Development requires activation but not phosphorylation of β1 integrins

Named after the ability of many of them to “integrate” the extracellular matrix with the intracellular cytoskeleton, integrins are well-characterized mediators of cellular adhesion and signaling (Giancotti and Ruoslahti 1999; Hynes 2002). Each integrin consists of an and a subunit and binds to a distinct, but often overlapping, spectrum of ligands. Whereas 1 integrins mediate cell adhesion to extracellular matrix components, 2 integrins play key roles in immune cell recognition and activation by interacting with counterreceptors of the Ig superfamily, and IIb 3 orchestrates platelet aggregation by binding to the polyvalent blood protein fibrinogen. Genetic studies using mouse models have demonstrated that 1 integrins have unique roles in embryonic development, hematopoiesis, wound healing, and cancer (Brakebusch et al. 1997; Grose et al. 2002; White et al. 2004). Successful execution of a number of physiological processes requires precisely choreographed changes in the activity of several integrins. The regulation of integrin functions is complex, and recent studies suggest that it hinges on long-range structural changes, which are propagated across the plasma membrane in both directions (Hynes 2002). In spite of its importance, genetic analysis of this aspect of integrin function has lagged behind. A new paper by Chen et al. (2006), published in the April 15 issue of Genes and Development, intends to remedy this imbalance. Many integrins are maintained in a default low-affinity state and, hence, need to be activated to exert their function. The past decade has seen the emergence and consolidation of two major paradigms for integrin activation and signaling. The first describes the process through which cytoplasmic signals trigger integrin activation (“inside-to-outside” integrin signaling) (Ginsberg et al. 1992). Most, perhaps all, activating signals promote the binding of talin to the cytoplasmic portion of the integrin subunit (Liddington and Ginsberg 2002). Talin binding promotes a separation of the integrin cytoplasmic tails and triggers a series of long-range conformation changes, including a “switch-blade” movement of the integrin head domains, resulting in increased availability of the extracellular ligand-binding site (Shimaoka et al. 2002). The immediate outcome of this form of regulation is an increase in the affinity of integrins for their matrix ligands. The second process, “outside-to-inside” integrin signaling, flows in the opposite direction. Upon binding to extracellular matrix components, integrins multimerize and recruit complexes of cytoskeletal and signaling molecules. Using this latter mechanism, integrins cooperate with receptors for cytokines and growth factors to control a multitude of cellular functions, including cell adhesion, migration, survival, differentiation, and proliferation (Assoian and Schwartz 2001; Miranti and Brugge 2002; Giancotti and Tarone 2003). Both inside-to-outside and outside-to-inside integrin signaling rely on a combination of regulatory controls that involves alterations in the structure of the receptors’ cytoplasmic domains, long-range conformational changes in integrin structure, a steep increase in ligand-binding affinity, and finally, clustering of integrins and downstream signal transduction. Mutagenesis and structural studies have painted a particularly vivid picture of integrin activation. Integrins are maintained inactive by interactions between the transmembrane domains and the membrane-proximal segments of the cytoplasmic tails of their constituent subunits (Fig. 1; Liddington and Ginsberg 2002). Substitution with Ala of an Asp residue in the membraneproximal segment of the subunit or an Arg residue in the corresponding region of the subunit causes constitutive integrin activation, suggesting that a salt bridge between the side chains of the two amino acids stabilizes the inactive conformation (Hughes et al. 1996). In agreement with this notion, forced dimerization of the and tails prevents integrin activation (Lu et al. 2001). What is the mechanism through which cytoplasmic signals disrupt the – tail “clasp”? The C-terminal portion of all subunits contains two tandem NPxY motifs, and compelling evidence indicates that the N-terminal motif functions to “unclasp” the integrin tails (Fig. 1). NMR structural analysis indicates that this motif forms a turn, creating a binding interface for a phospho-tyroCorrespondence. E-MAIL f-giancotti@ski.mskcc.org; FAX (212) 794-6236. E-MAIL yvp2001@med.cornell.edu; FAX (212) 794-6236. Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/ gad.1432006.