Cloning of the FAK cDNA and initial characterization of the kinase was accomplished independently by three groups in 1992 (Schaller et al., 1992; Hanks et al., 1992; Guan and Shalloway 1992). The cDNA of the human FAK homologue was first cloned by André and Becker-André (1993). The position of the human PTK2 gene encoding FAK on chromosome 8 was first predicted by Fiedorek and Kay (1995).

Initial expression studies using reverse transcriptase PCR detected FAK mRNA in a series of lymphoid cell lines as well as HeLa and SK-N-SH neuroblastoma cells indicating ubiquitous expression. Only a lymphocyte adhesion deficient cell line tested negative for the FAK transcript (André and Becker-André 1993). Transcripts of different sizes were detected in different human tissues in the same study with differential expression patterns for these transcripts noted in brain, lung, heart, liver and placenta.
Several transcript variants encoding different FAK isoforms have been found for the PTK2 gene. The full-length nature of the following three has been determined.
(Sources: http://useast.ensembl.org/index.html; http://www.ncbi.nlm.nih.gov/pubmed/)

- Variant 1 differs in the 5 UTR and coding sequence compared to variant 2. The resulting isoform (a) is shorter at the N-terminus compared to isoform (b).
4414 bp - 33 exons - 1065 aa.
Ave. residue weight: 113.521.
Charge: 3.5.
Isoelectric point: 6.7311.
Molecular weight: 120899.54.
Number of residues: 1065.

- Variant 2 encodes the longest isoform (b).
4286 bp - 31 exons - 1073 aa.
Ave. residue weight: 113.366.
Charge: 2.0.
Isoelectric point: 6.6317.
Molecular weight: 121641.45.
Number of residues: 1073.

- Variant 3 differs in the 5 UTR and coding sequence, and contains two additional in-frame segments near the 3 end of the coding sequence, compared to variant 2. The resulting isoform (c) is shorter at the N-terminus and contains two additional segments in the C-terminus compared to isoform b.

Focal adhesion kinase (FAK) is a cytoplasmic non-receptor protein tyrosine kinase which was isolated for the first time by co-immunoprecipitation of tyrosine-phosphorylated proteins from cells transformed with Rous sarcoma virus v-Src (Kanner et al., 1990).

FAK is a ubiquitously expressed protein composed of a N-terminal FERM domain (protein 4.1, ezrin, radixin, moesin), a kinase domain, three intervening proline-rich regions (PRR) and a C-terminal focal adhesion targeting (FAT) domain (Figure 1).

Ubiquitous.

Cytoplasmic and nuclear.

