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

Biological properties of chemokines are believed to be influenced by their association with glycosaminoglycans. Surface plasmon resonance kinetic anal ysis shows that the CXC chemokine stromal cell-derived factor-1 alpha (SDF- 1 alpha), which binds the CXCR4 receptor, associates with heparin with an a ffinity constant of 38.4 nM (k(on) = 2.16 x 10(6) m(-1) s(-1) and k(off) = 0.083 x s(-1)). A modified SDF-1 alpha (SDF-1 3/6) was generated by combine d substitution of the basic cluster of residues Lys(24), His(25), and Lys(2 7) by Ser. SDF-1 3/6 conserves the global native structure and functional p roperties of SDF-1 alpha, but it is unable to interact with sensor chip-imm obilized heparin. The biological relevance of these in vitro findings was i nvestigated. SDF-1 alpha was unable to bind in a CXCR4-independent manner o n epithelial cells that were treated with heparan sulfate (HS)-degrading en zymes or constitutively lack HS expression. The inability of SDF-1 3/6 to b ind to cells underlines the importance of the identified basic cluster for the physiological interactions of SDF-1 alpha with HS. importantly, the ami no-terminal domain of SDF-1 alpha which is required for binding to, and act ivation of, CXCR4 remains exposed after binding to HS and is recognized by a neutralizing monoclonal antibody directed against the first residues of t he chemokine. Overall, these findings indicate that the Lys(24), His(,)(25) and Lys(27) cluster of residues forms, or is an essential part of, the MS- binding site which is distinct from that required for binding to, and signa ling through, CXCR4.