The therapeutic targeting of intrinsically disordered proteins (IDPs) by small molecules has been a challenge
due to their heterogeneous conformational ensembles. A potential therapeutic strategy to alleviate
the aggregation of IDPs is to maintain them in their native monomeric state by small molecule binding.
This study investigates the structural basis of small molecule druggability of native monomeric Tau
whose aggregation is linked to the onset of Tauopathies such as Alzheimer’s disease. Initially, two available
monomeric conformational ensembles of a shorter Tau construct K18 (also termed Tau4RD) were
analyzed which revealed striking structural differences between the two ensembles, while similar number
of hot spots and small molecule binding sites were identified on monomeric Tau ensembles as on
tertiary folded proteins of similar size. Remarkably, some critical fibril forming sequence regions of Tau
(V306-K311, V275-K280) participated in hot spot formation with higher frequency compared to other
regions. As an example of small molecule binding to monomeric Tau, it was shown that methylene blue
(MB) bound to monomeric K18 and full-length Tau selectively with high affinity (Kd = 125.8 nM and
86.6 nM, respectively) with binding modes involving Cys291 and Cys322, previously reported to be
oxidized in the presence of MB. Overall, our results provide structure-based evidence that Tau can be a
viable drug target for small molecules and indicate that specific small molecules may be able to bind to
monomeric Tau and influence the way in which the protein interacts among itself and with other proteins