Effects of Glutamate Arginylation on α-Synuclein: Studying an Unusual Post-Translational Modification through Semisynthesis.

A variety of post-translational modifications (PTMs) are believed to regulate the behavior and function of α-synuclein (αS), an intrinsically disordered protein that mediates synaptic vesicle trafficking. Fibrils of αS are implicated in neurodegenerative disorders such as Parkinson's disease. In this study, we used chemical synthesis and biophysical techniques to characterize the neuroprotective effects of glutamate arginylation, a hitherto little characterized PTM in αS. We developed semisynthetic routes combining peptide synthesis, unnatural amino acid mutagenesis, and native chemical ligation (NCL) to site-specifically introduce the PTM of interest along with fluorescent probes into αS. We synthesized the arginylated glutamate as a protected amino acid, as well as a novel ligation handle for NCL, in order to generate full-length αS modified at various individual sites or a combination of sites. We assayed the lipid-vesicle binding affinities of arginylated αS using fluorescence correlation spectroscopy (FCS) and found that arginylated αS has the same vesicle affinity compared to control protein, suggesting that this PTM does not alter the native function of αS. On the other hand, we studied the aggregation kinetics of modified αS and found that arginylation at E83, but not E46, slows aggregation and decreases the percentage incorporation of monomer into fibrils in a dose-dependent manner. Arginylation at both sites also resulted in deceleration of fibril formation. Our study represents the first synthetic strategy for incorporating glutamate arginylation into proteins and provides insight into the neuroprotective effect of this unusual PTM.