Topology of the CD2-CD48 cell-adhesion molecule complex: implications for antigen recognition by T cells
van der Merwe PA, McNamee PN, Davies EA, Barclay AN, Davis SJ. (1995), Curr Biol. 5, 74-84
BACKGROUND: The T-lymphocyte cell-surface molecule, CD2, was the first heterophilic cell-adhesion molecule to be discovered and has become an important paradigm for understanding the structural basis of cell adhesion. Interaction of CD2 with its ligands. CD58 (in humans) and CD48 (in mice and rats), contributes to antigen recognition by T cells. CD2, CD48 and CD58 are closely related members of the immunoglobulin superfamily and their extracellular regions are predicted to have very similar structures. The three-dimensional crystal structure of this region of CD2 has been determined, revealing two immunoglobulin domains with the ligand-binding site situated on an exposed beta sheet in the membrane-distal domain. This GFCC’C” beta sheet is also involved in a homophilic ‘head-to-head’ interaction in the CD2 crystal lattice, which has been proposed to be a model for the interactions of CD2 with its ligands.
RESULTS: We show that the CD2-binding site on rat CD48 lies on the equivalent beta-sheet of its membrane-distal immunoglobulin domain. By making complementary mutations, we have shown that two charged residues in the CD48 ligand-binding site interact directly with two oppositely charged residues in CD2’s ligand-binding site. These results indicate that the amino-terminal immunoglobulin domains of CD2 and CD48 bind each other in the same orientation as the CD2-CD2 crystal lattice interaction, strongly supporting the suggestion that CD2 interacts head-to-head with its ligand. Modelling CD48 onto the CD2 structure reveals that the CD2-CD48 complex spans approximately the same distance (134 Å) as predicted for the complex between the T-cell receptor and the peptide-bound major histocompatibility complex (MHC) molecule.
CONCLUSIONS: Our results, together with recent structural studies of CD2, provide the first indication of the specific topology of a cell-adhesion molecule complex. The similar dimensions predicted for the CD2-CD48 complex and the complex between the T-cell receptor and the peptide-bound MHC molecule suggest that one of the functions of CD2 may be to position the plasma membranes of the T cell and the antigen-presenting (or target) cell at the optimal distance for the low-affinity interaction between the T-cell receptor and the peptide-bound MHC molecule.
Key figure: Models of the CD2–CD48, TCR–peptide–MHC and CD4–MHC complexes
The molecules shown are all Cα traces of crystal structures of proteins. The CD2–CD48 complex is essentially the same structure as the crystal homodimer of rat CD2 (yellow) with CD48 (white) modelled on one of the CD2 molecules . The two pairs of interacting residues identified in the present study are shown at the CD2–CD48 interface (coloured as in Figure 5). The TCR (blue) is represented by the IgG Fab′ fragment NEW . The MHC class II molecule (green) is HLA-DR1 with a bound influenza virus peptide antigen (pink)  and . The TCR model was docked on HLA-DR1 in an orientation that positions the CDR3 hypervariable loops over the peptide, and the CDR1 and CDR2 hypervariable loops over the α helices , , ,  and . The CD4 model (orange) was constructed as described  from the structures for human CD4 domains 1 and 2 and rat CD4 domains 3 and 4. Regions in CD4  and  and HLA-DR1  and  that are believed to interact are shown in red. The distance spanned by the proposed CD2–CD48, TCR–MHC, and CD4–MHC complexes are 134 Å, 135 Å, and 125 Å, respectively. The additional amino-acid residues that lie in the carboxy-terminal stalk region between the membrane-proximal immunoglobulin domains and the transmembrane domains (CD2, 4 residues; CD4, 8 residues; TCR α, ∼16 residues; TCR β, ∼34 residues; MHC class II α, 9 residues; MHC class II β, 8 residues) or GPI anchor (CD48, ∼5 residues) are not included in these measurements because the structures of these segments are not known.