For Microtubule Sliding, One Arm Is Better Than Two

BPJ_113_5.c1.inddThe versatile and dynamic network of the cytoskeleton scaffold would be stagnant and lifeless if not for the tiny nanoscopic machines called molecular motors. Kinesin motors, in particular, have captured the imagination of biologists and physicists because of their ability to transform ATP into anthropomorphic walking patterns on polar microtubule filaments, which make up a significant portion of the cytoskeleton. Recent experiments have shown that kinesin motors can crosslink adjacent microtubules and facilitate sliding between them resulting in cytoplasmic streaming in Drosophila cells. This facilitates faster distribution of molecules and organelles, and determines cell-shape.

But how do motors bring about microtubule sliding? How does the collective motion of microtubules depend on the movement of motor arms? In our work, we answer these questions by studying the effect of dimeric (one active arm, one anchored arm) and tetrameric (two active arms) kinesin motors on the dynamics of confined microtubules. Through our computer simulations we find that single-armed kinesins bring about much faster dynamics in specific regions of the confinement, compared to their two-armed counterpart. This goes against the intuitive idea that more arms pull more.

The cover image for the September 5th issue of the Biophysical Journal is our rendering of filament organization for two different motor types and the effects of these differences in the large-scale structure and dynamics of confined microtubules. The green shapes on the left represent the active motor heads that walk on polar microtubules. These are depicted as a linear array of dark-blue and yellow circles. The red blob depicts the anchor belonging to the single-armed, dimeric motor. Motor arms walk in specific directions on microtubules, and stretch, producing a sliding stress between microtubules.

The structures shown in the circular confinement on top consist of sluggish filament packages formed by tetrameric motors. The arrows at the bottom represent the highly dynamic microtubule arrangement formed by dimeric motors. Here, we also depicted the trajectories that three selected microtubules have taken. The cover image was crafted to highlight the large-scale biophysical implications of seemingly trivial and counterintuitive details in biology. Through this work we emphasize the vastly different cytoskeletal dynamics due to dimeric and tetrameric motors. By way of the trajectories, we capture the active layer of microtubules close to the circular confinement we observed for the single-armed motor systems.

– Arvind Ravichandran, Gerard Vliegenthart, Guglielmo Saggiorato, Gerhard Gompper, Thorsten Auth