Stromal cell-derived factor-1α associates with heparan sulfates through the first β-strand of the chemokine

Abstract

Biological properties of chemokines are believed to be influenced by their association with glycosaminoglycans. Surface plasmon resonance kinetic analysis shows that the CXC chemokine stromal cell-derived factor-1α (SDF- 1α), which binds the CXCR4 receptor, associates with heparin with an affinity constant of 38.4 nM (k(on) = 2.16 x 106 M-1 s-1 and k(off) = 0.083 x s-1). A modified SDF-1α (SDF-1 3/6) was generated by combined substitution of the basic cluster of residues Lys24, His25, and Lys27 by Ser. SDF-1 3/6 conserves the global native structure and functional properties of SDF-1α, but it is unable to interact with sensor chip- immobilized heparin. The biological relevance of these in vitro findings was investigated. SDF-1α was unable to bind in a CXCR4-independent manner on epithelial cells that were treated with heparan sulfate (HS)-degrading enzymes or constitutively lack HS expression. The inability of SDF-1 3/6 to bind to cells underlines the importance of the identified basic cluster for the physiological interactions of SDF-1α with HS. Importantly, the amino- terminal domain of SDF-1α which is required for binding to, and activation of, CXCR4 remains exposed after binding to HS and is recognized by a neutralizing monoclonal antibody directed against the first residues of the chemokine. Overall, these findings indicate that the Lys24, His25, and Lys27 cluster of residues forms, or is an essential part of, the HS-binding site which is distinct from that required for binding to, and signaling through, CXCR4.