SUMMARYApical constriction is a major mechanism underlying tissue internalization during development. This cell constriction typically requires actomyosin contractility. Thus, understanding apical constriction requires characterization of the mechanics and regulation of actomyosin assemblies. We have analyzed the relationship between myosin and the polarity regulators Par-6, aPKC and Bazooka (Par-3) (the PAR complex) during amnioserosa apical constriction at Drosophila dorsal closure. The PAR complex and myosin accumulate at the apical surface domain of amnioserosa cells at dorsal closure, the PAR complex forming a patch of puncta and myosin forming an associated network. Genetic interactions indicate that the PAR complex supports myosin activity during dorsal closure, as well as during other steps of embryogenesis. We find that actomyosin contractility in amnioserosa cells is based on the repeated assembly and disassembly of apical actomyosin networks, with each assembly event driving constriction of the apical domain. As the networks assemble they translocate across the apical patch of PAR proteins, which persist at the apical domain. Through loss-and gain-of-function studies, we find that different PAR complex components regulate distinct phases of the actomyosin assembly/disassembly cycle: Bazooka promotes the duration of actomyosin pulses and Par-6/aPKC promotes the lull time between pulses. These results identify the mechanics of actomyosin contractility that drive amnioserosa apical constriction and how specific steps of the contractile mechanism are regulated by the PAR complex.

BackgroundThe development and homeostasis of multicellular organisms depends on sheets of epithelial cells. Bazooka (Baz; PAR-3) localizes to the apical circumference of epithelial cells and is a key hub in the protein interaction network regulating epithelial structure. We sought to identify additional proteins that function with Baz to regulate epithelial structure in the Drosophila embryo.Methodology/Principal FindingsThe baz zygotic mutant cuticle phenotype could be dominantly enhanced by loss of known interaction partners. To identify additional enhancers, we screened molecularly defined chromosome 2 and 3 deficiencies. 37 deficiencies acted as strong dominant enhancers. Using deficiency mapping, bioinformatics, and available single gene mutations, we identified 17 interacting genes encoding known and predicted polarity, cytoskeletal, transmembrane, trafficking and signaling proteins. For each gene, their loss of function enhanced adherens junction defects in zygotic baz mutants during early embryogenesis. To further evaluate involvement in epithelial polarity, we generated GFP fusion proteins for 15 of the genes which had not been found to localize to the apical domain previously. We found that GFP fusion proteins for Drosophila ASAP, Arf79F, CG11210, Septin 5 and Sds22 could be recruited to the apical circumference of epithelial cells. Nine of the other proteins showed various intracellular distributions, and one was not detected.Conclusions/SignificanceOur enhancer screen identified 17 genes that function with Baz to regulate epithelial structure in the Drosophila embryo. Our secondary localization screen indicated that some of the proteins may affect epithelial cell polarity by acting at the apical cell cortex while others may act through intracellular processes. For 13 of the 17 genes, this is the first report of a link to baz or the regulation of epithelial structure.