Although the expression and altered function of non-receptor tyrosine kinases (NRTKs) in various tumor types have been described, the normal function of many NRTKs has remained elusive. Members of the mammalian fes/fps/fer gene family encode for NRTKs originally identified as viral oncogenes. Although the fes/fps genes have been studied extensively, most studies of Fer kinase have been performed in vitro, and it's in vivo functions have not yet been identified. We are investigating the function of the C. elegans homologue of Fer kinase,
frk-1 (Fer related kinase -1). FRK-1 is most similar to the truncated isoform of the Fer kinase, FerT, which lacks the coiled-coiled domains found in the N-terminus of the longer mammalian isoform. FRK-1 antibodies detect only a single 45kDa protein in C. elegans extracts, suggesting that C. elegans, unlike mammals, does not express a long isoform. FRK-1 is detectable throughout the life of the worm, including very early embryos, suggesting that it is maternally provided. In early embryos, immunoreactive FRK-1 localizes to the cell boundaries and nuclei; however, it is detectable exclusively at the plasma membrane later in embryogenesis, at the time that most cells have become post-mitotic and just prior to enclosure. Expression is seen in most epithelial tissue types including epidermis, pharynx, intestine, and muscle. Consistent with a role in epithelial morphogenesis, RNAi of
frk-1 leads to embryonic lethality with severe defects in morphogenesis resulting from a failure in epidermal enclosure. mDf7, which deletes
frk-1, results in a similar morphogenetic phenotype and a failure to express seam cell-specific differentiation markers (Terns et al., 1997). We found that the enclosure defect of mDf7 is rescued by
frk-1(+); rescued animals elongate partially and express markers of seam differentiation. In cultured mammalian cells, Fer kinase has been shown to interact with -catenin in the plasma membrane cadherin complex, and associates with the -integrin complex in the absence of cadherin function. We found that membrane localization of FRK-1 is severely reduced in mutants of
hmp-2 and that it becomes primarily nuclear-localized in epidermal cells. While HMP-2 is a non-nuclear -catenin that has not been implicated in Wnt signaling, we found that loss of
frk-1 rescues the gutless phenotype caused by the absence of WRM-1 (the nuclear, Wnt-involved -catenin). This observation suggests that HMP-2 can function in the Wnt signaling pathway, but does not normally do so because FRK-1 retains it at the plasma membrane. Surprisingly, FRK-1 also localizes to the nuclei of epidermal cells in the
pat-3 ( -integrin) mutant, revealing a potential integrating function in both cadherin- and integrin-mediated cell adhesion systems. We are currently investigating the role of FRK-1 in cellular adhesion and cell rearrangements that occur during morphogenesis by examining how FRK-1 interacts with cadherin and integrin complexes at the cell membrane and whether FRK-1 mediates cross-talk between these two adhesion systems. We are also further examining interactions of FRK-1 with the Wnt signaling pathway.