Stewart, Alasdair Gwilym; (2005) Studies of focal adhesion kinase in epithelial cells: Involvement in cell-cell adhesion. Doctoral thesis , UCL (University College London).

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Abstract

Epithelial cell-cell adhesion is mediated by tight junctions, adherens junctions and desmosomes. Epithelial cell-matrix adhesion is mediated by hemidesmosomes and focal contacts. These complexes exhibit great plasticity, and each contains molecular components which are able to participate in one or more of the other adhesive complexes. Focal adhesion kinase (FAK/pl25FAK) is a non-receptor tyrosine kinase which transduces signals from integrins at sites of focal contact to promote adhesion, spreading and migration. FAK possesses a central kinase domain which is flanked by large, non-catalytic, amino- and carboxy-terminal domains. Whereas the functions of the carboxy-terminal and kinase domains of FAK are well understood, the role of the amino-terminal domain remains unclear. FAK expression was examined in the human epithelial cell line, HEK 293. Amino-terminal FAK immunoreactivity was noted at sites of cell-cell contacts and in the nucleus, in contrast to carboxy-terminal immunoreactivity, which was largely cytoplasmic and perinuclear. Western blot analysis of endogenous FAK revealed expression of a presumptive proteolytic cleavage fragment corresponding to the amino- terminal domain. A series of FAK constructs was generated to test the hypothesis that the observed amino-terminal FAK localisation was due to this proteolytic fragment. Epitope- tagged Amino-Terminal FAK (ATF) constructs localised primarily at areas of cell-cell contact and in the nucleus in HEK 293 cells. This localisation was independent of Tyrosine 397, the major FAK autophosphorylation site. This sub-cellular distribution was confirmed in another epithelial cell line, MDCK, in which transiently transfected ATF constructs also localised primarily to the nucleus and at cell-cell contacts. HEK 293 cells were characterised with respect to expression of adhesive proteins, and ATF was found to co- localise with the tight junction protein occludin, with cortical actin and with junctional ?1 integrin. Immunoprecipitation data suggests that none of these proteins forms a precipitable complex with ATF. These findings indicate that the amino-terminal domain of FAK is capable of localising at epithelial cell-cell contacts and suggest a novel role for FAK in mediating cross-talk between focal contacts and cell-cell contacts through endogenously expressed amino-terminal FAK fragments.

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