BINDING STOICHIOMETRY OF THE CYTOTOXIC T-LYMPHOCYTE-ASSOCIATED MOLECULE-4 (CTLA-4) - A DISULFIDE-LINKED HOMODIMER BINDS 2 CD86 MOLECULES

CD28 and CTLA-4 are homologous T cell receptors of the immunoglobulin (Ig) superfamily, which bind B7 molecules (CD80 and CD86) on antigen-p resenting cells and transmit important costimulatory signals during T cell activation. Here we have investigated the subunit structure of CT LA-4 and the stoichiometry of its binding to B7 molecules. We demonstr ate CTLA-4 is a homodimer interconnected by one disulfide bond in the extracellular domain at cysteine residue 120. Each monomeric polypepti de chain of CTLA-4 contains a high affinity binding site for B7 molecu les; soluble CTLA-4 and CD86 form complexes containing equimolar amoun ts of monomeric CTLA-4 and CD86 (i.e. a 2:2 molecular complex). Thus, CTLA-4 and probably CD28 have a receptor structure consisting of preex isting covalent homodimers with two binding sites. Dimerization of CTL A-4 and CD28 is not required for B7 binding, nor is it sufficient to t rigger signaling.