Effects of confinement on the self-organization of microtubules and motors.

The regulation of the cytoskeleton is essential for the proper organization and function of eukaryotic cells. For instance, radial arrays of microtubules (MTs), called asters, determine the intracellular localization of organelles. Asters can be generated through either MT organizing center (MTOC)-dependent regulation or self-organization processes. In vivo, this occurs within the cell boundaries. How the properties of these boundaries affect MT organization is unknown. To approach this question, we studied the organization of microtubules inside droplets of eukaryotic cellular extracts with varying sizes and elastic properties. Our results show that the size of the droplet determined the final steady-state MT organization, which changed from symmetric asters to asymmetric semi-asters and, finally, to cortical bundles. A simple physical model recapitulated these results, identifying the main physical parameters of the transitions. The use of vesicles with more elastic boundaries resulted in very different morphologies of microtubule structures, such as asymmetrical semi-asters, "Y-branching" organizations, cortical-like bundles, "rackets," and bundled organizations. Our results highlight the importance of taking into account the physical characteristics of the cellular confinement to understand the formation of cytoskeleton structures in vivo.

Self-organization of microtubules and motors.

Cellular structures are established and maintained through a dynamic interplay between assembly and regulatory processes. Self-organization of molecular components provides a variety of possible spatial structures: the regulatory machinery chooses the most appropriate to express a given cellular function. Here we study the extent and the characteristics of self-organization using microtubules and molecular motors as a model system. These components are known to participate in the formation of many cellular structures, such as the dynamic asters found in mitotic and meiotic spindles. Purified motors and microtubules have previously been observed to form asters in vitro. We have reproduced this result with a simple system consisting solely of multi-headed constructs of the motor protein kinesin and stabilized microtubules. We show that dynamic asters can also be obtained from a homogeneous solution of tubulin and motors. By varying the relative concentrations of the components, we obtain a variety of self-organized structures. Further, by studying this process in a constrained geometry of micro-fabricated glass chambers, we demonstrate that the same final structure can be reached through different assembly 'pathways.