Self recognition in the Ig superfamily - Identification of precise subdomains in carcinoembryonic antigen required for intercellular adhesion

The homophilic binding of extracellular domains of membrane-bound immunoglo bulin superfamily (IgSF) molecules is often required for intercellular adhe sion and signaling. Carcinoembryonic antigen (CEA), a member of the IgSF, i s a widely used tumor marker that functions in vitro as a homotypic interce llular adhesion molecule, CEA has also been shown to contribute to tumorige nicity by inhibiting cellular differentiation, an effect that requires the hemophilic binding of its extracellular domains, It was of interest, theref ore, to identify small subdomain sequences In CEA that could serve as a foc us in the design of peptides that disrupt CEA-mediated intercellular adhesi on. Three subdomains in the N-terminal domain of CEA, identified by site-di rected deletions and point mutations, were shown to be required for interce llular adhesion. Cyclized peptides representing two of these subdomains, (4 2)NRQII and (80)QNDTG, were found to be effective in blocking CEA-mediated cellular aggregation when added to CEA-expressing transfectants in suspensi on. Intermolecular binding involving each of these subdomains is therefore essential for intercellular adhesion and cannot be compensated for by known binding contributions of other regions in the CEA molecule. In further sup port of this assumption, the binding epitope of an anti-CEA monoclonal anti body (monoclonal antibody A20) known to block CEA-mediated adhesion, was sh own to bridge two of the three required subdomains: (42)NRQII and (30)GYSWY K.