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Publication: Secondary structure and solvent accessibility of a calmodulin-binding C-terminal segment of membrane-reconstituted myelin basic protein

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Title Secondary structure and solvent accessibility of a calmodulin-binding C-terminal segment of membrane-reconstituted myelin basic protein
Authors/Editors* L. Homchaudhuri, M. De Avila, S.B. Nilsson, G.S.T. Smith, V.V. Bamm, A.A. Musse, G. Harauz, J.M. Boggs
Where published* Biochemistry
How published* Journal
Year* 2010
Volume 49
Number 41
Pages 8955-8966
Publisher ACS
Keywords
Link doi: 10.1021/bi100988p
Abstract
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 acquires ordered structure in these associations. These associations are modulated by post-translational modifications of MBP, as well as by interactions of MBP with Ca2+-calmodulin (CaM). Enzymatic deimination of usually 6 arginine residues to citrulline, results in a decrease in net positive charge of the protein from +19 to +13 or less. 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 (pseudo-deiminated). We demonstrate here by site-directed spin-labeling and electron paramagnetic resonance (EPR) spectroscopy that segment Y142-L154 in membrane-associated rmC1 forms an amphipathic α-helix, with high accessibility to O2 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.
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