Structure of NCAM

NCAM is a molecule belonging to the immunoglobulin (Ig) superfamily (for a review, see ref. 3). The extracellular part of NCAM contains five Ig-like domains and two fibronectin type III-like repeats (Figure 6.1). NCAM sequences from vertebrates, ranging from human to frog, share residue identities of 70-98%. More distantly related are fasciclin II of Drosophila melonagaster and the Aplysia californica cell adhesion molecule, apCAM, which have 25% identity to vertebrate NCAM proteins. NCAM exists in several isoforms, due to alternative splicing of a single gene consisting of at least 26 exons. In the mouse, NCAM exists in three major membrane-bound isoforms derived from one gene: NCAM120, NCAM140, and NCAM180. The last two forms are integral membrane proteins differing in the size of their intracellular domains. NCAM120, on the other hand, is linked to the membrane via a GPI anchor. GPI-linked proteins appear to be relatively free to move within the plane of the lipid bilayer, in contrast to membrane-spanning proteins that may interact with cytoskeletal components and are thus more rigidly inserted in the membrane. Additionally, several soluble forms of NCAM are present in the brain4. Some isoforms contain (on their fifth Ig-like domain) PSA in an unusual a2,8 linkage in chains which can be up to 200 residues long. Attachment of the highly negatively charged PSA to NCAM results in a large hydration sphere and modulates the function of the molecule. For example, homophilic binding of nonpolysialylated NCAM is much stronger than of PSA-NCAM.

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