αVβ6 is a novel receptor for human fibrillin-1 -: Comparative studies of molecular determinants underlying integrin-RGD affinityand specificity

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 beta-binding protein-like (TB) domains. TB4 contains a flexible RGD loop that mediates cell adhesion via alpha V beta 3 and alpha 5 beta 1 integrins. This study identifies integrin alpha V beta 6 as a novel cellular receptor for fibrillin-1 with a K-d of similar to 0.45 mu M. Analyses of this interaction by surface plasmon resonance and immunocytochemistry reveal different module requirements for alpha V beta 6 activation compared with those of alpha V beta 3, suggesting that a covalent linkage of an N-terminal calcium binding epidermal growth factor-like domain to TB4 can modulate alpha V integrin binding specificity. Furthermore, our data suggest alpha 5 beta 1 is a low affinity fibrillin-1 receptor (K-d > 1 mu M), thus providing a molecular explanation for the different alpha 5 beta 1 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 alpha 5 beta 1 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.