Structure of the Calcium-Rich Signature Domain of Human Thrombospondin-2.

There are five members of the thrombospondin (TSP) family in vertebrates. This family of proteins comprises two separate subsets, group A and group B. Group A includes TSP-1 and TSP-2. TSP-1 is a major platelet α-granule component and contains a polymorphism that has been linked to coronary artery disease (CAD). TSP-2, although not found in platelets, is important for megakaryopoiesis. TSPs contain a signature domain that is found with high fidelity throughout the TSP family and consists of tandem epidermal growth factor (EGF)-like modules, 13 aspartate-rich calcium-binding repeats and a C-terminal lectin-like module. The signature domains of TSP-1 and TSP-2, which have three EGF-like modules, share 77% identity, without any insertions or deletions. In this study, we describe the 2.6 angstrom resolution crystal structure of the glycosylated signature domain of human TSP-2. Components of the domain interact extensively to form three striking structural features: a stalk that consists of the two N-terminal EGF-like modules, a wire that consists of the asparate-rich repeats, and a globe that consists of the lectin-like module. The third EGF-like module serves as a clasp to tie together the globe and the ends of the wire. The TSP-2 signature domain is stabilized by 30 bound Ca²⁺ ions and 18 disulfide bonds. The structure reveals functions of the residues at the position of the CAD-linked polymorphism in TSP-1 and many of the positions of the over 100 skeletal dysplasia-linked mutations in TSP-5. Finally, the structure allows for a better understanding of interactions between TSPs and a number of binding partners, including integrins, CD47, vonWillebrand factor, type V collagen, fibroblast growth factor, cathepsin-G and neutrophil elastase.