Accumulation of microtubule-associated protein Tau in neurofibrillary tangles is a major hallmark of Alzheimer’s disease (AD). In vivo, Tau is chemically modified with a variety of post-translational modifications, many associated with its pathological fibrillar state (the paired helical filaments PHFs). Beside phosphorylation, polyubiquitin (polyUb), with chain linkages at lysine (K)6, K11, and K48, marks Tau at K254, K311 and K353 in PHFs purified from AD brains. Since these modifications are located within the microtubule binding domain, that includes the region of Tau involved in fibril formation, and it was recently shown that polyUb chains are able to form amyloid-like fibrils, we expect that polyubiquitination greatly impact Tau functions and aggregation process with effect on the development of AD. In this framework this project aims to define the effect of polyubiquitination on the structural properties of Tau and on its aggregation pathway to fibrils. The planned studies require a method to obtain high yield of the protein Tau homogeneously modified via regiospecific isopeptide bond with polyUb chains of defined linkage and length. In our knowledge, no structural information is today available for polyubiquitinated-Tau (polyUb-Tau). To obtain Tau modified with polyUb at specific positions we will exploit an innovative method that combines a cutting-edge semi-synthetic protocol with the genetic-expansion technology to covalently link the C-terminus of a polyUb to specific lysines of Tau protein. Then, with advanced NMR techniques and biophysical studies we aim to provide detailed information on the aggregation mechanism of polyUb-Tau. The study of the effects of polyubiquitination on Tau structure and aggregation can reveal structural changes that impair the proteasomal-dependent degradation of Tau ubiquitinated species, with possible novel feedback for medical research.