Biology subjects

Betterton, M.

Publications and source records attributed to Betterton, M..

8 recordsLinked to original sources

bioRxiv2026

Feedback between filament spacing, crosslinker binding, and self-organization in cytoskeletal bundles

The lateral spacing between filaments in crosslinked cytoskeletal bundles is a critical yet poorly understood physical parameter that affects force generation, transport, and bundle architecture. We develop a biophysical model of crosslinkers and crosslinking motors on filament pairs. Motor/crosslinker binding sets the filament spacing, which in turn biases which motors/crosslinkers can bind. Crosslinking motors generate pulling forces that bring antiparallel filaments closer together, while non-motor crosslinkers with a preferred binding angle exert repulsive torques that maintain larger spacing. We demonstrate these principles in a model of the fission yeast anaphase mitotic spindle midzone, where microtubules form a square array with nearest-neighbor spacing 2-5 times smaller than the length of crosslinking proteins. Our model shows that motor-crosslinker interactions alone are sufficient to drive self-organization into this experimentally observed geometry. Furthermore, the feedback between geometry and binding creates strong indirect cooperativity, because crosslinkers establish spacing that favors binding of similar-length proteins, leading to history-dependent states that persist long after individual protein binding equilibration. This feedback mechanism in which crosslinkers control geometry and geometry controls crosslinker binding should operate in any multi-crosslinker-filament system and represents a general self-organizing principle for cytoskeletal networks.

biophysics↗

bioRxiv2025

Micron-scale protein transport along microtubules by kinesin-driven shepherding

How microscopic interactions give rise to cellular-scale order in the crowded environment of cells remains an open problem. Far-from-equilibrium mixtures of active and passive molecules self-organize, but the principles remain unclear. Microtubules are cytoskeletal polymers composed of 13 protofilaments bound by mixtures of actively moving motors and passively diffusing microtubule-associated proteins (MAPs), serving as a model system for self-organization on a multi-lane lattice. Here, we combine computational modeling, analytic theory, and in vitro reconstitution to demonstrate that motors can "shepherd" diffusive MAPs by rectifying Brownian motion into net directional drift without motor-MAP binding. Our model reveals that biased movement increases when fewer protofilaments are accessible or when lateral diffusion between protofilaments is limited, identifying local dimensionality and diffusion as key parameters governing spatial patterning. Notably, shepherding depends not on individual motor processivity but on the total number of motors bound to microtubules. We experimentally recapitulate micron-scale shepherding with a kinesin-1 motor (K401) and a diffusive MAP (PRC1), which show no detectable binding. These findings reveal shepherding as an emergent mechanism by which ensembles of motors generate micron-scale spatial patterning and transport of MAPs without direct binding interactions.

biophysics↗

bioRxiv2025

PRC1 resists microtubule sliding in two distinct resistive modes due to variations in the separation between overlapping microtubules

Crosslinked cytoskeletal filament networks provide cells with a mechanism to regulate cellular mechanics and force transmission. An example in the microtubule cytoskeleton is mitotic spindle elongation. The three-dimensional geometry of these networks, including the overlap length and lateral microtubule spacing, likely controls how forces can be regulated, but how these parameters evolve during filament sliding is unknown. Recent evidence suggests that the crosslinker PRC1 can resist microtubule sliding by two distinct modes: a braking mode and a less resistive coasting mode. To explore how molecular-scale mechanisms influence network geometry in this system, we developed a computational model of sliding microtubule pairs crosslinked by PRC1 that reproduces the experimentally observed braking and coasting modes. Surprisingly, we found that the braking mode was associated with a substantially smaller lateral separation between the crosslinked microtubules than the coasting mode. This closer separation aligns the PRC1-mediated forces against sliding, increasing the resistive PRC1 force and dramatically reducing sliding speed. The model also finds an emergent similar average sliding speed due to PRC1 resistance, because higher initial sliding speed favors the transition to braking. Together, our results highlight the importance of the three-dimensional geometric relationships between crosslinkers and microtubules.

biophysics↗

bioRxiv2023

Distinct regions of the kinesin-5 C-terminal tail are essential for mitotic spindle midzone localization and sliding force

Kinesin-5 motor proteins play essential roles during mitosis in most organisms. Their tetrameric structure and plus-end-directed motility allow them to bind to and move along antiparallel microtubules, thereby pushing spindle poles apart to assemble a bipolar spindle. Recent work has shown that the C-terminal tail is particularly important to kinesin-5 function: the tail affects motor domain structure, ATP hydrolysis, motility, clustering, and sliding force measured for purified motors, as well as motility, clustering, and spindle assembly in cells. Because previous work has focused on presence or absence of the entire tail, the functionally important regions of the tail remain to be identified. We have therefore characterized a series of kinesin-5/Cut7 tail truncation alleles in fission yeast. Partial truncation causes mitotic defects and temperature-sensitive growth, while further truncation that removes the conserved BimC motif is lethal. We compared the sliding force generated by cut7 mutants using a kinesin-14 mutant background in which some microtubules detach from the spindle poles and are pushed into the nuclear envelope. These Cut7-driven protrusions decreased as more of the tail was truncated, and the most severe truncations produced no observable protrusions. Our observations suggest that the C-terminal tail of Cut7p contributes to both sliding force and midzone localization. In the context of sequential tail truncation, the BimC motif and adjacent C-terminal amino acids are particularly important for sliding force. In addition, moderate tail truncation increases midzone localization, but further truncation of residues N-terminal to the BimC motif decreases midzone localization.

