Detection of Conformation Change in Specific Residue for Membrane Protein in Monolayer

Abstract

Membrane proteins cause challenges for the major techniques (i.e., X-ray crystallography, NMR, and cryo-Electron Microscopy) to determine protein’s structure with high resolution. For example, it is difficult for membrane proteins to form single crystal structure required by X-ray. For NMR, the tumbling rate will be decreased substantially for membrane proteins. As a recent developed technique, cryo-Electron Microscopy (cryo-EM) requires mono-dispersed and unified samples in frozen state. Therefore, coexisting conformations in residues in protein’s structure are not welcome. In addition, all the techniques above cannot provide high resolution results for monolayer structure, which is the native state of membrane proteins. α-Synuclein (α-syn) is a membrane protein with 140 amino acids in the sequence and its abnormal aggregation is related to Parkinson’s disease (PD). In the sequence of α-syn, the non-amyloid component (NAC) spanning residues 61 to 95 is critical for the aggregation. In this thesis, p-polarized Multiple Incidence Angle Resolution Spectrometry (pMAIRS) was used to examine the NAC monolayer formed at interface. After two hours compression, coexisting conformation (i.e., the transition stage of the conformation change) was detected at residue 68G in NAC by pMAIRS. With residue level resolution for coexisting conformations, pMAIRS power for monolayers was exhibited. Furthermore, 68G was found to completely change its conformation from α-helix to β-sheet after three days compression. To our best knowledge, this is the first report to detect the conformation change of a specific residue in a peptide in monolayer structure. Therefore, pMAIRS can supplement the major techniques mentioned above to address protein’s (especially membrane proteins) structure.