The role of charged residues mediating low affinity protein-protein recognition at the cell surface by CD2
Davis SJ, Davies EA, Tucknott MG, Jones EY, van der Merwe PA. (1998), Proc Natl Acad Sci U S A. 95, 5490-4
Insights into the structural basis of protein–protein recognition have come principally from the analysis of proteins such as antibodies, hormone receptors, and proteases that bind their ligands with relatively high affinity (Ka ≈ 109 M−1). In contrast, few studies have been done on the very low affinity interactions mediating cell adhesion and cell–cell recognition. As a site of protein–protein recognition, the ligand binding face of the T lymphocyte cell–cell recognition molecule, CD2, which binds its ligands 104– to 105-fold more weakly than do antibodies and proteases, is unusual in being both very flat and highly charged. An analysis of the effect of mutations and ionic strength on CD2 binding to its ligand, CD48, indicates that these charged residues contribute little, if any, binding energy to this interaction. However, the loss of these charged residues is shown to markedly reduce ligand-binding specificity. Thus, the charged residues increase the specificity of CD2 binding without increasing the affinity. This phenomenon is likely to result from a requirement for electrostatic complementarity between charged binding surfaces to compensate for the removal, upon binding, of water interacting with the charged residues. It is proposed that this mode of recognition is highly suited to biological interactions requiring a low affinity because it uncouples increases in specificity from increases in affinity.
Key figure: Mutagenesis of domain 1 of CD2
The crystal structure of domain 1 of CD2 (residues 1–99) is shown in Corey, Pauling, and Koltun format drawn using RASMOL (31). In each panel, the view is approximately perpendicular to the ligand-binding GFCC′C” β-sheet surface. (A) Residues whose nonconservative substitution significantly interfered with, or had no effect on, ligand (CD48) binding by CD2 are colored red and light blue, respectively. All of the unmutated residues are colored yellow, except for the sites ofN-glycosylation (N67, N77, and N84) which are colored green. (B) The chemical composition of domain 1 of CD2 is indicated by coloring acidic residues red, basic residues dark blue, uncharged polar residues light blue, and hydrophobic residues green. (C) Residues whose substitution with alanine reduced ligand-binding affinity >20-fold are colored red and those for which the reduction in affinity was twofold or less are colored light blue. The details of the substitutions, and their effects, are given in Table 1.