Abstract
Actomyosin networks generate contractile force that changes cell and tissue shape. In muscle cells, actin filaments and myosin II appear in a polarized structure called a sarcomere, in which myosin II is localized in the center. Nonmuscle cortical actomyosin networks are thought to contract when nonmuscle myosin II (myosin) is activated throughout a mixed-polarity actin network. Here, we identified a mutant version of the myosin-activating kinase, ROCK, that localizes diffusely, rather than centrally, in epithelial cell apices. Surprisingly, this mutant inhibits constriction, suggesting that centrally localized apical ROCK/myosin activity promotes contraction. We determined actin cytoskeletal polarity by developing a barbed end incorporation assay for Drosophila embryos, which revealed barbed end enrichment at junctions. Our results demonstrate that epithelial cells contract with a spatially organized apical actomyosin cortex, involving a polarized actin cytoskeleton and centrally positioned myosin, with cell-scale order that resembles a muscle sarcomere.