IDENTIFICATION OF A HUMAN CD4-CDR3-LIKE SURFACE INVOLVED IN CD4(-CELLFUNCTION() T)

The CD4 molecule is expressed on the surface of helper T cells, This m olecule contains four tandem external immunoglobulin-like domains (D1- D4), a transmembrane domain, and a cytoplasmic tail, Through the use o f molecular modeling techniques, peptide analogs of the CDR3-like regi on of the human CD4 molecule, analog hPGP, a cyclized peptide 13 amino acids long, was synthesized and tested for its ability to inhibit pro liferation in human mixed lymphocyte reactions. A conservative amino a cid substitution was made at position 5 (D --> N) to increase its acti vity and designated hPGP(N), A series of alanine substitution peptides were synthesized based on the sequence of hPGP(N) to determine the im portance of each residue to the peptide's function, The substitutions of amino acids in positions 3, 7, and 8 had essentially no effect on t he inhibitory activity of hPGP(N), while substitutions of amino acids in positions 4 and 6 increased its inhibitory effect. Alanine substitu tions of amino acids in positions 2, 5, and 9 dramatically decreased t he inhibitory effect of analog hPGP(N), Molecular modeling of the nati ve CD4-CDR3-like domain suggested that the residues corresponding to p ositions 2, 5, and 9 of the peptide formed a contiguous surface repres enting the active site.