Biophysical journal最新文献

Kinesin-5 motors are bipolar tetramers that cross-link and slide antiparallel microtubules during mitotic spindle assembly. Fungal kinesin-5 motors, such as Cin8, exhibit bidirectional motility, switching between minus- and plus-end-directed stepping in response to environmental conditions; however, the molecular basis of this directional switching remains unclear. To better understand the origin of this bidirectional behavior, we investigated the motility and ATPase kinetics of two Cin8 dimers, created by fusing the motor domains to a stable coiled-coil domain from kinesin-1. To investigate the role of the proximal neck coiled-coil region in coordinating motor activity, we compared Cin8 dimers that included or lacked the first four heptads of the Cin8 neck-coil domain. By analyzing the stepping kinetics, microtubule residence times, and directional switching dynamics, we found that these Cin8 dimers move processively with a net plus-end directionality along with undirected movements, behaviors that mimic the plus-ended motility state of wild-type Cin8. However, fast minus-ended motility seen in wild-type Cin8 tetramers was not observed in the dimers. The instantaneous velocity distributions and ATPase rates were inconsistent with the undirected movement being solely due to passive diffusion, suggesting that they reflect random bidirectional stepping. Fewer undirected movements were seen on yeast microtubules, their native physiological substrate, compared with on bovine microtubules. Replacing the Cin8 neck-coil domain with a stable coiled-coil led to faster plus-end stepping, fewer undirected movements, a reduction in the microtubule binding duration, and enhanced coupling between ATP hydrolysis and plus-end stepping. Our results suggest that the native Cin8 neck coil confers flexibility between the two motor domains that contributes to bidirectional stepping, and that sustained minus-end movement requires regions outside the motor domain.

Conduction velocity along axons reflects geometric and biophysical influences whose joint statistical organization remains largely uncharacterized. Using high-resolution time-of-arrival measurements along hundreds of identified axonal branches in vitro, we quantified how propagation speed changes along trajectories. The ratio between terminal and initial velocities, ρ=v_end/v_start, follows a right-skewed distribution whose shape remains invariant across branch lengths, positions within neurons, and hierarchical aggregation levels. Local conduction profiles reveal a predominantly progressive deceleration along branches. These observations indicate a simple and robust statistical organization of slowdown, suggesting that proportional modulation of propagation speed is a consistent feature of axonal signaling in structurally variable substrates.

Animal morphogenesis involves complex tissue deformation processes, which require tight control over tissue rheology. Yet, it remains insufficiently understood how tissue rheology results from the interplay between cellular packing and forces, such as cortical tension or cell-cell adhesion. We follow a biomimetic approach to study this interplay, using oil droplets with tunable adhesion strength to mimic adhesive cells. We expose emulsions to cyclic shear and use a geometric method to quantify their rheology using only imaging data. We find that emulsions made of two droplet types change yielding behavior across subsequent shear cycles. Combining this with vertex model simulations, we show that this shift is due to a progressive compaction, which only occurs with a high adhesion differential and only under oscillatory shear. Our work thus demonstrates how gradients observed during development can lead to gradients in tissue rheology. Moreover, progressive compaction suggests the emergence of a pumping mechanism, which potentially acts in many cellular materials, from foams to tissues.