The Anti-adhesive Activity of Thrombospondin Is Mediated by the N-terminal Domain of Cell Surface Calreticulin*

Abstract

Thrombospondin (TSP) induces reorganization of the actin cytoskeleton and restructuring of focal adhesions through binding of amino acids (aa) 17–35 (hep I peptide) of thrombospondin to a cell surface form of calreticulin (CRT). In this report we provide further evidence for the involvement of calreticulin in thrombospondin signaling and characterize thrombospondin-calreticulin interactions. Wild type but notcrt ^−/− cells respond to hep I/TSP. Responsiveness can be restored by incubation of cells with exogenous calreticulin or by transfection with calreticulin. Thrombospondin forms complexes with the CRT-N-domain that are enhanced by physiologic levels of calcium and zinc. Consistent with thrombospondin/CRT-N-domain binding, only the CRT-N-domain blocks hep I- and thrombospondin-stimulated focal adhesion disassembly. A series of glutathione S-transferase-N-domain mutants were used to map the sequence within the N-domain that interacts with TSP/hep I. A construct containing aa 1–43 but not a construct of aa 1–31 supported thrombospondin binding and focal adhesion disassembly. A series of overlapping peptides were used to further map the thrombospondin-binding site. Peptides spanning aa 19–36 (RWIESKHKSDFGKFVLSS) blocked hep I-stimulated focal adhesion disassembly, indicating that the TSP/hep I-binding site is located to this sequence in calreticulin. A mutant fusion protein lacking aa 19–36 (glutathione S-transferase-CRTΔhep I) failed to restore responsiveness to hep I in crt ^−/−cells, bind thrombospondin, or competitively block focal adhesion disassembly, providing evidence for the role of this calreticulin sequence in mediating thrombospondin signaling.