Molecular analysis of the epidermal growth factor-like short consensus repeat domain-mediated protein-protein interactions - Dissection of the CD97-CD55 complex

Epidermal growth factor-like (EGF) and short consensus repeat (SCR) domains are commonly found in cell sill-face and soluble proteins that mediate spe cific protein-protein recognition events. Unlike the immunoglobulin (Ig) su perfamily, very Little is known about the general properties of intermolecu Iar interactions encoded by these common modules, and in particular, how sp ecificity of binding is achieved. We have dissected the binding of CD97 (a member of the EGF-TM7 family) to the complement regulator CD55, two cell su rface modular proteins that contain EGF and SCR domains, respectively. We d emonstrate that the interaction is mediated solely by these domains and is characterized by a low affinity (86 mum) and rapid off-rate (at least 0.6 s (-1)). The interaction is Ca2+-dependent but is unaffected by glycosylation of the EGF domains. Using biotinylated multimerized peptides in cell bindi ng assays and surface plasmon resonance, we show that a CD97-related EGF-TM 7 molecule (termed EMR2), differing by only three amino acids within the EG F domains, binds CD55 with a K-D at least an order of magnitude weaker than that of CD97, These results suggest that low affinity cell-cell interactio ns may be a general feature of highly expressed cell surface proteins and t hat specificity of SCR-EGF binding can be finely tuned by a small number of amino acid changes on the EGF module surface.