IDENTIFICATION OF A KEY INTEGRIN-BINDING SEQUENCE IN VCAM-1 HOMOLOGOUS TO THE LDV ACTIVE-SITE IN FIBRONECTIN

The integrin adhesion receptor alpha(4) beta(1) binds two ligands, the extracellular matrix glycoprotein fibronectin and the immunoglobulin superfamily member VCAM-1. Ligand-inding sites are contained with the HepII/IIICS domain of fibronectin, and within the homologous immunoglo bulin domains 1 and 4 of VCAM-1. Previous studies have shown that the binding of each ligand to alpha(4) beta(1) is mutually exclusive, sugg esting that they may employ similar mechanisms to bind receptor. Fibro nectin contains at least three distinct peptide sequences that are act ive sites for alpha(4) beta(1) binding, two homologous sequences Leu-A sp-Val-Pro (LDVP) and Ile-Asp-Ala-Pro (IDAP), and a third related to A rg-Gly-Asp (RGD). Using a combination of site-directed mutagenesis and synthetic peptide approaches in conjunction with VCAM-1-dependent cel l adhesion assays, we now report the identification of a key alpha(4) beta(1)-binding' sequence in both domains 1 and 4 of VCAM-1 as the tet rapeptide ne-Asp-Ser-Pro (IDSP). Mutagenesis studies also suggest that an additional sequence in domain 1, KLEK, participates in receptor bi nding. Since IDSP is homologous to the LDVP and IDAP fibronectin pepti des, this therefore provides a molecular explanation for the promiscui ty of ligand binding by alpha(4) beta(1) and has implications for the design of synthetic VCAM-1 antagonists. The extrapolation of these fin dings to other integrin-binding immunoglobulin ligands is also discuss ed.