FUNCTIONAL TOPOGRAPHY OF THE MYELIN-ASSOCIATED GLYCOPROTEIN .1. MAPPING OF DOMAINS BY ELECTRON-MICROSCOPY

The functional topography of the myelin-associated glycoprotein (MAG) was investigated by electron microscopic analysis of rotary-shadowed m olecules of a MAG fragment (MAG 90) comprising the five immunoglobulin -like domains of the extracellular part of the molecule. MAG 90 molecu les appeared as rodlike structures (18.5 +/- 1.2 nm long and 4.0 +/- 0 .8 nm wide) with a globular domain at one end. Antibodies directed aga inst 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 ap position 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 t o 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 immuno globulin-like domains.