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 immunoglobulin superfamily (IgSF) molecules is often required for intercellular adhesion and signaling. Carcinoembryonic antigen (CEA), a member of the IgSF, is a widely used tumor marker that functions in vitro as a homotypic intercellular adhesion molecule. CEA has also been shown to contribute to tumorigenicity by inhibiting cellular differentiation, an effect that requires the homophilic binding of its extracellular domains. It was of interest, therefore, to identify small subdomain sequences in CEA that could serve as a focus in the design of peptides that disrupt CEA-mediated intercellular adhesion. Three subdomains in the N-terminal domain of CEA, identified by site-directed deletions and point mutations, were shown to be required for intercellular adhesion. Cyclized peptides representing two of these subdomains, (42)NRQII and (80)QNDTG, were found to be effective in blocking CEA-mediated cellular aggregation when added to CEA-expressing transfectants in suspension. 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 support of this assumption, the binding epitope of an anti-CEA monoclonal antibody (monoclonal antibody A20) known to block CEA-mediated adhesion, was shown to bridge two of the three required subdomains: (42)NRQII and (30)GYSWYK.

Synthetic CD4 exocyclics inhibit binding of human immunodeficiency virus type 1 envelope to CD4 and virus replication in T lymphocytes.

CD4 functions as a major T-cell surface receptor for human immunodeficiency virus by binding the human immunodeficiency virus type 1 (HIV-1) envelope protein gp120 with relatively high affinity. We have developed constrained aromatically modified analogs of the secondary structures of the first domain of CD4 in order to analyze surfaces involved in binding of gp120. Complementarity determining-like regions (CDRs) of the D1 domain of CD4 were reproduced as synthetic aromatically modified exocyclic (AMEs) forms. The exocyclic CDR3.AME(82-89), derived from the CDR3 (residues 82-89) region of CD4 D1 domain, specifically inhibited binding of recombinant gp120 to both recombinant soluble CD4, and CD4+ Jurkat cells, and blocked syncytium formation and virus particle production caused by HIV-1 infection. We have previously shown that the CDR3.AME analog binds to the CD4 CDR3 region and creates a disabled CD4 heterodimer. We propose that the AME prevents the formation of an essential homodimeric surface needed for efficient HIV binding. Additionally the disabled CD4 receptor may be less able to signal the cell to allow HIV replication and HIV infection. Such compounds may represent a new receptor specific approach to modulate biological functions.