A Comparison of the Conformational Changes of Tau Isoforms in the Tau-Tubulin Complex

Abstract

Tau is an intrinsically disordered protein found in the axons of neurons, where it functions to maintain microtubules and stabilize microtubule growth. It is present in the human nervous system as six differentially spliced isoforms, most noticeably half of which possess four imperfect repeats (4R) within the microtubule binding region (MTBR) and the others possess three of these repeats (3R). For poorly understood reasons, tau can form intracellular aggregates known as neurofibrillary tangles (NFTs) which have been implicated in the pathology of Alzheimer's disease and other tauopathies. This aggregation of tau in disease is thought to be precipitated by altered interactions between tau and tubulin (or microtubules). Previous studies indicate that some tauopathies possess isoform-specific aggregates, leading us to hypothesize that differences in microtubule binding between isoforms may be important to understanding the isoform-specific transition to aggregation. To investigate this, we determined the average conformational changes of tau 3R and 4R isoforms upon binding to tubulin via single molecule Forster resonance energy transfer (smFRET). Our constructs were labeled at several sets of residues within the MTBR with donor and acceptor fluorophores to elucidate the changes in residue distance between tau's solution conformation and its tubulin-bound conformation. The results provide insight into differences in the topological features of the tubulin-bound isoforms. Moreover, they may also elucidate mechanisms of association and dissociation of tau to microtubules, relevant to the initiation of aggregation.