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Functional topography of the myelin-associated glycoprotein. I. Mapping of domains by electron microscopy
T Fahrig1, R Probstmeier, E Spiess
1Department of Neurobiology, Swiss Federal Institute of Technology, Zürich.
Abstract:
The functional topography of the myelin-associated glycoprotein (MAG) was investigated by electron microscopic analysis of rotary-shadowed molecules of a MAG fragment (MAG 90) comprising the five immunoglobulin-like domains of the extracellular part of the molecule. MAG 90 molecules appeared as rod-like structures (18.5 +/- 1.2 nm long and 4.0 +/- 0.8 nm wide) with a globular domain at one end. Antibodies directed against the amino- and carboxy-terminus of MAG 90 interacted with the non-globular terminal region, indicating that the molecule is bent in the globular region with the amino- and carboxy-terminal arms in close apposition to each other. An antibody which interferes with the binding of MAG to neurons interacted predominantly with the globular domain of MAG 90. The fibril-forming collagen types I, III and V bound mainly to the non-globular terminal region of MAG 90, whereas the majority of heparin molecules interacted with the globular region of the molecule. The L2/HNK-1 carbohydrate structure was localized at the non-globular region in the protein fragment comprising the fourth and fifth immunoglobulin-like domains.
Insights
This study maps the structure of myelin-associated glycoprotein (MAG) domains. Researchers found MAG
Area of Science:
- Neuroscience
- Molecular Biology
- Biochemistry
Background:
- Myelin-associated glycoprotein (MAG) is crucial for myelin sheath formation and maintenance in the central nervous system.
- Understanding the functional topography of MAG is essential for elucidating its role in neural development and repair.
- Previous studies have provided insights into MAG's interactions, but its precise structural organization and binding sites remain incompletely characterized.
Purpose of the Study:
- To investigate the functional topography of the myelin-associated glycoprotein (MAG) using a recombinant protein fragment.
- To determine the spatial arrangement of MAG's extracellular domains and identify regions involved in specific molecular interactions.
- To map the binding sites for neuronal elements, collagen, and heparin on the MAG molecule.
Main Methods:
- Electron microscopic analysis of rotary-shadowed MAG 90 fragment, comprising the five immunoglobulin-like domains.
- Immuno-electron microscopy using antibodies against MAG 90 termini and functional epitopes.
- Binding assays with fibril-forming collagens (types I, III, V) and heparin molecules.
Main Results:
- MAG 90 molecules presented as rod-like structures with a globular domain at one end.
- Antibodies against MAG 90 termini bound to the non-globular region, suggesting a bent conformation.
- An antibody inhibiting MAG-neuronal binding localized to the globular domain, while collagens bound to the non-globular region and heparin to the globular region.
- The L2/HNK-1 carbohydrate epitope was found in the non-globular region of the fourth and fifth immunoglobulin-like domains.
Conclusions:
- The MAG 90 fragment adopts a bent conformation, with globular and non-globular regions mediating distinct molecular interactions.
- The globular domain is crucial for MAG's interaction with neurons, while the non-globular region binds collagens and contains the L2/HNK-1 epitope.
- These findings provide a structural basis for understanding MAG's role in neural development, myelination, and potential regeneration processes.