Structure and Interactions of NCAM Modules 1 and 2, Basic Elements in Neural Cell Adhesion

The structure in solution of the second Ig-module fragment of residues 117-208 of NCAM has been determined. Like the first Ig-module of residues 20-116, it belongs to the I set of the immunogloblin superfamily. Module 1 and module 2 interact weakly, and the binding sites of this interaction have been identified. The two-module fragment NCAM(20-208) is a stable dimer. Removal of the charged residues in these sites in NCAM(20-208) abolishes the dimerization. Modeling the dimer of NCAM(20-208) to fit the interactions of these charges produces one coherent binding site for the formation of two antiparallel strands of the first two NCAM modules. This mode of binding could be a major element in trans-cellular interactions in neural cell adhesion.