STIP1/HOP promotes the formation of cytotoxic α-synuclein oligomers

Background: The accumulation of alpha-synuclein (a-Syn) as toxic oligomers, and subsequently in Lewy bodies, is a pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Molecular chaperones and cochaperones are expected to act in concert to maintain physiological activities of proteins, including a-Syn, but in neurodegeneration this process can become mal-adaptive. Transcript levels of Stress inducible phosphoprotein 1 (STIP1), a co-chaperone of Hsp90/Hsp70, are elevated in brain samples from PD patients. In synucleinopathy mouse models, STIP1 has unexpected bidirectional effects on a-Syn, with overexpression of STIP1 aggravating a-Syn toxicity, whereas knockdown of STIP1 improves toxicity and behavioural phenotypes. However, it is unclear how STIP1 enhances the toxicity of a-Syn. Methods: Here we investigate the direct impact of the interaction between STIP1 and a-Syn on the aggregation kinetics of a-Syn using a diverse and integrated set of techniques, including Nuclear Magnetic Resonance (NMR), molecular dynamics simulation, aggregation kinetics assays, electron microscopy, atomic force microscopy, and dynamic light scattering. The toxicity of a-Syn aggregates formed in the presence of STIP1 was assessed using yeast models and SH-SY5Y cell assays. Results: We unravel the mechanisms by which STIP1/HOP regulates the neurotoxicity of a-Syn. Specifically, two binding motifs in the C-terminus of a-Syn directly interact with the TPR2A domain of STIP1/HOP in a dynamic manner, competing for a shared interface on TPR2A. Binding of STIP1/HOP to a-Syn attenuates the formation of a-Syn fibrils while promoting the accumulation of high molecular weight amorphous a-Syn species. Samples of a-Syn aggregated in the presence of STIP1/HOP contain significantly more A11-positive oligomeric species and cause a greater reduction in cell viability than a-Syn aggregated in the absence of STIP1/HOP in neuronal cells. Conclusions: Biological Sciences - Biochemistry. Supplementary Information: The online version contains supplementary material available at 10.1186/s44477-026-00030-3.