Unambiguous Tracking of Protein Phosphorylation by Fast High?Resolution FOSY NMR**

Dysregulation of post-translational modifications (PTMs) like phosphorylation is often involved in disease. NMR may elucidate exact loci and time courses PTMs at atomic resolution near-physiological conditions but requires signal assignment to individual atoms. Conventional methods for this base on tedious global that fail, as large intrinsically disordered proteins (IDPs). We present a sensitive, robust alternative rapidly obtain only the local near affected signals, based FOcused SpectroscopY (FOSY) experiments using selective polarisation transfer (SPT). prove its efficiency by identifying two sites glycogen synthase kinase 3 beta (GSK3?) human Tau40, an IDP 441 residues, where extreme spectral dispersion FOSY revealed unprimed also Ser409. broadly benefit studies other hotspots IDPs, including molecular interactions.