Dissecting the Interactions between Human Serum Albumin and α-Synuclein: New Insights on the Factors Influencing α-Synuclein Aggregation in Biological Fluids

α-Synuclein (α-syn) is found to be naturally present in biofluids such as cerebrospinal fluid (CSF) and serum. Human serum albumin (HSA) is the most abundant protein found in these biofluids, which, beyond transporting hormones and drugs, also exerts a chaperone-like activity binding other proteins in blood and inhibiting their aggregation. Contrasting results are reported in the literature about the effects of albumin on α-syn aggregation. We characterized the binding region of HSA on α-syn by high-field solution NMR spectroscopy and the effect of HSA on α-syn aggregation by thioflavin-T (ThT) fluorescence under both low-ionic-strength and physiological conditions at the albumin concentration in serum and CSF. We found that HSA, at the concentration found in human serum, slows the aggregation of α-syn significantly. α-Syn interacts with HSA in an ionic strength- and pH-dependent manner. The binding is driven by hydrophobic interactions at the N-terminus under physiological experimental conditions and by electrostatic interactions at the C-terminus at low ionic strength. This work provides novel information about the proteostasis of α-syn in biofluids and supports the hypothesis of a chaperone-like behavior of HSA.