Integrin structure: new twists and turns in dynamic cell adhesion

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Abstract
Summary: The divalent-cation-dependent binding of αβ heterodimeric integrins to their ligands regulates most cellular processes. Integrin–ligand interactions are tightly controlled by inside-out activation signals. Ligand-bound integrins transduce outside-in signals typical of other receptors. Precise information of how ligands bind to integrins is restricted to that of a small vWF A-type domain present in some α-subunits (αA). Both inside-out and outside-in signals elicit tertiary and quaternary changes in integrins, but the precise nature and scope and of these changes are unknown. The recently solved structures of the extracellular segment of integrin αVβ3 in its unliganded and liganded states are generating exciting new insights into the design, wiring, function and regulation of this protein family. The structures reveal a surprising degree of flexibility at defined regions in the structure that is potentially controlled by cations. The quaternary structure of the ligand-binding region bears a striking resemblance to the nucleotide-binding pocket of G-proteins, implying analogous signaling mechanisms. Structural links exist through which ligand-induced tertiary changes may be translated into quaternary changes and vice versa. The structures also raise the tantalizing hypothesis that αA is a regulated endogenous integrin ligand, so that no special regulatory features are needed in this integrin. These findings provide the framework for new investigations of structure–activity relationships in integrins, with important implications for targeting these receptors therapeutically.