The structure of a Ca 2+-binding epidermal growth factor-like domain: Its role in protein-protein interactions

Abstract

Various diverse extracellular proteins possess Ca 2+-binding epidermal growth factor (EGF)-like domains, the function of which remains uncertain. We have determined, at high resolution (1.5 Å), the crystal structure of such a domain, from human clotting factor IX, as a complex with Ca 2+. The Ca 2+ ligands form a classic pentagonal bipyramid with six ligands contributed by one polypeptide chain and the seventh supplied by a neighboring EGF-like domain. The crystal structure identifies the role of Ca 2+ in maintaining the conformation of the N-terminal region of the domain, but more importantly demonstrates that Ca 2+ can directly mediate protein-protein contacts. The observed crystal packing of the domains provides a plausible model for the association of multiple tandemly linked EGF-like domains in proteins such as fibrillin-1, Notch, and protein S. This model is consistent with the known functional data and suggests a general biological role for these domains.