Characterization of the AT180 epitope of phosphorylated Tau protein by a combined nuclear magnetic resonance and fluorescence spectroscopy approach

► pThr231 of the Tau protein is necessary for the binding of the AT180 antibody. ► pSer235 of the Tau protein does not interfere with the AT180 recognition of pThr231. ► Epitope mapping is efficiently achieved by combining NMR and FRET spectroscopy. We present here the characterization of the epitope recognized by the AT180 monoclonal antibody currently used to define an Alzheimer’s disease (AD)-related pathological form of the phosphorylated Tau protein. Some ambiguity remains as to the exact phospho-residue(s) recognized by this monoclonal: pThr231 or both pThr231 and pSer235. To answer this question, we have used a combination of nuclear magnetic resonance (NMR) and fluorescence spectroscopy to characterize in a qualitative and quantitative manner the phospho-residue(s) essential for the epitope recognition. Data from the first step of NMR experiments are used to map the residues bound by the antibodies, which were found to be limited to a few residues. A fluorophore is then chemically attached to a cystein residue introduced close-by the mapped epitope, at arginine 221, by mutagenesis of the recombinant protein. The second step of Förster resonance energy transfer (FRET) between the AT180 antibody tryptophanes and the phospho-Tau protein fluorophore allows to calculate a dissociation constant Kd of 30nM. We show that the sole pThr231 is necessary for the AT180 recognition of phospho-Tau and that phosphorylation of Ser235 does not interfere with the binding.