The Drosophila DOCK family protein sponge is involved in differentiation of R7 photoreceptor cells

The Drosophila sponge (spg)/CG31048 gene belongs to the dedicator of cytokinesis (DOCK) family genes that are conserved in a wide variety of species. DOCK family members are known as DOCK1–DOCK11 in mammals. Although DOCK1 and DOCK2 involve neurite elongation and immunocyte differentiation, respectively, the functions of other DOCK family members are not fully understood. Spg is a Drosophila homolog of mammalian DOCK3 and DOCK4. Specific knockdown of spg by the GMR-GAL4 driver in eye imaginal discs induced abnormal eye morphology in adults. To mark the photoreceptor cells in eye imaginal discs, we used a set of enhancer trap strains that express lacZ in various sets of photoreceptor cells. Immunostaining with anti-Spg antibodies and anti-lacZ antibodies revealed that Spg is localized mainly in R7 photoreceptor cells. Knockdown of spg by the GMR-GAL4 driver reduced signals of R7 photoreceptor cells, suggesting involvement of Spg in R7 cell differentiation. Furthermore, immunostaining with anti-dpERK antibodies showed the level of activated ERK signal was reduced extensively by knockdown of spg in eye discs, and both the defects in eye morphology and dpERK signals were rescued by over-expression of the Drosophila raf gene, a component of the ERK signaling pathway. Furthermore, the Duolink in situ Proximity Ligation Assay method detected interaction signals between Spg and Rap1 in and around the plasma membrane of the eye disc cells. Together, these results indicate Spg positively regulates the ERK pathway that is required for R7 photoreceptor cell differentiation and the regulation is mediated by interaction with Rap1 during development of the compound eye.