Structural and functional insights into the nuclear role of Parkinson's disease-associated α-synuclein as a histone chaperone.

α-Synuclein (αSyn) plays a critical role in the pathogenesis of 'Synucleinopathies'. Although increased nuclear αSyn localization induces neurotoxicity, its definitive physiological role remains elusive. Previous studies on nuclear αSyn are limited to its interactions with individual histones and dsDNA, leaving a significant gap in understanding its interactions with assembled histone H2a-H2b dimer and (H3-H4)2 tetramer, as well as its role in chromatin regulation. Here, we demonstrate that αSyn binds specifically to both H2a-H2b and (H3-H4)2 with high affinity. Truncation studies reveal that αSyn(1-103) region interacts with (H3-H4)2, while the acidic (121-140) C-terminal end is crucial for H2a-H2b binding and contains a conserved DEF/YxP motif present in other dimer-binding histone chaperones. High-resolution structure of αSyn(121-140) with H2a-H2b complex reveals that αSyn adopts two binding modes (BM-1 and BM-2). Nonetheless, the αSyn C-terminal end in both modes overlap but runs in opposite orientations, specifically interacting with the H2a-L2 and H2b-L1 loop regions of the dimer and cap the H2a-R78 residue. Mutational analysis confirms that αSyn-Y136 and P138 residues, part of the DEF/YxP motif, together with H2a-R78, are critical for αSyn-(H2a-H2b) interaction. The chaperoning assay supports αSyn's function as a histone chaperone, suggesting the potential role of αSyn in the nucleosome assembly/disassembly process.