Cryo-EM structures of human α-synuclein filaments reveal the molecular basis of PET tracer ACI-12589 recognition

Abstract Misfolded α-synuclein adopts distinct amyloid architectures across synucleinopathies, complicating the development of PET tracers for in vivo detection. Here, we used cryo-EM to determine how the clinically evaluated tracer ACI-12589 recognizes disease-derived α-synuclein filaments isolated from human brain. In MSA type I and II filaments, ACI-12589 occupies a conserved C-terminal cleft in protofilament A, forming a diagonal array stabilized by hydrogen bonds and cooperative aromatic stacking without detectable backbone rearrangement. Recognition of PD filaments was strongly architecture dependent: no ordered ligand density was observed in singlet Lewy folds from idiopathic PD or SNCA H50Q-associated PD, whereas ACI-12589 bound an exposed β-sheet surface in the G51D dimerised Lewy fold in a geometry distinct from that observed in MSA. These structures reveal the molecular basis of fold-dependent tracer recognition and provide a framework for interpreting variable imaging signals and developing next-generation α-synuclein PET agents.