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Requirements for the Protein Specific Polysialylation of NCAM N‐Glycans
Author(s) -
Colley Karen J.,
Mendiratta Shalu Shiv,
Sekulic Nikolina,
Close Brett,
Lavie Ar
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.4.a56-b
Subject(s) - polysialic acid , neural cell adhesion molecule , glycan , chemistry , glycoprotein , linker , sialic acid , glycosylation , cell adhesion , biochemistry , cell , computer science , operating system
The neural cell adhesion molecule, NCAM, is modified by the anti‐adhesive glycan, α2, 8‐polysialic acid that caps two of its six N‐glycans. Polysialic acid negatively modulates both NCAM‐dependent and NCAM‐independent cell adhesion and has been shown to promote neurite outgrowth, axon guidance and pathfinding, and general cell migration. The restricted addition of polysialic acid to only five mammalian glycoproteins is unusual among glycosylation events, and implies an additional level of specificity in substrate recognition. NCAM is the most abundant polysialylated protein and we have used it as a model to test the hypothesis that the protein specificity of polysialylation requires an initial protein‐protein contact between the polysialyltransferases and their glycoprotein substrates. We have found that NCAM polysialylation requires its first fibronectin type III (FN III ) repeat (FN1), which is adjacent to the fifth Ig domain (Ig5) that carries the polysialylated N‐glycans. This domain cannot be replaced by other FN III repeats such as FN2 of NCAM. Modeling of the FN1 structure using the NMR structure of FN2 revealed that FN1 has an acidic surface patch that is missing from FN2. Mutagenesis studies showed that, while the acidic patch plays a role in polysialylation, additional contact sites are likely to be involved. We solved the crystal structure of the FN1 and found that it possesses a unique α helical linker between the strands 4 and 5 of its β sandwich structure. Surprisingly, replacement of this linker changes the sites of polysialic acid addition from the N‐glycans on Ig5 to O‐glycans that are predicted to be on FN1. We conclude that NCAM requires specific features of its FN1 domain for recognition and polysialylation of the N‐glycans on the adjacent Ig5 domain.