title: Cytoplasmic domain of human myelin protein zero likely folded as beta-structure in compact myelin.
abstract: Myelin protein zero (P0 or P0 glycoprotein), the major integral membrane protein in peripheral nervous system myelin, plays a key role in myelin membrane compaction and stability. While the structure of P0 extracellular domain was determined by crystallography, the paucity of any structural data on the highly positive-charged P0 cytoplasmic domain (P0-cyt) has greatly limited our understanding of the mechanism of P0 function. Here, using circular dichroism and intrinsic fluorescence spectroscopy, we attempted to elucidate the structure of human P0-cyt (hP0-cyt) in membrane mimetic environments composed of detergents or lipid vesicles. We found that the secondary structure of P0-cyt was polymorphic-at the lipid/protein ratio corresponding to that of mature peripheral myelin ( approximately 50:1), hP0-cyt mainly adopted a beta-conformation, whereas when the proportion of lipid increased, the structure underwent a beta-->alpha transition. By contrast, the secondary structure of the major isoform of myelin basic protein, another myelin protein with a very large positive charge, remained unchanged across a wide range of lipid/protein ratios. We propose that when hP0-cyt is bound at sufficient concentration to lamellar lipid bilayers such as myelin, it folds into a beta-conformation; before this threshold lipid/protein ratio is reached, the domain is alpha-helical. We suggest that the cytoplasmic apposition (major dense line) in compact myelin may be stabilized via the hydrogen-bonding of beta-strands formed as a result of local P0-P0 aggregation.
authors.affiliation: Department of Biology, Boston College, Chestnut Hill, Massachusetts, USA.
authors.forename: Xiaoyang
authors.initials: X
authors.lastname: Luo
authors.forename: Deepak
authors.initials: D
authors.lastname: Sharma
authors.forename: Hideyo
authors.initials: H
authors.lastname: Inouye
authors.forename: Daniel
authors.initials: D
authors.lastname: Lee
authors.forename: Robin L
authors.initials: RL
authors.lastname: Avila
authors.forename: Mario
authors.initials: M
authors.lastname: Salmona
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Kirschner
mesh_terms: D000595:Amino Acid Sequence; D019281:Dimerization; D006801:Humans; D006860:Hydrogen Bonding; D008055:Lipids / Q000737:chemistry; D008567:Membranes, Artificial; D008969:Molecular Sequence Data; D004676:Myelin Basic Protein / Q000737:chemistry* / Q000378:metabolism; D018993:Myelin P0 Protein / Q000737:chemistry* / Q000378:metabolism; D009186:Myelin Sheath / Q000378:metabolism*; D017510:Protein Folding; D017433:Protein Structure, Secondary
chemical_list: D008055:Lipids; D008567:Membranes, Artificial; D004676:Myelin Basic Protein; D018993:Myelin P0 Protein