Yr i

FIGURE 7-7

The membrane-distal, peptide-binding cleft of a human class II MHC molecule, HLA-DR1 (blue), superimposed over the corresponding regions of a human class I MHC molecule, HLA-A2 (red). [From J. H. Brown et al., 1993, Nature 364:33.]

Hla Class Structure

FIGURE 7-8

| Antigen-binding cleft of dimeric class II DR1 molecule in (a) top view and (b) side view. This molecule crystallized as a dimer of the ap heterodimer. The crystallized dimer is shown with one DR1 molecule in red and the other DR1 molecule in blue. The bound peptides are yellow. The two peptide-binding clefts in the dimeric molecule face in opposite directions. [From J. H. Brown et al., 1993, Nature 364:33]

FIGURE 7-8

| Antigen-binding cleft of dimeric class II DR1 molecule in (a) top view and (b) side view. This molecule crystallized as a dimer of the ap heterodimer. The crystallized dimer is shown with one DR1 molecule in red and the other DR1 molecule in blue. The bound peptides are yellow. The two peptide-binding clefts in the dimeric molecule face in opposite directions. [From J. H. Brown et al., 1993, Nature 364:33]

latter occurred as a dimer of ap heterodimers, a "dimer of dimers" (Figure 7-8). The dimer is oriented so that the two peptide-binding clefts face in opposite directions. While it has not yet been determined whether this dimeric form exists in vivo, the presence of CD4 binding sites on opposite sides of the class II molecule suggests that it does. These two sites on the a2 and p2 domains are adjacent in the dimer form and a CD4 molecule binding to them may stabilize class II dimers.

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