A Novel α-Helix in the First Fibronectin Type III Repeat of the Neural Cell Adhesion Molecule Is Critical for N-Glycan Polysialylation

• Published in: The Journal of biological chemistry 2006-11, Vol.281 (47), p.36052
• Main Authors: Shalu Shiv Mendiratta, Nikolina Sekulic, Francisco G. Hernandez-Guzman, Brett E. Close, Arnon Lavie, Karen J. Colley
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M608073200

Polysialic acid is a developmentally regulated, anti-adhesive glycan that is added to the neural cell adhesion molecule, NCAM. Polysialylated NCAM is critical for brain development and plays roles in synaptic plasticity, axon guidance, and cell migration. The first fibronectin type III repeat of NCAM, FN1, is necessary for the polysialylation of N -glycans on the adjacent immunoglobulin domain. This repeat cannot be replaced by other fibronectin type III repeats. We solved the crystal structure of human NCAM FN1 and found that, in addition to a unique acidic surface patch, it possesses a novel α-helix that links strands 4 and 5 of its β-sandwich structure. Replacement of the α-helix did not eliminate polysialyltransferase recognition, but shifted the addition of polysialic acid from the N -glycans modifying the adjacent immunoglobulin domain to O -glycans modifying FN1. Other experiments demonstrated that replacement of residues in the acidic surface patch alter the polysialylation of both N - and O -glycans in the same way, while the α-helix is only required for the polysialylation of N -glycans. Our data are consistent with a model in which the FN1 α-helix is involved in an Ig5-FN1 interaction that is critical for the correct positioning of Ig5 N -glycans for polysialylation.