Cooperative extraction of membrane nanotubes by molecular motors

Abstract

In eukaryotic cells, nanotubes represent a substantial fraction of transport intermediates between organelles. They are extracted from membranes by molecular motors walking along microtubules. We previously showed that kinesins fixed on giant unilamellar vesicles in contact with microtubules are sufficient to form nanotubes in vitro. Motors were attached to the membrane through beads, thus facilitating cooperative effects. Koster et al. proposed that motors could dynamically cluster at the tip of tubes when they are individually attached to the membrane. We demonstrate, in a recently designed experimental system, the existence of an accumulation of motors allowing tube extraction. We determine the motor density along a tube by using fluorescence intensity measurements. We also perform a theoretical analysis describing the dynamics of motors and tube growth. The only adjustable parameter is the motor binding rate onto microtubules, which we measure to be 4.7 ؎ 2.4 s ؊1 . In addition, we quantitatively determine, for a given membrane tension, the existence of a threshold in motor density on the vesicle above which nanotubes can be formed. We find that the number of motors pulling a tube can range from four at threshold to a few tens away from it. The threshold in motor density (or in membrane tension at constant motor density) could be important for the understanding of membrane traffic regulation in cells.giant unilamellar vesicle ͉ intracellular transport ͉ kinesin ͉ membrane tubule ͉ traffic jam

Bidirectional membrane tube dynamics driven by nonprocessive motors

“…The Kinesin motors along the bulk of membrane tube are moving freely in a fluid lipid bilayer, do not feel any force, and may walk at maximum speed toward the membrane tube tip. However, the motors at the tip experience the load of the membrane tube and their speeds are damped (7,8,16). The Gaussian-like distribution of speeds we find for Kinesin elucidates the influence of load on the cluster of motors accumulating at the tip of the membrane tube.…”

“…Colocalization of these membrane tubes with the underlying cytoskeleton has led to the finding that cytoskeletal motor proteins can extract membrane tubes (6). Motors must work collectively to extract membrane tubes (7,8), because the force needed to form a tube, F tube (9), is larger than the mechanical stall force of an individual motor (10).…”

Endoplasmic reticulum architecture: structures in flux

“…Several studies in which network formation was observed on a glass surface demonstrated a direct role for microtubules and microtubule motors in tubule formation (reviewed in [2]). Recently, two groups [40,41] have analyzed tubule generation from giant unilamellar lipid vesicles tethered to microtubules via microtubule motors. Interestingly, in the presence of energy and in the absence of any other additional proteins, branching tubules were readily created in these in vitro systems.…”

Chromosome Oscillations in Mitosis

“…All the available models for the oscillatory movement of the chromosome (often referred to as "kinetochore directional instability" [8]) have as a common feature that the kinetochore somehow controls the switching between P and AP phases [8,9,10,11]. However, the collective behavior of molecular motors can give rise to dynamical instabilities [14] which have been observed in biological and biomimetic systems [15,16]. Indeed, inhibiting chromokinesin activity leads to the disappearance of the oscillations and to the collapse of mono-oriented chromosomes onto the centrosome [4].…”