Taviare L. Hawkins

Mechanics of microtubules

Microtubules are rigid cytoskeletal filaments, and their mechanics affect cell morphology and cellular processes. For instance, microtubules for the support structures for extended morphologies, such as axons and cilia. Further, microtubules act as tension rods to pull apart chromosomes during cellular division. Unlike other cytoskeletal filaments (e.g., actin) that work as large networks, microtubules work individually or in small groups, so their individual mechanical properties are quite important to their cellular function. In this review, we explore the past work on the mechanics of individual microtubules, which have been studied for over a quarter of a century. We also present some prospective on future endeavors to determine the molecular mechanisms that control microtubule rigidity.

Control of molecular shuttles by designing electrical and mechanical properties of microtubules

Molecular shuttles can be autonomously sorted by designing electromechanical properties of microtubules. Kinesin-driven microtubules have been focused on to serve as molecular transporters, called “molecular shuttles,” to replace micro/nanoscale molecular manipulations necessitated in micro total analysis systems. Although transport, concentration, and detection of target molecules have been demonstrated, controllability of the transport directions is still a major challenge. Toward broad applications of molecular shuttles by defining multiple moving directions for selective molecular transport, we integrated a bottom-up molecular design of microtubules and a top-down design of a microfluidic device. The surface charge density and stiffness of microtubules were controlled, allowing us to create three different types of microtubules, each with different gliding directions corresponding to their electrical and mechanical properties. The measured curvature of the gliding microtubules enabled us to optimize the size and design of the device for molecular sorting in a top-down approach. The integrated bottom-up and top-down design achieved separation of stiff microtubules from negatively charged, soft microtubules under an electric field. Our method guides multiple microtubules by integrating molecular control and microfluidic device design; it is not only limited to molecular sorters but is also applicable to various molecular shuttles with the high controllability in their movement directions.

The Rigidity of Aging Microtubules

Microtubules are the most rigid filament of the cytoskeleton. They define the shape of the cell, function as routes for intracellular transport, and aid during cell division. The microtubule must be rigid enough to structurally support the cell, yet dynamic to reorganize during mitosis. We are investigating the effect of “old age” on microtubule rigidity using freely fluctuating taxol-stabilized, fluorescent microtubules in vitro. We find that the persistence length does not depend on the contour length when the measurements are all taken within several hours, but the persistence length does change on the order of a day. We also find that the noise floor is higher for new (within hours of polymerization) microtubules, perhaps due to the presence of unpolymerized dimers. After 24 hours, the noise decreases and the data is the most reproducible. After 48 hours, the noise rises again, likely due to disintegration of old microtubules and aggregation of dimers. We have also tested the effects of tubulin type (bovine and porcine) and rhodamine content on the persistence length value and the error in the measurement.