Myelin-Associated Glycoprotein Interacts with Neurons via a Sialic Acid Binding Site at ARG118 and a Distinct Neurite Inhibition Site

• Published in: The Journal of cell biology 1997-09, Vol.138 (6), p.1355-1366
• Main Authors: Tang, Song, Shen, Ying Jing, DeBellard, Maria Elena, Mukhopadhyay, Gitali, Salzer, James L., Crocker, Paul R., Filbin, Marie T.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amino acids; Animals; Antibodies; Arginine - metabolism; Binding sites; Binding Sites - physiology; Cell Adhesion - physiology; Cellular biology; Chimeras; CHO cells; CHO Cells - physiology; Cricetinae; Glycoproteins; Molecular Sequence Data; Mutagenesis - physiology; Myelin; Myelin-Associated Glycoprotein - chemistry; Myelin-Associated Glycoprotein - genetics; Myelin-Associated Glycoprotein - metabolism; N-Acetylneuraminic Acid - chemistry; N-Acetylneuraminic Acid - metabolism; Neurites; Neurites - chemistry; Neurites - physiology; Neurons; Neurons - cytology; Neurons - metabolism; Neurons - ultrastructure; Protein Structure, Tertiary; Proteins; Schwann cells; Schwann Cells - cytology; Schwann Cells - physiology; Transfection
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.138.6.1355

Inhibitory components in myelin are largely responsible for the lack of regeneration in the mammalian CNS. Myelin-associated glycoprotein (MAG), a sialic acid binding protein and a component of myelin, is a potent inhibitor of neurite outgrowth from a variety of neurons both in vitro and in vivo. Here, we show that MAG's sialic acid binding site is distinct from its neurite inhibitory activity. Alone, sialic acid-dependent binding of MAG to neurons is insufficient to effect inhibition of axonal growth. Thus, while soluble MAG-Fc (MAG extracellular domain fused to Fc), a truncated form of MAG-Fc missing Ig-domains 4 and 5, MAG(d1-3)-Fc, and another sialic acid binding protein, sialoadhesin, each bind to neurons in a sialic acid-dependent manner, only full-length MAG-Fc inhibits neurite outgrowth. These results suggest that a second site must exist on MAG which elicits this response. Consistent with this model, mutation of arginine 118 (R118) in MAG to either alanine or aspartate abolishes its sialic acid-dependent binding. However, when expressed at the surface of either CHO or Schwann cells, R118-mutated MAG retains the ability to inhibit axonal outgrowth. Hence, MAG has two recognition sites for neurons, the sialic acid binding site at R118 and a distinct inhibition site which is absent from the first three Ig domains.