Serine-65 phosphorylated ubiquitin associates with soluble tau and seeding activity in Alzheimer’s disease

Tau propagation is a key driver of Alzheimer’s disease (AD), yet selective markers of seeding-competent tau remain elusive. Ubiquitin phosphorylated at Serine-65 (pS65-Ub) is a PINK1-derived cellular stress signal that accumulates in AD with elevated tau pathology, but its relationship to tau seeding is unknown. Here, using postmortem brain tissue from female and male donors, we show that a subset of tau is covalently modified by pS65-Ub in AD brain. The pS65-Ub-associated tau population is elevated in AD and is enriched in soluble, high-molecular-weight, oligomeric, and hyperphosphorylated tau species. Across AD cases, soluble pS65-Ub-associated tau levels strongly correlate with tau seeding activity. Tau species isolated with pS65-Ub antibodies exhibit seeding activity, whereas removal of pS65-Ub-associated material markedly reduces seeding from AD brain extracts. Together, these findings identify pS65-Ub as a disease-enriched molecular feature of a seed-competent tau population and link cellular stress signaling with pathological tau propagation in AD. Not all selective markers of seeding-competent Tau are known. Here, the authors identify pS65-Ub-associated tau linked to pathological Tau spreading and elevated in Alzheimer’s disease. Removal of pS65-Ub reduces tau seeding activity.