Microtubules (MTs) are anionic hollow cylindrical nanotubes assembled from tubulin heterodimers as building blocks, and they perform crucial functions in our cells such as cell structure maintenance, mitotic spindles in cell division process, and road for intracellular trafficking.
As a microtubule-associated protein (MAP) found exclusively and abundant in neurons, tau regulates the dynamics of and stabilizes axonal MTs. Abnormalities of tau due to such mutations as hyperphosphorylation is known to disintegrate the stabilized MTs and make tangled clumps of paired helical filaments (PHFs) of tau, which lead to neuronal death. Neuronal losses from such abnormalities are shown as lesions in many neurodegenerative diseases. Being an intrinsically disordered protein (IDP), there are no secondary nor tertiary structures of tau, which poses difficulties on understanding how tau interacts with MTs under normal and abnormal circumstances. Still. Despite the numerous structural researches on tau and MTs, where tau binds on MTs and how it stabilizes them are not well understood.
Inspired by a previous study suggesting tau binding inside of MTs in a co-assembly system led our group to conduct small angle X-ray scattering experiments on tau co-assembled MTs with variations in Taxol, a therapeutic cancer drug that is well known to stabilize MTs by binding to the inside of MTs. With respect to increasing $\Phi$, the mixing molar ratio of tau to tubulin, the inner radius of the co-assembled MTs increased until $\Phi=0.2~0.4$, and then decreased for the rest of the mixing ratio up to $\Phi=1$.
Due to the characteristics of tau being an IDP, a such more fundamental study regarding the binding ratio of tau and tubulin is necessary to understand the structure of tau co-assembled MTs. In this thesis, a study on the binding affinity of human short (3RS, 4RS)/long (3RL, 4RL) tau isoforms and tubulin was done using purified tau and tubulin, sucrose cushioning ultracentrifugation and binding assays. Interestingly, the binding behavior of tau to tubulin was isoform-dependent, the differences of isoforms being short or long accounting more than the number of repeat regions each isoform have. The differences in the mixing ratio of Taxol suggests a possibility of tau binding to tubulin competing with Taxol, maybe even for the same inner binding site.