FAK is characterized by a functional duality, serving as a kinase as well as a molecular scaffold. These two functions may be required independently or in concert depending on the context in which FAK-mediated signaling occurs (Sieg et al., 1999; Sieg et al., 2000). FAK regulates the dynamic of focal adhesion complexes which are sites of attachment between cells and extracellular matrix. Cyclic assembly and disassembly of these complexes at the leading and trailing edge of the cell is required for the migration of mesenchymal cells and those that adopt mesenchymal-like characteristics as a consequence of developmental processes or during disease states. The latter mainly encompasses forms of tissue repair (i.e. wound healing and fibrosis) as well as neoplasia (i.e. tumor invasion and metastasis). As an example, Figure 2 shows immunohistochemical staining for FAK in metastatic cancer of the uterine cervix. FAK has been implicated with the establishment of a front-back polarity (Tilghman et al., 2005), lamellipodial persistence at the leading edge (Owen et al., 2007) and release of adhesions at the trailing edge (Iwanicki et al., 2008).
Integrin engagement with the extracellular matrix results in integrin clustering and a sequence of inter- and intramolecular events that permit the autophosphorylation of FAK at Tyr397 (Dunty et al., 2004). Subsequent recruitment of Src-family kinases through SH2-domain binding is followed by a mutual activation of both kinases. In the case of FAK this further activation is accomplished by phosphorylation of other tyrosine residues, specifically Tyr407, 576, 577, 861 and 925. Phosphorylation of Tyr576 and 577 increases FAK kinase activity whereas the remaining tyrosine residues serve as docking sites for SH2-containing factors such as Grb2 which links FAK into the MAPK pathway. Tyr397 also constitutes a docking site for the p85 subunit of PI3K (Chen and Guan, 1994) and phospholipase C gamma (Zhang et al., 1999). The PRRs are sites of interaction with SH3-containing factors which transmit signals downstream of the kinase and regulate the activity of Rho-family GTPases in charge of cell motility through the formation of stress fibres (RhoA), lamellipodia (Rac) and filopodia (Cdc42). Crk-associated substrate (p130Cas), initially identified in a two-hybrid screen, is one of the main downstream factors that bind to the PRRs of FAK (Polte and Hanks, 1995). Signaling via p130Cas towards Crk, DOCK180 and Rac has been linked to membrane ruffling, lamellipodia formation and cell motility (Harte et al., 1996; Cho and Klemke, 2002). A second essential downstream target of FAK is paxillin (Bellis et al., 1995), an adaptor protein lacking intrinsic kinase activity which can be phosphorylated at two sites, Tyr31 and Tyr118, and binds FAK within the C-terminal focal adhesion targeting (FAT) region (Hayashi et al., 2002). Mutations in FAK that disrupt binding to paxillin affect the localization of the kinase to focal contacts. Paxillin may also be involved in regulation of MAPK downstream signaling due to an overlapping binding site with Grb2 which binds to Tyr925 within the FAT region (Liu et al., 2002).
More recent studies link FAK to the regulation of cell-cell contacts, microtubule stability and control of gene transcription. Conflicting results have been reported from different experimental systems implicating FAK either with the dissolution (Avizienyte et al., 2002; Cicchini et al., 2008) or the promotion (Yano et al., 2004; Playford et al., 2008) of cell-cell contacts which suggests a dependency of this feature on the specific cellular context. Ezratty et al. (2005) reported on the role of FAK, Grb2 and dynamin in microtubule-induced focal adhesion disassembly. Earlier studies demonstrated an integrin-mediated activation of FAK at the leading edge of migrating cells as requirement for microtubule stabilization mediated by Rho and mDia (Palazzo et al., 2004). This mechanism also involves localization of a lipid raft marker, ganglioside GM1, to the leading edge. Xie et al. (2003) showed that Cdk5-mediated serine-phosphorylation of FAK was linked to the localization of the kinase at microtubule fork structures which contribute to nuclear repositioning in migrating neuronal cells. Recently, serine-phosphorylated FAK was shown to co-localize with centrosomes in mitotic endothelial cells. In this study by Park et al. (2009), FAK was also found associated with cytoplasmic dynein, and deletion of FAK resulted in mitotic defects. In 2005, Golubovskaya et al. reported results indicating a physical interaction between the N-terminal fragment of FAK and the N-terminal transactivation domain of p53. This interaction led to suppression of p53-mediated apoptosis and inhibition of the transcriptional activity of p53. Lim et al. (2008) subsequently provided data demonstrating a scaffolding role of nuclear FAK for MDM2-mediated p53 degradation mediated by the different lobes of the FERM domain. Also, basic sequences in the F2 lobe of the FERM domain were implicated in the nuclear localization of FAK (Lim et al., 2008), but alternative mechanisms independent from this putative nuclear localization signal are thought to exist (Schaller, 2010). Another nuclear function was recently uncovered by Luo et al. (2009) who described a role of FAK in chromatin remodelling via its interaction with MBD2 leading to increased myogenin expression and muscle-terminal differentiation. Liu et al. (2004), Li et al. (2004) and Ren et al. (2004) reported an involvement of FAK in netrin-1 signaling downstream of the netrin receptor DCC with consequences for axonal outgrowth and guidance in the developing brain.
