All Wired Up: An Exploration of the Electrical Properties of Microtubules and Tubulin

Abstract

Microtubules are hollow, cylindrical polymers of the protein α, β tubulin, that interact mechano-chemically with a variety of macromolecules. Due to their mechanically robust nature, microtubules have gained attention as tracks for precisely directed transport of nanomaterials within lab-on-a-chip devices. Largely because of the unusually negative charge on the tail-like C-termini of tubulin, recent work demonstrates that these biopolymers are also involved in a broad spectrum of intracellular electrical signaling. This thesis discusses the electrochemical properties of microtubules. Impedance spectroscopy was used to measure the impedance of microtubule networks at physiologically relevant tubulin concentrations. While polymerized microtubules increased solution capacitance, it was seen that unpolymerized tubulin at the same concentrations did not. This work indicates the role of microtubules as potential intracellular ion storage devices. Next, we aimed to understand how tubulin and microtubules respond to the presence of dimethyl sulfoxide (DMSO), a solvent commonly used in the fabrication of organic optoelectronic devices. Dynamic light scattering (DLS) and fluorescence microscopy showed that the effective size of tubulin increases in the presence of increasing DMSO volume fractions due to the formation of oligomers. In the presence of >80 % DMSO however, zeta potential experiments showed that tubulin reversibly acquired a net positive charge, causing it to form two-dimensional sheets and aggregates instead of cylindrical microtubules. Fluorescence microscopy showed that tubulin sheets and aggregates co-localize with g-C3N4 sheets while microtubule do not, further verifying the presence of a positive surface charge. This study illustrates that tubulin and its polymers, in addition to being mechanically robust, are also electrically tunable. These findings indicate the utility of DLS for monitoring early-state microtubule polymerization and tubulin oligomerization. Experiments in the future, using different contact geometries can be used to determine the solid-state properties of microtubules, and their properties in different solvents, for applications within electrically oriented nanodevices.