Residual structure, backbone dynamics, and interactions within the synuclein family.

The human synuclein protein family includes alpha-synuclein, which has been linked to both familial and sporadic Parkinson's disease, and the highly homologous beta and gamma-synuclein. Mutations in alpha-synuclein cause autosomal dominant early onset Parkinson's, and the protein is found deposited in a fibrillar form in hereditary and idiopathic forms of the disease. No genetic link between beta and gamma-synuclein, and any neurodegenerative disease has been established, and it is generally considered that these proteins are not highly pathogenic. In addition, beta and gamma-synuclein are reported to aggregate less readily than alpha-synuclein in vitro. Indeed, beta-synuclein has been reported to protect against alpha-synuclein aggregation in vitro, as well as alpha-synuclein-mediated toxicity in vivo. Earlier, we compared the structural properties of the highly helical states adopted by all three synucleins in association with detergent micelles in an attempt to delineate the basis for functional differences between the three proteins. Here, we report a comparison of the structural and dynamic properties of the free states of all three proteins in order to shed light on differences that may help to explain their different propensities to aggregate, which in turn may underlie their differing contributions to the etiology of Parkinson's disease. We find that gamma-synuclein closely resembles alpha-synuclein in its free-state residual secondary structure, consistent with the more similar propensities of the two proteins to aggregate in vitro. beta-Synuclein, however, differs significantly from alpha-synuclein, exhibiting a lower predisposition towards helical structure in the second half of its lipid-binding domain, and a higher preference for extended structures in its C-terminal tail. Both beta and gamma-synuclein show less extensive transient long-range structure than that observed in alpha-synuclein. These results raise questions regarding the role of secondary structure propensities and transient long-range contacts in directing synuclein aggregation reactions.