The intrinsically disordered protein tau becomes excessively phosphorylated and aggregates into neurofibrillary tangles in Alzheimer's disease. To obtain insight into the structural consequences of phosphorylation, we characterized a mutant protein of tau in which epitopes recognized by Alzheimer diagnostic antibodies were mimicked by mutation to glutamic acid [AT8 (S199E, S202E, T205E), AT100 (T212E and S214E), and PHF1 (S396E and S404E)]. A large number of distance restraints obtained from NMR. paramagnetic relaxation enhancement in combination with ensemble conformer calculations demonstrate that pseudophosphorylation causes an opening of the transient folding of tau. Together with previous studies on the Parkinson-related protein alpha-synuclein, our data indicate that networks of transient long-range interactions are common properties of intrinsically disordered proteins and that their modulation is important for aggregation.