Secondary structure and solvent accessibility of a calmodulin-binding C-terminal segment of membrane-associated myelin basic protein.
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.
Homchaudhuri
Lopamudra
L
Department of Molecular Structure and Function, Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada.
De Avila
Miguel
M
Nilsson
Stina B
SB
Bessonov
Kyrylo
K
Smith
Graham S T
GS
Bamm
Vladimir V
VV
Musse
Abdiwahab A
AA
Harauz
George
G
Boggs
Joan M
JM
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D002147:Calmodulin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D051379:Mice; D004676:Myelin Basic Protein / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D000089842:Xenotropic and Polytropic Retrovirus Receptor
D002147:Calmodulin; D004676:Myelin Basic Protein; D000089842:Xenotropic and Polytropic Retrovirus Receptor; C000715268:Xpr1 protein, mouse; D002118:Calcium