ANALYSIS OF THE CONTACT SITES ON THE CD4 MOLECULE WITH CLASS-II MHC MOLECULE - CO-LIGAND VERSUS CORECEPTOR FUNCTION

The CD4 molecule interacts with the alpha(2) and beta(2) domains of th e MHC class II molecules. However, the class II contact sites on CD4 a re less clear, Involvement of different regions throughout D1, D2, and D3 domains have been suggested, To further delineate the class II MHC contact sites on CD4, a crystal structure-informed mutagenesis was pe rformed, Alanine scan mutants were generated for exposed residues loca ted throughout D1 and the FC loop of D2, and in the ''hinge-like'' reg ion, a short and flexible region between D2 and D3, Mutants were teste d in a co-ligand (Dd stimulation) and a co-receptor (staphylococcal en terotoxin B stimulation) assay, In the co-ligand assay, TCR and CD4 in teract with two distinct ligands (Dd Or HLA-DR), while in the co-recep tor assay both molecules interact with the same ligand, namely HLA-DR, Results show that residues from both lateral faces of D1 and the FC l oop of D2 are implicated in interaction with class II, although a bigg er surface of CD4 was involved in co-receptor compared with co-ligand function, The potential involvement of residues on both the top and tw o side faces of CD4 is consistent with a bivalent model, which involve s the interaction between a single CD4 and two class II molecules, Alt ernatively, our results can be interpreted with a model of a specifica lly organized CD4 and/or class II oligomerization event, Finally, resu lts from the hinge region mutants revealed a very important role in ma intaining the overall structural integrity of CD4, its topology, and f unction.