The A-domain of integrin alpha 2 binds specifically to a range of collagens but is not a general receptor for the collagenous motif

Integrin alpha 2 beta 1 is a major cellular receptor for collagens, but the molecular basis of its function is unknown. The alpha 2 subunit contains a von Willebrand factor A-domain (I-domain) in its N-terminal region, and it has been demonstrated recently that this domain binds specifically to collagen I. This interaction requires divalent cations (e.g., Mg2+) and native collagen conformation, as does binding of the parent integrin to collagen. The alpha 2 A-domain therefore has a number of functional similarities to the parent integrin, alpha 2 beta 1. However, while sequence specificity has been demonstrated for the parent integrin, no such observations have been made for the A-domain. In particular, it is not known whether the A-domain is responsible for sequence-specific recognition of collagens or whether it binds to the genetic collagenous motif. To investigate the ligand specificity of the alpha 2 A-domain, its binding to a range of collagenous ligands has been studied, with cation dependence, collagen triple-helicity, and inhibition by function-blocking antibodies as criteria for specificity. Binding of the parent integrin was examined for comparison. The alpha 2 A-domain was found to bind specifically to collagens I, II, IV and XI. The complement component C1q has a collagenous domain but this was unable to support specific binding of alpha 2 A-domain or alpha 2 beta 1. Furthermore, synthetic triple-helical collagenous peptides failed to act as specific ligands. In conclusion, the alpha 2 A-domain binds specifically to a range of extracellular matrix collagens, but it is not a receptor for all collagenous triple helices. By inference, these findings indicate the existence of an integrin-specific sequence motif within collagenous ligands recognised by the alpha 2 A-domain.