Characterizing Biophysical Features Driving the Self-Assembly of Microtubule Nano-Arrays

Abstract

Microtubules (MTs) are biopolymeric filaments important in several cellular functions, including maintaining cell structure and providing platforms for intracellular transport. Biopolymers, including MTs, are being explored as nanostructure templates (i.e., for nanowire synthesis) due to their dimensions, self-assembly dynamics and versatile chemical functionalization. MTs have unique self-assembly processes scaling from polymerization of tubulin at the nanometer, to the head-to-tail micron-scale assembly of mature MTs, forming segmented nano-arrays. Controlling nano-array assembly requires characterization of the physicochemical properties driving self-assembly. Modelling studies have mapped the electrostatic potential of MTs and found that they are overall negatively charged with oppositely charged ends, suggesting electrostatically attractive interactions may drive assembly. Structural studies, however, suggest that end-to-end interactions of tubulin are driven by hydrophobic interactions. We demonstrate that biophysical features including electrostatics, hydrophobicity and temperature regulate the assembly of MT nano-arrays. Electrostatically shielding MTs, thus shielding the electrostatically polar ends, using salts resulted in increased rate of nano-array assembly. These results suggest that (1) decreasing the overall negative charge of MTs decreases electrostatic repulsion, increasing the probability of end-to-end interactions and (2) shielding electrostatically attractive ends of the MTs does not inhibit end-to-end interactions, suggesting that self-assembly is driven predominately by hydrophobic interactions. Finally, data exploring the role of temperature suggests that increasing temperature causes MT filaments to undergo a phase transition resulting in a discontinuous Arrhenius behavior. Together, these results provide insights into tunable parameters driving self-assembly of MT nano-arrays. Adjusting these parameters to optimize assembly will greatly aid in the application of biopolymeric templates for use in nanomaterials applications. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.