HOMOPHILIC ADHESION BETWEEN IG SUPERFAMILY CARCINOEMBRYONIC ANTIGEN MOLECULES INVOLVES DOUBLE RECIPROCAL BONDS

Both carcinoembryonic antigen (CEA) and neural cell adhesion molecule (NCAM) belong to the immunoglobulin supergene family and have been dem onstrated to function as homotypic Ca++-independent intercellular adhe sion molecules. CEA and NCAM cannot associate heterotypically indicati ng that they have different binding specificities. To define the domai ns of CEA involved in homotypic interaction, hybrid cDNAs consisting o f various domains from CEA and NCAM were constructed and were transfec ted into a CHO-derived cell line; stable transfectant clones showing c ell surface expression of CEA/NCAM chimeric-proteins were assessed for their adhesive properties by homotypic and heterotypic aggregation as says. The results indicate that all five of the Ig(C)-like domains of NCAM are required for intercellular adhesion while the COOH-terminal d omain containing the fibronectin-like repeats is dispensable. The resu lts also show that adhesion mediated by CEA involves binding between t he Ig(V)-like amino-terminal domain and one of the Ig(C)-like internal repeat domains: thus while transfectants expressing constructs contai ning either the N domain or the internal domains alone were incapable of homotypic adhesion, they formed heterotypic aggregates when mixed. Furthermore, peptides consisting of both the N domain and the third in ternal repeat domain of CEA blocked CEA-mediated cell aggregation, thu s providing direct evidence for the involvement of the two domains in adhesion. We therefore propose a novel model for interactions between immunoglobulin supergene family members in which especially strong bin ding is effected by double reciprocal interactions between the V-like domains and C-like domains of antiparallel CEA molecules on apposing c ell surfaces.