Oligomers, fibrils and aggregates formed by alpha-synuclein: role of solution conditions.

The classical Hofmeister series orders ions into kosmotropes and chaotropes, based on their interaction with the solvent, water. The role of protein is mostly ignored probably because most of the proteins studied are natively folded and broadly follow this classification pattern. Recent reports suggest that the interaction of ions is different with solvent molecules of proximal layer and bulk. Intrinsically disordered proteins (IDPs) differ from globular proteins in the fraction of polar vis-à-vis hydrophobic amino acids and the absence of distinct secondary and tertiary structures. The kosmotrope, ammonium sulphate, increases the compactness of the polypeptide conformation, with differing effects for globular proteins and IDPs. For globular proteins, lowered flexibility corresponds to a more stable native structure. Using oligomer-specific and aggregation-specific antibodies and comparing with fibrillation results, we show for alpha-synuclein, an IDP, ammonium sulphate-induced compaction results in the formation of the aggregation-prone hydrophobic core, which combines with other similar moieties to form the fibrillar 'seed'. SEC-HPLC and SAXS analysis show the presence of the threshold oligomers. In the presence of the aggregation suppressor, arginine too, an oligomer is formed. This oligomer, however, is 'dead', and does not move further along the aggregation pathway. Thus, alpha-synuclein undergoes compaction in the presence of protein stabilisers, with differing consequences. In case of the chaotropes, KSCN and urea, aggregation of alpha-synuclein is partially inhibited. However, the amounts and types of aggregates formed are different in the two cases. Thus, the classical catalogue of molecules into protein stabilisers and destabilisers requires a relook for IDPs.Communicated by Ramaswamy H. Sarma.