Integrin-mediated cell adhesion: the cytoskeletal connection

Members of the integrin family of cell adhesion molecules play a pivotal role in the interaction between animal cells and the extracellular matrix. This article reviews the evidence (i) that the integrin beta-subunit cytoplasmic domain is important in the localization of integrins to focal adhesions, and for integrin-mediated cell adhesion/spreading; and (ii) that the integrin beta-subunit can be linked to F-actin via the actin-binding proteins talin, alpha-actinin and filamin. Talin has two or more actin-binding sites, and three binding sites for the cytoskeletal protein vinculin. Because vinculin can also bind F-actin, it may cross-link talin and actin, thereby stabilizing the interaction. In addition, vinculin contains a binding site for VASP (vasodilator-stimulated phospho-protein), a protein which may serve to recruit a profilin/G-actin complex to talin, which has actin-nucleating activity. Evidence that talin, vinculin and alpha-actinin are important in the assembly of focal adhesions, obtained using antisense technology and protein microinjection, is reviewed. To analyse the role of talin in focal adhesions, we have disrupted both copies of the talin gene in mouse embryonic stem (ES) cells. Undifferentiated talin (-/-) ES cell mutants are unable to assemble focal adhesions when plated on fibronectin, whereas vinculin (-/-) ES cells are able to do so. Finally, the role of small GTP-binding proteins in the assembly of focal adhesions is discussed, along with our recent studies using streptolysin-O-permeabilized Swiss 3T3 cells which suggest that the GTP-binding protein ADP-ribosylation factor-1 (ARF-1) is important in targeting the protein paxillin to focal adhesions.