αVβ6 Is a Novel Receptor for Human Fibrillin-1 COMPARATIVE STUDIES OF MOLECULAR DETERMINANTS UNDERLYING INTEGRIN-RGD AFFINITY AND SPECIFICITY

Abstract Human fibrillin-1, the major structural protein of connective tissue 10-12 nm microfibrils, contains multiple calcium binding epidermal growth factor-like domains interspersed with transforming growth factor β-binding protein-like (TB) domains. TB4 contains a flexible RGD loop that mediates cell adhesion via αVβ3 and α5β1 integrins. This study identifies integrin αVβ6 as a novel cellular receptor for fibrillin-1 with a Kd of ∼0.45 μm. Analyses of this interaction by surface plasmon resonance and immunocytochemistry reveal different module requirements for αVβ6 activation compared with those of αVβ3, suggesting that a covalent linkage of an N-terminal calcium binding epidermal growth factor-like domain to TB4 can modulate αV integrin binding specificity. Furthermore, our data suggest α5β1 is a low affinity fibrillin-1 receptor (Kd > 1 μm), thus providing a molecular explanation for the different α5β1 distribution patterns seen when human keratinocytes and fibroblasts are plated on recombinant fibrillin fragments versus those derived from the physiological ligand fibronectin. Non-focal contact distribution of α5β1 suggests that its engagement by fibrillin-1 may elicit a lesser degree and/or different type of intracellular signaling compared with that seen with a high affinity ligand.