cell biology↗

bioRxiv2020

Motor guidance by long-range communication through the microtubule highway

Coupling of motor proteins within arrays drives muscle contraction, flagellar beating, chromosome segregation, and other biological processes. Current models of motor coupling invoke either direct mechanical linkage or protein crowding, which rely on short-range motor-motor interactions. In contrast, coupling mechanisms that act at longer length scales remain largely unexplored. Here we report that microtubules can physically couple motor movement in the absence of short-range interactions. The human kinesin-4 Kif4A changes the run-length and velocity of other motors on the same microtubule in the dilute binding limit, when 10-nm-sized motors are separated by microns. This effect does not depend on specific motor-motor interactions because similar changes in Kif4A motility are induced by kinesin-1 motors. A micron-scale attractive interaction potential between motors is sufficient to recreate the experimental results in a computational model. Unexpectedly, our theory suggests that long-range microtubule-mediated coupling not only affects binding kinetics but also motor mechanochemistry. Therefore, motors can sense and respond to motors bound several microns away on a microtubule. These results suggest a paradigm in which the microtubule lattice, rather than being merely a passive track, is a dynamic medium responsive to binding proteins to enable new forms of collective motor behavior.

biophysics↗

bioRxiv2019

Length regulation of microtubule antiparallel overlaps by molecular motors

During cell division, microtubules in the mitotic spindle form antiparallel overlaps near the center of the spindle. Kinesin motor proteins alter microtubule polymerization dynamics to regulate the length of these overlaps to maintain spindle integrity. Length regulation of antiparallel overlaps has been reconstituted with purified microtubules, crosslinkers, and motors. Here we develop a theory of steady-state overlap length which depends on the filament plus-end motor concentration, determined by a balance between motor arrival (motor binding and stepping in the overlap) and motor departure (motor unbinding from filament tips during depolymerization) in the absence of motor-driven sliding. Assuming that motors processively depolymerize and exhibit altered binding kinetics near MT plus-ends improves the agreement between theory and experiment. Our theory explains the origin of the experimentally observed critical concentration, a minimum motor concentration to observe a steady-state overlap length.

biophysics↗

bioRxiv2019

Design principles of selective transport through biopolymer barriers

In biological systems, polymeric materials block the movement of some macromolecules while allowing the selective passage of others. In some cases, binding enables selective transport, while in others the most inert particles appear to transit most rapidly. To study the general principles of filtering, we develop a model motivated by features of the nuclear pore complex (NPC) which are highly conserved and could potentially be applied to other biological systems. The NPC allows selective transport of proteins called transport factors which transiently bind to disordered, flexible proteins called FG Nups. While the NPC is tuned for transport factors and their cargo, we show that a single feature is sufficient for selective transport: the bound-state motion resulting from transient binding to flexible filaments. Interchain transfer without unbinding can further improve selectivity, especially for crosslinked chains. We generalize this observation to model nanoparticle transport through mucus and show that bound-state motion accelerates transport of transient nanoparticle application, even with clearance by mucus flow. Our model provides a framework to control binding-induced selective transport in bipolymeric materials.

biophysics↗

bioRxiv2018

Theory of cytoskeletal reorganization during crosslinker-mediated mitotic spindle assembly

Cells grow, move, and respond to outside stimuli by large-scale cytoskeletal reorganization. A prototypical example of cytoskeletal remodeling is mitotic spindle assembly, during which micro-tubules nucleate, undergo dynamic instability, bundle, and organize into a bipolar spindle. Key mechanisms of this process include regulated filament polymerization, crosslinking, and motor-protein activity. Remarkably, using passive crosslinkers, fission yeast can assemble a bipolar spindle in the absence of motor proteins. We develop a torque-balance model that describes this reorganization due to dynamic microtubule bundles, spindle-pole bodies, the nuclear envelope, and passive crosslinkers to predict spindle-assembly dynamics. We compare these results to those obtained with kinetic Monte Carlo-Brownian dynamics simulations, which include crosslinker-binding kinetics and other stochastic effects. Our results show that rapid crosslinker reorganization to microtubule overlaps facilitates crosslinker-driven spindle assembly, a testable prediction for future experiments. Combining these two modeling techniques, we illustrate a general method for studying cytoskeletal network reorganization.

cell biology↗