[摘要]：Myelin basic protein (MBP), specifically the 18.5 k Da 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 M BP with Ca2+-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 k Da 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 a-helix, with high accessibility to O-2 and low accessibility to NiEDDA. In membrane-associated rmC8, this segment assumed a structure distorted from an alpha-helix. Spin-labeled residues in rmC1 in solution were more immobilized on binding Ca2+-CaM than those in rmC8. Furthermore, rmC8 was dissociated more readily from a lipid bilayer by Ca2+-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 Ca2+-CaM than the unmodified form.
