Vinculin activation by talin through helical bundle conversion

Abstract

Vinculin is a conserved component and an essential regulator of both cell-cell (cadherin-mediated) and cell-matrix (integrin-talin-mediated focal adhesions) junctions, and it anchors these adhesion complexes to the actin cytoskeleton by binding to talin in integrin complexes or to alpha-actinin in cadherin junctions. In its resting state, vinculin is held in a closed conformation through interactions between its head (Vh) and tail (Vt) domains. The binding of vinculin to focal adhesions requires its association with talin. Here we report the crystal structures of human vinculin in its inactive and talin-activated states. Talin binding induces marked conformational changes in Vh, creating a novel helical bundle structure, and this alteration actively displaces Vt from Vh. These results, as well as the ability of alpha-actinin to also bind to Vh and displace Vt from pre-existing Vh-Vt complexes, support a model whereby Vh functions as a domain that undergoes marked structural changes that allow vinculin to direct cytoskeletal assembly in focal adhesions and adherens junctions. Notably, talin's effects on Vh structure establish helical bundle conversion as a signalling mechanism by which proteins direct cellular responses.

Paxillin: Adapting to Change

“…With respect to vinculin binding, the face of the vinculin H2/5 helix, which corresponds to H1/4 of FAK and could potentially function as a second paxillin LD binding site, is blocked. Evidence exists, however, that this region may be regulatable by acidic phospholipids and/or other vinculin binding partners (10,96,114). Whether access is controlled and the proper hydrophobic interface is available for paxillin LD interactions under physiological conditions to perhaps stabilize this association and provide a platform for actin assembly remains to be determined.…”

“…The vinculin tail and FAK FAT domain share a parallel up-down-up-down four-helix bundle. This general structural organization is shared by ␣-catenin, apolipoprotein E, and the p130Cas family of proteins (7,10,91,114), although only vinculin and FAK bind paxillin. Interestingly, structural predictions suggest that the PBS-FIG.…”

“…The crystal structures of the vinculin tail and the FAK FAT domain, which contain the PBS, have been solved (7,10,91,114). FAK FAT solution structures have also been reported (66, 152).…”

Structural Basis for Amplifying Vinculin Activation by Talin

“…Crystallization and X-ray Data Collection-The His 6 -tagged human Vh (residues 1-258) and Vt (residues 879 -1066) expression constructs and the purification of Vh and Vt proteins have been described previously (32).…”

“…However, other models are now equally plausible because talin VBS3 and the vinculin binding site of ␣-actinin can displace Vt from preexisting Vh⅐Vt complexes (32). Furthermore, the crystal structure of the Vh⅐VBS3 complex established that talin VBS3 distorts the binding site for Vt in Vh from a distance, by provoking dramatic alterations in the structure and positions of the ␣-helices of the N-terminal helical bundle of Vh, by a process coined as helical bundle conversion (32).…”