- Mouse Models
Several mouse models have been generated to elucidate the functions of FAK both during normal development and neoplasia. A role of FAK in embryonal development was first observed in fak-/- mice which displayed defects in mesoderm development and anterior-posterior axis formation with embryonic lethality around day E8.5 (Ilic et al., 1995).
A conditional knockout model using a Cre-loxP system with Cre recombinase under the control of the nkx2-5 promoter was generated by Hakim et al. (2007). The major abnormality reported from this study was a profound disturbance of the development of the cardiac outflow tract. Knockout mice from this study died shortly after birth and displayed a range of cardiac defects which resemble the human congenital heart defects Tetralogy of Fallot and persistent truncus arteriosus. Peng et al. (2006) and DiMichele et al. (2006) reported results obtained with conditional knockout mice which carried Cre-recombinase under the control of the myosin light chain 2v promoter. Peng et al. (2006) found that knockout mice developed eccentric cardiac hypertrophy upon stimulation with angiotensin II or pressure overload. In contrast, the results by DiMichele et al. (2006) suggest that FAK functions to promote cardiac hypertrophy. In a later study by Peng at al. (2008) with myosin light chain-2a Cre mice they observed cardiac developmental abnormalities with thin ventricular walls and ventricular septal defects in the knockout mice, the majority of which died in the embryonic stage. Endothelial cell-specific knockout of FAK, again using a Cre-loxP approach, has been reported by Shen et al. (2005) and Braren et al. (2006). The observed phenotypes in knockout mice from both studies strongly suggest a role of FAK in vascular morphogenesis, particularly vascular remodelling and sprouting angiogenesis. The roles of FAK in the cardiovascular system were reviewed by Vadali et al. (2007).
A series of mouse models suggest an essential role of FAK during the development of the central nervous system. Beggs et al. (2003) created dorsal forebrain-specific conditional knockout mice using the Cre/loxP approach and observed an essential function of FAK for the formation of a normal basal lamina at the interface between radial glial end-feet and meningeal fibroblasts. They noted that the cortical changes seen in their study resembled lissencephaly phenotypes seen in some forms of human congenital muscular dystrophy.
Van Miltenburg et al. (2009) investigated the role of FAK in normal mammary gland using a conditional FAK-knockout mammary epithelial cell transplantation model based on FAK(lox/lox)/Rosa26Cre-ERT2 donor mice with loss of FAK in all mammary cells. They observed an abnormal mammary duct development with a disruption of myoepithelial and luminal epithelial cell layer and aberrant ductal morphogenesis during pregnancy.
Comprehensive reviews focused on the cellular functions of FAK have been published by Mitra et al. (2005) and Schaller (2010). Figure 3 schematically summarizes some of the diverse cellular functions of FAK.
- Regulation
The level of FAK expression is negatively and positively regulated by several transcription factors including p53, NF-kB and N-Myc (Golubovskaya et al., 2004; Beierle et al., 2007). Aside from the tyrosine residues implicated with FAK activation, at least four different serine phosphorylation sites (Ser722, 840, 843 and 910) have been recognized within FAK. Although the function of these serine sites has been examined less comprehensively, their phosphorylation has generally been associated with FAK inactivation, such as during mitosis (Ma et al., 2001), in suspension and under conditions that disturb the integrity of the actin cytoskeleton (Jacamo et al., 2007). FAK signaling is subject to additional levels of regulation which involve proteolytic cleavage (Dourdin et al., 2001), sumoylation (Kadaré et al., 2003), inhibition by FAK family interacting protein of 200 kDa (FIP200) (Abbi et al., 2002), dephosphorylation by protein-tyrosine phosphatases (Zeng et al., 2003), and generation of alternatively spliced isoforms such as FAK-related non-kinase (FRNK) (Schaller et al., 1993).
- Other protein family members: Pyk2.

Homo sapiens PTK2	% Identity for	Protein	DNA
vs. Pan troglodytes PTK2		99.8	99.7
vs. Canis lupus familiaris PTK2		97.0	91.7
vs. Mus musculus Ptk2		97.2	90.7
vs. Rattus norvegicus Ptk2		97.0	90.8
vs. Gallus gallus PTK2		94.9	83.9
vs. Danio rerio ptk2.1		83.2	74.2
vs. Drosophila melanogaster Fak56D		42.9	48.9
vs. Caenorhabditis elegans kin-32		36.5	47.8

(Source : http://www.ncbi.nlm.nih.gov/pubmed/)