Localization of focal adhesion kinase isoforms in cells of the central nervous system

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase which in non-neuronal cells is localized to focal adhesions, where it participates to adhesion-dependent intracellular signalling. FAK is highly expressed in the central nervous system both during development and in the adult. FAK(+), a splice isoform of FAK selectively enriched in neurons, contains a three-amino acid insertion in the carboxy-terminal sequence responsible for the localization of FAK to focal adhesions. Enhanced green fluorescent protein-tagged constructs were used to study the targeting of FAK and FAK(+) in neuronal and non-neuronal cells of the central nervous system. In transfected non-neuronal cells, both fusion proteins colocalized with vinculin in focal contacts. When expressed in hippocampal neurons in culture, both chimeras were locally concentrated in the growth cone, where they overlapped with F-actin enrichments but not with vinculin. In the growth cone of living neurons, the FAK(+) chimera showed a dynamic relocalization to membrane ruffles and to the tips of the membrane protrusions induced by cytochalasin D treatment, indicating a dependence of FAK distribution on F-actin organization. Since virtually identical patterns of distribution were found for FAK and FAK(+) chimeras, it follows that the additional insertion in FAK(+) is not responsible for the localization of the kinase. Finally, we showed that the carboxy-terminal domain of both FAK and FAK(+) is sufficient to mediate the localization of the proteins to focal adhesions in non-neuronal cells and to maintain their correct intracellular targeting in neurons.