Immobilization approaches can affect protein dynamics: a surface-enhanced infrared spectroscopic study on lipid–protein interactions

Abstract

The intrinsically disordered Parkinson disease protein α-synuclein (αS) performs conformational changes induced by intermolecular protein–protein as well as by protein-membrane interactions. Aggregation of αS is a hallmark for the disease, however the role of the membrane in the aggregation process still needs to be clarified. We used a surface-enhanced infrared absorption (SEIRA) spectroscopic approach to investigate the effect of lipid interactions on αS conformation. The near-field detection of SEIRA allows to study exclusively structural changes of immobilized αS with the advantage that the supernatant remains undetected and thus does not interfere with the spectral read-out. self-assembled monolayer (SAMs) of mixed NHS-PEG-SH linker and MT(PEG)₄ spacer molecules were utilized to immobilize αS. The linker/spacer composition of the SAM was adjusted to prevent αS–αS interactions. Two different methods were applied for site-specific (C-terminal and N-terminal) αS immobilization. The immobilized protein was then exposed to lipid vesicles and SEIRA difference spectra were recorded to monitor the αS conformation over time. Irrespective of the used immobilization method, αS tethering hindered lipid-induced conformational changes. The spectra also indicate that a fraction of the immobilized αS eventually desorbs from the surface into the supernatant solution. Desorbed αS performs conformational changes and formation of β-structured aggregates is observed upon interaction with either lipid vesicles or supplementary αS. Our study demonstrates that αS aggregates only when the protein is free in solution and that surface immobilization procedures, commonly used in many analytical applications, can change the dynamic behavior of proteins thereby affecting protein structure and function.