B. Huang et al., ANALYSIS OF THE CONTACT SITES ON THE CD4 MOLECULE WITH CLASS-II MHC MOLECULE - CO-LIGAND VERSUS CORECEPTOR FUNCTION, The Journal of immunology, 158(1), 1997, pp. 216-225
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.