Cytoskeletal Interactions with the Leukocyte Integrin beta 2 Cytoplasmic Tail: Activation-dependent Regulation of Associations with Talin and alpha -actinin

Circulating leukocytes are nonadherent but bind tightly to endothelial cells following activation. The increased avidity of leukocyte integrins for endothelial ligands following activation is regulated, in part, by interaction of the beta 2 subunit cytoplasmic tail with the actin cytoskeleton. We propose a mechanism to explain how tethering of the actin cytoskeleton to leukocyte integrins is regulated. In resting leukocytes, beta 2 integrins are constitutively linked to the actin cytoskeleton via the protein talin. Activation of cells induces proteolysis of talin and dissociation from the beta 2 tail. This phase is transient, however, and is followed by reattachment of actin filaments to integrins that is mediated by the protein alpha -actinin. The association of alpha -actinin with integrins may stabilize the cytoskeleton and promote firm adhesion to and migration across the endothelium. Glutathione S- transferase- beta 2 tail fusion protein/mutagenesis experiments suggest that the affinity of alpha -actinin binding to the beta 2 tail is regulated by a change in the conformation of the tail that unmasks a cryptic alpha -actinin binding domain. Positive and inhibitory domains within the beta 2 tail regulate alpha -actinin binding: a single 11-amino acid region (residues 736-746) is necessary and sufficient for alpha -actinin binding, and a regulatory domain between residues 748-762 inhibits constitutive association of the beta 2 tail with alpha -actinin.