SummaryCell junctions have both structural and morphogenetic roles, and contain complex mixtures of proteins whose interdependencies are still largely unknown. Junctions are also major signaling centers that signify correct integration into a tissue, and modulate cell survival. During Drosophila eye development, the activity of the immunoglobulin cell adhesion molecule Roughest (also known as Irregular chiasm C-roughest protein) mediates interommatidial cell (IOC) reorganization, leading to an apoptotic event that refines the retinal lattice. Roughest and the cadherin-based zonula adherens (ZA) are interdependent and both are modulated by the apical polarity determinant, Crumbs. Here we describe a novel relationship between the Crumbs partner Heavy -spectrin ( H ), the ZA and Roughest. Ectopic expression of the C-terminal segment 33 of H (H33) induces defects in retinal morphogenesis, resulting the preferential loss of IOC. This effect is associated with ZA disruption and Roughest displacement. In addition, loss-of-function karst and roughest mutations interact to cause a synergistic and catastrophic effect on retinal development. Finally, we show that H coimmunoprecipitates with Roughest and that the distribution of Roughest protein is disrupted in karst mutant tissue. These results suggest that the apical spectrin membrane skeleton helps to coordinate the Cadherin-based ZA with Roughest-based morphogenesis.Key words: Drosophila, Cell adhesion, Apoptosis, Spectrin, Membrane skeleton, Crumbs, IrreC, Roughest
Journal of Cell Science 278Phillips and Thomas, 2006;Stabach et al., 2009) activities, spectrins can variously stabilize proteins at the membrane, nucleate protein complexes and modulate protein delivery and recycling. The Crumbs- H complex has been implicated in the development and maintenance of the apical domain via the modulation of endocytosis in both lossof-function and gain-of-function experiments (Pellikka et al., 2002;Williams et al., 2004). This activity appears to be centered on the Cterminal segment 33 of H (H33), the overexpression of which causes membrane growth and the sequestration of Dynamin (Williams et al., 2004). In some tissues, expression of H33 also induces apoptosis (Williams et al., 2004). These phenotypes are H specific, because overexpression of the equivalent region of the basolateral -spectrin does not have this effect (Williams et al., 2004). Modulation of endocytosis or recycling of junctional proteins through interactions with segment 33 of H might thus provide the normal mechanism whereby the ZA is stabilized.Here, we further characterize the effects of H33 in the developing eye. We find that H33 expression preferentially results in the loss of IOC. We further show that these defects in morphogenesis are associated with fragmentation of the ZA and disruption of the Ig-CAM Roughest. In addition, we show that H genetically and physically interacts with Roughest to maintain its wild-type distribution. Because H also supports the ZA, we suggest that the ac...