Dynein self-organizes while translocating the centrosome in T-cells

Centrosome translocation upon T-cell activation is constrained by actin reorganization. Consequently, we propose that dynein is attached to mobile anchors in the membrane. Using computer simulations, we find that dynein self-organizes into clusters to form a stalklike microtubule bundle that connects to the centrosome.

The EB1–Kinesin-14 complex is required for efficient metaphase spindle assembly and kinetochore bi-orientation

Kinesin-14s are conserved molecular motors required for high-fidelity chromosome segregation, but their specific contributions to spindle function have not been fully defined. Here, we show that key functions of budding yeast Kinesin-14 Cik1-Kar3 are accomplished in a complex with Bim1 (yeast EB1). Genetic complementation of mitotic phenotypes identifies a novel KLTF peptide motif in the Cik1 N-terminus. We show that this motif is one element of a tripartite binding interface required to form a high-affinity Bim1–Cik1-Kar3 complex. Lack of Bim1-binding by Cik1-Kar3 delays cells in mitosis and impairs microtubule bundle organization and dynamics. Conversely, constitutive targeting of Cik1-Kar3 to microtubule plus ends induces the formation of nuclear microtubule bundles. Cells lacking the Bim1–Cik1-Kar3 complex rely on the conserved microtubule bundler Ase1/PRC1 for metaphase spindle organization, and simultaneous loss of plus-end targeted Kar3 and Ase1 is lethal. Our results reveal the contributions of an EB1–Kinesin-14 complex for spindle formation as a prerequisite for efficient kinetochore clustering and bi-orientation.

Nano-positioning and tubuline conformation determine transport of mitochondria along microtubules

Correct spatiotemporal distribution of organelles and vesicles is crucial for healthy cell functioning and is regulated by intracellular transport mechanisms. Controlled transport of bulky mitochondria is especially important in polarized cells such as neurons that rely on these organelles to locally produce energy and buffer calcium. Mitochondrial transport requires and depends on microtubules which fill much of the available axonal space. How mitochondrial transport is affected by their position within the microtubule bundles is not known. Here, we found that anterograde transport, driven by kinesin motors, is susceptible to the molecular conformation of tubulin both in vitro and in vivo. Anterograde velocities negatively correlate with the density of elongated tubulin dimers, similar to GTP-tubulin, that are more straight and rigid. The impact of the tubulin conformation depends primarily on where a mitochondrion is positioned, either within or at the rim of microtubule bundle. Increasing elongated tubulin levels lowers the number of motile anterograde mitochondria within the microtubule bundle and increases anterograde transport speed at the microtubule bundle rim. We demonstrate that the increased kinesin step processivity on microtubules consisting of elongated dimers underlies increased mitochondrial dynamics. Our work indicates that the molecular conformation of tubulin controls mitochondrial motility and as such locally regulates the distribution of mitochondria along axons.

Spatio-temporal correlations between catastrophe events in a microtubule bundle: a computational study

We explore correlations between dynamics of different microtubules in a bundle, via numerical simulations, using a one-dimensional stochastic model of a microtubule. The GTP-bound tubulins undergo diffusion-limited binding to the tip. Random hydrolysis events take place along the filament, and converts GTP-tubulin to GDP-tubulin. The filament starts depolymerising when the monomer at the tip becomes GDP-bound; in this case, detachment of GDP-tubulin ensues and continues until either GTP-bound tubulin is exposed or complete depolymerisation is achieved. In the latter case, the filament is defined to have undergone a “catastrophe”. Our results show that, in general, the dynamics of growth and catastrophe in different filaments are coupled to each other; closer the filaments are, the stronger the coupling. In particular, all filaments grow slower, on average, when brought closer together. The reduction in growth velocity also leads to more frequent catastrophes. More dramatically, catastrophe events in the different filaments forming a bundle are found to be correlated; a catastrophe event in one filament is more likely to be followed by a similar event in the same filament. This propensity of bunching disappears when the filaments move farther apart.