The loss of inhibitory C-terminal conformations in disease associated P123H β-synuclein

beta-synuclein (beta S) is a homologue of alpha-synuclein (alpha S), the major protein component of Lewy bodies in patients with Parkinson's disease. In contrast to alpha S, beta S does not form fibrils, mitigates alpha S toxicity in vivo and inhibits alpha S fibril formation in vitro. Previously a missense mutation of beta S, P123H, was identified in patients with Dementia with Lewy Body disease. The single P123H mutation at the C-terminus of beta S is able to convert beta S from a nontoxic to a toxic protein that is also able to accelerate formation of inclusions when it is in the presence of alpha S in vivo. To elucidate the molecular mechanisms of these processes, we compare the conformational properties of the monomer forms of alpha S, beta S and P123H-beta S, and the effects on fibril formation of coincubation of alpha S with beta S, and with P123H-beta S. NMR residual dipolar couplings and secondary structure propensities show that the P123H mutation of beta S renders it more flexible C-terminal to the mutation site and more alpha S-like. In vitro Thioflavin T fluorescence experiments show that P123H-beta S accelerates alpha S fibril formation upon coincubation, as opposed to wild type beta S that acts as an inhibitor of alpha S aggregation. When P123H-beta S becomes more alpha S-like it is unable to perform the protective function of beta S, which suggests that the extended polyproline II motif of beta S in the C-terminus is critical to its nontoxic nature and to inhibition of alpha S upon coincubation. These studies may provide a basis for understanding which regions to target for therapeutic intervention in Parkinson's disease.