title: A novel alpha-helix in the first fibronectin type III repeat of the neural cell adhesion molecule is critical for N-glycan polysialylation.
abstract: 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 alpha-helix that links strands 4 and 5 of its beta-sandwich structure. Replacement of the alpha-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 alpha-helix is only required for the polysialylation of N-glycans. Our data are consistent with a model in which the FN1 alpha-helix is involved in an Ig5-FN1 interaction that is critical for the correct positioning of Ig5 N-glycans for polysialylation.
authors.affiliation: Department of Biochemistry and Molecular Genetics, College of Medicine, University of Illinois at Chicago, Chicago, Illinois 60607, USA.
authors.forename: Shalu Shiv
authors.initials: SS
authors.lastname: Mendiratta
authors.forename: Nikolina
authors.initials: N
authors.lastname: Sekulic
authors.forename: Francisco G
authors.initials: FG
authors.lastname: Hernandez-Guzman
authors.forename: Brett E
authors.initials: BE
authors.lastname: Close
authors.forename: Arnon
authors.initials: A
authors.lastname: Lavie
authors.forename: Karen J
authors.initials: KJ
authors.lastname: Colley
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D018360:Crystallography, X-Ray; D018076:DNA, Complementary / Q000378:metabolism; D005353:Fibronectins / Q000737:chemistry*; D006801:Humans; D008958:Models, Molecular; D019006:Neural Cell Adhesion Molecules / Q000737:chemistry*; D011134:Polysaccharides / Q000737:chemistry; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012794:Sialic Acids / Q000737:chemistry / Q000378:metabolism; D012799:Sialyltransferases / Q000737:chemistry
chemical_list: D018076:DNA, Complementary; D005353:Fibronectins; D019006:Neural Cell Adhesion Molecules; D011134:Polysaccharides; D012794:Sialic Acids; C021319:polysialic acid; D012799:Sialyltransferases; C515117:ST8SIA4 protein, human