Title :
Opalin , a transmembrane
sialylglycoprotein located in the central nervous system
myelin paranodal loop membrane
Abstract :
- In contrast to compact myelin , the series of paranodal loops located in the outermost lateral region of myelin is non-compact; the intracellular space is filled by a continuous channel of cytoplasm, the extracellular surfaces between neighboring loops keep a definite distance, but the loop membranes have junctional specializations
- Although the proteins that form compact myelin have been well studied, the protein components of paranodal loop membranes are not fully understood
- This report describes the biochemical characterization and expression of Opalin as a novel membrane protein in paranodal loops
- Mouse Opalin is composed of a short N-terminal extracellular domain (amino acid residues 1-30 ), a transmembrane domain ( residues 31-53 ), and a long C-terminal intracellular domain ( residues 54-143 )
- Opalin is enriched in myelin of the central nervous system, but not that of the peripheral nervous system of mice
- Enzymatic deglycosylation showed that myelin Opalin contained N- and O-glycans , and that the O-glycans , at least, had negatively charged sialic acids
- We identified two N-glycan sites at Asn-6 and Asn-12 and an O-glycan site at Thr-14 in the extracellular domain
- Site-directed mutations at the glycan sites impaired the cell surface localization of Opalin
- In addition to the somata and processes of oligodendrocytes, Opalin immunoreactivity was observed in myelinated axons in a spiral fashion, and was concentrated in the paranodal loop region
- Immunogold electron microscopy demonstrated that Opalin was localized at particular sites in the paranodal loop membrane
- These results suggest a role for highly sialylglycosylated Opalin in an intermembranous function of the myelin paranodal loops in the central nervous system