The Structural Basis of Localization and Signaling by the Focal Adhesion Targeting Domain

Abstract

The localization of focal adhesion kinase (FAK) to sites of integrin clustering initiates downstream signaling. The C-terminal focal adhesion targeting (FAT) domain causes this localization by interacting with talin and paxillin. FAT also mediates signaling through Grb2 via phosphorylated Y925. We report two crystal structures of the FAT domain. Large rearrangements of the structure are indicated to allow phosphorylation of Y925 and subsequent interaction with Grb2. Sequence homology and structural compatibility suggest a FAT-like fold for the C-terminal domains of CAS, Efs/Sin, and HEF1. A structure-based alignment including these proteins and the vinculin tail domain reveals a conserved region that could play a role in focal adhesion targeting. Previously postulated "paxillin binding subdomains" may contribute to structural integrity rather than directly to paxillin binding.

Paxillin: Adapting to Change

“…Upon their identification, the leucine-rich paxillin LD motifs were modeled and suggested to fold as amphi-pathic ␣-helices with the leucines providing a hydrophobic interface with its binding partners (23, 228, 274), a prediction subsequently confirmed experimentally (7,91,152). The individual paxillin (and Hic-5) LD motifs are flanked by proline-and glycine-rich segments (average composition of 25%), which may also contribute to the global folding, presentation, regulation, and function of the individual paxillin protein binding domains.…”

“…containing PKL carboxy terminus also folds in a similar manner (7). In contrast, this structural organization is unlikely in the case of the ILK and actopaxin PBS domains (7,183).…”

“…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).…”

“…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.…”

“…In contrast, this structural organization is unlikely in the case of the ILK and actopaxin PBS domains (7,183). Such differences are likely to dictate the optimal binding parameters for each protein and contribute to selection of the "appropriate" paxillin binding partner(s) in response to a particular extracellular cue.…”