FAK

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Introduction

Cell migration is a complex and coordinated process that involves rapid changes in the dynamics of actin filaments, as well as the formation and decomposition of cell adhesion sites. The complex interaction between the actin cytoskeleton and cell adhesion sites leads to membrane protrusion and traction forces. Transmembrane integrins transduce external stimuli for cell migration into intracellular biochemical signals.

For cells to correctly handle these different environmental motility-promoting stimuli correctly (such as shear stress that promotes the deformation of the actin cytoskeleton), it must rely on intracellular signalling proteins, which are stimulated by multiple extracellular inputs and that function to regulate multiple signalling pathway outputs. Here, we describe the unique molecular connections of focal adhesion kinase (FAK) that enable this tyrosine kinase act as an important receptor-proximal regulator of cell shape, adhesion, and motility.

The FERM and FAT domains of FAK

FAK is a 125kDa protein tyrosine kinase consisting of an N-terminal FERM domain, a central kinase domain, a proline-rich region, and a C-terminal focal-adhesion targeting (FAT) domain. The FERM domain of FAK promotes signalling linkage such as epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR). When analyzing the cell-motility-promoting signals initiated by the G protein-coupled receptor (GPCR), the overexpression of the FAK FERM domain prevented FAK activation and ultimately inhibited cell migration stimulated by the G-protein. FAK is post-translationally modified by covalently adding a small ubiquitin-related modifier (SUMO) to the ε-amino position of Lys152. In most cases, sumoylation is related to the nuclear import of proteins. Although the use of leptomycin B to block nuclear export promotes the nuclear accumulation of FAK, and the exogenous expression of the FAK FERM domain exhibits strong nuclear localization, it is not clear whether these events are dependent on sumoylation.

The C-terminal domain of FAK contains two proline-rich regions, which are binding sites for proteins containing SRC-homology (SH)3-DOMAIN. The SH3 domain-mediated binding of the adaptor protein p130Cas and FAK promotes cell migration through coordinated activation of Rac at the membrane extension.

The best-characterized FAK phosphorylation event is autophosphorylation at Tyr397, which can occur in cis or trans. Various SH2-DOMAIN-containing proteins can recognize motifs produced by phosphorylation at Tyr397, these proteins include Phospholipase Cγ (PLCγ), SRC- FAMILY Kinase (SFKs), growth-factor-receptor-bound protein-7 (GRB7), the Shc adaptor protein, p120 RasGAP, and the p85 subunit of phosphatidylinositol 3-kinase (PI3K). In this regard, there is a sequential association mode in which the combination of cellular Src (hereinafter referred to as Src) and FAK initiates signal transduction, and the association of SOCS and FAK is a terminal event leading to ubiquitin-mediated FAK degradation.

Figure 1. Focal adhesion kinase domain structure and phosphorylation sites (Mitra, S.K.; et al.2005)

FAK as a biosensor

Integrins can "sense" environmental cues and control anchorage-dependent cell proliferation and survival. Integrins are also closely involved in the conversion of physical signals (such as contractile forces or external mechanical perturbations) into chemical signalling events, and FAK activation is an important part of this " mechanosensing" of cells. Early observations indicate that FAK can be activated by the tangential fluid shear stress of endothelial cells, which is related to the formation of the FAK-Src signaling complex, and FAK Tyr925 phosphorylation, etc. FAK phosphorylation is enhanced at the leading edge of motile cells.

Another means of "mechanoperception" is the ability of cells to sense the "rigidity" of the surrounding ECM. This response requires FAK expression, and FAK-/- cells are not sensitive to changes in substrate flexibility. Interestingly, the Tyr397Phe mutant of FAK showed a similar durotaxis response to wild-type cells. Although it is unclear whether the intrinsic FAK catalytic activity or the role of FAK as a scaffold protein at the focal contact is a determinant of durotaxis response, this observation is still one of the few examples in which Tyr397 FAK phosphorylation does not require a signalling response.

Figure 2. Focal adhesion kinase influences phospholipid and microtubule structure (Mitra, S.K.; et al.2005)