Structural characterization of the intrinsically unfolded protein β-synuclein, a natural negative regulator of α-synuclein aggregation

The synuclein family of intrinsically unfolded proteins is composed of three highly homologous. members, alpha-synuclein (alpha S), beta-synuclein (beta S) and gamma-synuclein (gamma S), which are linked to neurodegenerative disorders and cancer. alpha S has been studied intensively after its identification as the major protein component of amyloid-like deposits in Parkinson's disease and dementia with Lewy bodies. OS, on the other hand, was found to act as a potent inhibitor of alpha S amyloid formation, and it is proposed as a natural regulator of its neurotoxicity. It is then of particular interest to elucidate the structural and dynamic features of the soluble state of beta S as a first step to understand the molecular basis of its anti-amyloidogenic effect on alpha S We present here the characterization of natively unstructured beta S by high resolution heteronuclear NMR techniques. A combination of pulse-field gradient, three-dimensional heteronuclear correlation, residual dipolar couplings, paramagnetic relaxation enhancement and backbone relaxation experiments were employed to characterize the ensemble of conformations populated by the protein. The results indicate that beta S adopts extended conformations in its native state, characterized by the lack of the long-range contacts as previously reported for alpha S. Despite the lack of defined secondary structure, we found evidence for transient polyproline II conformations clustered at the C-terminal region. The structuring of the backbone at the C terminus residues embedded in a polypeptide stretch rich in hydrophilic and negatively charged amino acids. The structural and functional implications of these findings are analyzed via a thorough comparison with its neurotoxic homolog alpha S.