👤 Abdiwahab A Musse

Myelin basic protein (MBP), specifically the 18.5 kDa isoform, is a peripheral membrane protein and a major component of mammalian central nervous system myelin. It is an intrinsically disordered and multifunctional protein that binds cytoskeletal and other cytosolic proteins to a membrane surface and thereby acquires ordered structure. These associations are modulated by post-translational modifications of MBP, as well as by interactions of MBP with Ca(2+)-calmodulin (CaM). Enzymatic deimination of usually six arginine residues to citrulline results in a decrease in the net positive charge of the protein from 19 to ≤13. This deiminated form is found in greater amounts in normal children and in adult patients with the demyelinating disease multiple sclerosis. In this paper, we examine the secondary structure of a calmodulin-binding domain, residues A141-L154, when associated with a lipid bilayer in recombinant murine 18.5 kDa forms rmC1 (unmodified) and rmC8 (pseudodeiminated). We demonstrate here by site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy that the Y142-L154 segment in membrane-associated rmC1 forms an amphipathic α-helix, with high accessibility to O(2) and low accessibility to NiEDDA. In membrane-associated rmC8, this segment assumed a structure distorted from an α-helix. Spin-labeled residues in rmC1 in solution were more immobilized on binding Ca(2+)-CaM than those in rmC8. Furthermore, rmC8 was dissociated more readily from a lipid bilayer by Ca(2+)-CaM than was rmC1. These results confirm both a predicted induced ordering upon membrane association in a specific segment of 18.5 kDa MBP, and that this segment is a CaM-binding site, with both interactions weakened by deimination of residues outside of this segment. The deiminated form would be more susceptible to regulation of its membrane binding functions by Ca(2+)-CaM than the unmodified form. Show less

The degradation of myelin in the CNS is the hallmark of multiple sclerosis. Reduction in the net positive charge of myelin basic protein (MBP), through deimination, correlates strongly with disease severity and may mediate myelin instability and loss of compaction. Using Cys scanning, spin labeling, EPR spectroscopy, and site-specific proteolysis, we show that in the membrane-bound state the primary immunodominant epitope, V83-T92, of the less cationic recombinant murine MBP C8 mimic (rmC8) forms a more highly surface-exposed and shorter amphipathic alpha-helix than in the unmodified form, recombinant murine MBP C1 mimic (rmC1), analogous to the most cationic and abundant isomer of MBP in normal myelin. Moreover, cathepsin D digested lipid-associated rmC8 3-fold faster than rmC1, and cleavage at F86-F87 occurred more readily in rmC8 than rmC1. These findings suggest a mechanism for initial loss of myelin stability and the autoimmune pathogenesis of multiple sclerosis. Show less