity-sorted microtubule patterns”, Proc. Natl. Acad. Sci. U.S.A. 121 (49) e2406985121 (2024). 2. “Pattern formation in a ternary mixture of two opposing molecular motors and polar microtubule laments” (in preparation).