| Multiple Sclerosis (MS) is a chronic demyelinating disease that affects the central nervous system and is thought to be an autoimmune disorder triggered by both genetic and environmental factors. Myelin basic protein (MBP) is a candidate autoantigen in MS and maintains the compactness of the myelin sheath. MBP undergoes an assortment of post-translational modifications, which leads to a variety of charge isomers. Deimination of arginine residues (charge +1) to citrulline (charge 0) is one of the principal modifications that gives rise to the multivalency of MBP. Charge isoforms are termed C1 to C8, with C8 being the most heavily deiminated and least charged. In the last decade a number of studies have found an association with the more heavily deiminated C8 with victims of MS.; A recombinant murine MBP was used to represent the least modified form of MBP/C1, and was called quasi-C1 (qC1). Using site-directed mutagenesis, Gln substitutions were used to engineer a mimic of the C8 isoform (subsequently called qC8). The qC8 form was found to be less effective at compacting vesicles and to have slightly less structure when bound to micellar or negatively charged lipids. Fluorescence microscopy showed that qC1 and qC8 could bind to G-actin, but only qC1 could organize F-actin into branched bundles.; Long-range molecular dynamics was done on three theoretical models of MBP, with one representing qC1, and the other two representing the citrullinated form found naturally and the quasi-deiminated form (qC8). The molecular dynamics showed that the less charged forms were less globular, and that all forms of the protein showed spontaneous generation of alpha-helices.; Site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) of the two forms of MBP when bound to vesicles emulative of the myelin sheath revealed that the C-terminus of qC8 was significantly more exposed to the aqueous phase than qC1, suggesting increased availability for immune or ligand recognition. Another site of significant importance in the MS field is an immunodominant region between Pro83 to Pro92. SDSL of every residue between Pro83 and Pro92 showed that this important site is an amphipathic alpha-helix when bound to the membrane. |
