The dASPP-dRASSF8 Complex Regulates Cell-Cell Adhesion during Drosophila Retinal Morphogenesis

Background: Adherens junctions (AJs) provide structure to epithelial tissues by connecting adjacent cells through homophilic E-cadherin interactions and are linked to the actin cytoskeleton via the intermediate binding proteins beta-catenin and alpha-catenin. Rather than being static; structures, AJs are extensively remodeled during development, allowing the cell rearrangements required for morphogenesis. Several noncore AJ components have been identified, which modulate AJs to promote this plasticity but are not absolutely required for cell-cell adhesion. Results: We previously identified dASPP as a positive regulator of dCsk (Drosophila C-terminal Src kinase). Here we show that dRASSF8, the Drosophila RASSF8 homolog, binds to dASPP and that this interaction is required for normal dASPP levels. Our genetic and biochemical data suggest that dRASSF8 acts in concert with dASPP to promote dCsk activity. Both proteins specifically localize to AJs and are mutually required for each other's localization. Furthermore, we observed abnormal E-cadherin localization in mutant pupal retinas, correlating with aberrant cellular arrangements. Loss of dCsk or overexpression of Src elicited similar AJ defects. Conclusions: Because Src is known to regulate AJs in both Drosophila and mammals, we propose that dASPP and dRASSF8 fine tune cell-cell adhesion during development by directing dCsk and Src activity. We show that the dASPP-dRASSF8 interaction is conserved in humans, suggesting that mammalian ASPP1/2 and RASSF8, which are candidate tumor-suppressor genes, restrict the activity of the Src protooncogene.