Abstract
Microtubules are dynamic protein filaments that are involved in a number of cellular processes. Here, we report the development of a novel localized surface plasmon resonance (LSPR) biosensing approach for investigating one aspect of microtubule dynamics that is not well understood, namely, nucleation. Using a modified Mie theory with radially variable refractive index, we construct a theoretical model to describe the optical response of gold nanoparticles when microtubules form around them. The model predicts that the extinction maximum wavelength is sensitive to a change in the local refractive index induced by microtubule nucleation within a few tens of nanometers from the nanoparticle surface, but insensitive to a change in the refractive index outside this region caused by microtubule elongation. As a proof of concept to demonstrate that LSPR can be used for detecting microtubule nucleation experimentally, we induce spontaneous microtubule formation around gold nanoparticles by immobilizing tubulin subunits on the nanoparticles. We find that, consistent with the theoretical model, there is a redshift in the extinction maximum wavelength upon the formation of short microtubules around the nanoparticles, but no significant change in maximum wavelength when the microtubules are elongated. We also perform kinetic experiments and demonstrate that the maximum wavelength is sensitive to the microtubule nuclei assembly even when microtubules are too small to be detected from an optical density measurement.
Highlights
Microtubules (MTs), hollow protein filaments consisting of polymerized α/β-tubulin subunits, are involved in a number of biological processes including cell division, intracellular transport and cell motility
To investigate how the formation of MTs around AuNPs affect the characteristics of Localized Surface Plasmon Resonance (LSPR), we developed a Mie theory model of the nanoparticle system illustrated in Figure 1b, and calculated how the extinction spectrum depends on the MT layer thickness
The model was developed as a first-order approximation tool that can be used to understand how the sensing volumes of the nanoparticles depend on various physical parameters and to predict how the extinction spectrum maximum λmax depends on the degree of MT formation
Summary
Microtubules (MTs), hollow protein filaments consisting of polymerized α/β-tubulin subunits, are involved in a number of biological processes including cell division, intracellular transport and cell motility. The end of a MT is stable when it is capped by subunits whose β-tubulins are bound to GTP, and it can elongate by incorporating additional GTP-bound subunits at the end. GTP is hydrolyzed to GDP, which destabilizes the MT. Loss of the GTP cap exposes unstable GDP-tubulin subunits at the end, and results in MT disassembly. This allows a MT to cycle back and forth between phases of growth and shrinkage, a phenomenon named dynamic instability [1], and this dynamic behavior is critical to the proper functions of MTs
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