The structure of the CD28 homodimer
(A) Ribbon representation of the CD28 dimer observed in the crystal lattice, with oligomannose, bi- or tri-antennary N-glycans modeled at each of the potential glycosylation sites. (B) GRASP surface representation of the CD28 and CTLA-4 homodimers, with their ligand binding surfaces coloured red and blue respectively and potential glycosylation sites coloured green. (C) Two orthogonal views of alpha carbon representations of the CD28 (red) and CTLA-4 (blue) homodimers after superposition by one monomer. The top view of CD28 is identical to that in A and B. (D) B7-1 binding to the CD28 dimer is modelled using the mode of interaction observed in complexes of either B7-1 (dark blue) or B7-2 (light blue) with CTLA-4. This model explains the observed monovalence of the CD28 dimer (Collins et al., 2001) as two ligands cannot bind simultaneously without steric interference of their membrane proximal domains.