Echnoid (Ed) is a cell adhesion molecule (CAM), which regulates many function durng Drosophila development. To examine how this cell adhesion molecule exerts its biological functions, several screens have been performed. One major protein was identified to be Receptor of Activated protein C Kinase 1 (RACK1). The molecular interaction between Ed and RACK1 was confirmed both in vivo and in vitro (Hsu, unpublished data). RACK1 thus raised our interests to investigate.
We obsreved that RACK1 mutant cell form rough and concaved compound eye. The RACK1 mutant photoreceptors located basally in third instar larvae eye disc. These mislocalized photoreceptors express the epithelial markers such as aPKC, apical junction proteins (Patj), Disc large (Dlg), and E-cadherin above their nucleus. These proteins are present apical to mislocalized nuclei, but a more basal localization. The apical constriction in RACK1 mutant clones of furrow was normal, and no apical constriction were observed in more posterior region. We found that RACK1 exerts its effects on eye development by regulating the accumulation of F-actin and phosphorylation of MRLC of nonmuscle myosin II. The accumulation of F-actin in ectopic clones were present both anterior and posterior to the furrow, and the excessive F-actin present in RACK1 mutant photoreceptors causes them to adopt a more basal location in the eye disc. Also the level of F-actin and phosphorylated MRLC is increased in RACK1 mutant clones were also found in wing discs. During oogenesis, RACK1 mutant follicle cell sometimes form multiple layer in later stages with decreased F-actin expression.

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