FIGURE 10.

Model for the role of FAT conformational dynamics in FAK function at focal adhesions. The FAT four-helix bundle exists in closed (1) and open (2) conformations in a dynamic equilibrium that is strongly shifted toward the closed state. By decreasing or increasing the propensity of the H1-H2 hinge to open, R-FAK and T-FAK mutants favor the closed and open FAT domain conformation, respectively. The scheme depicts the proposed role of these forms in FAK function at FAs. 1, the closed conformation of FAT has a strong affinity for paxillin LD motifs (two binding sites in FAT) and FAK can be strongly recruited to FAs through this interaction. However, in this configuration, Tyr⁹²⁵ is buried in the four-helix bundle. Ser⁹¹⁰ and Tyr⁸⁶¹ are also poorly accessible, possibly because of their masking by intramolecular interactions. FAT may also interact with FERM (20) (not shown in the scheme). 2, local accumulation of FAK at FAs promotes FAK dimerization through FERM-FERM interactions (20) (only the FERM domain of the second FAK molecule is drawn in dark gray). Possibly with the help of co-activators, the association between FERM and kinase domains in the dimer is loosened, which promotes autophosphorylation by intermolecular transphosphorylation of Tyr³⁹⁷. The Src homology 3 and 2 domains of SFK bind to a proline-rich motif 1 region and Tyr(P)³⁹⁷, respectively, in the FERM:kinase linker peptide. Either spontaneously (in a stochastic manner) or in response to unknown factors, H1 dissociates from the rest of the bundle. In the open conformation of FAT, the affinity of paxillin for the H1/H4 binding site on FAT is completely lost. Opening of FAT H1 is also expected to alter the stability and disposition of the other three helices, compromising paxillin binding to the H2/H3 site (32, 33); Tyr⁹²⁵ is exposed and can be phosphorylated by SFKs and then bind Grb2, which activates the ERK pathway. Through unfolding and/or unmasking of the linker region between FAT and the kinase domain, Ser⁹¹⁰ and Tyr⁸⁶¹ become exposed and accessible to ERK and SFKs, respectively. Together with the loss of paxillin affinity, phosphorylation of the above sites contributes to detachment of FAK from FAs for degradation or recycling.