Transition from rolling to firm adhesion can be mimicked by extension of integrin αLβ2 in an intermediate affinity state

Abstract

αLβ2 affinity for intercellular adhesion molecule-1 (ICAM-1) is regulated by the conformation of the αL I domain, which is in turn controlled by the conformation and orientation of other adjacent domains. Additionally, overall integrin conformation (bent versus straightened) influences the orientation of the I domain and access to its ligands, influencing adhesive efficiency. The open or high affinity I domain conformation supports strong adhesion, whereas the closed, low affinity conformation mediates weak interactions or rolling. We have previously suggested that αLβ2 can also exist on the cell surface in an intermediate affinity state. Here we have studied the adhesive properties of integrin αLβ2 containing mutant I domains with intermediate affinities for ICAM-1. In an overall bent conformation, the intermediate affinity state of αLβ2 is hardly detected by conventional adhesion assays, but robust adhesion is seen when an extended conformation is induced by a small molecule α/β I allo-steric antagonist. Intermediate affinity αLβ 2 supports more stable rolling than wild-type α Lβ2 under shear conditions. Moreover, antagonist-induced extension transforms rolling adhesion into firm adhesion in amanner reminiscent of chemokine activation of integrin α Lβ2. These findings suggest the relevance of intermediate affinity states of αLβ2 to the transition between inactive and active states and demonstrate the importance of both I domain affinity and overall integrin conformation for cell adhesion. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.