Practice question

Dorsal protein activity gradient depends on physical sequestration by Cactus inhibitor forming stable cytoplasmic heterodimer covering Dorsal nuclear localization signal and DNA-binding domain. Complex concentration high dorsally where Cactus unphosphorylated, low ventrally where Toll signaling triggers Pelle kinase phosphorylation and Slimb-mediated degradation. Gurken-Torpedo regulates follicle Pipe transcription upstream, Pipe-Spatzle regulates Toll ligand processing extracellularly, Wingless-Frizzled regulates segment polarity maintenance. Dorsal-Cactus interaction therefore represents final intracellular checkpoint directly restricting transcription factor availability. Stoichiometric excess of Cactus can ventralize embryo, while Cactus degradation dynamics sharpen Dorsal nuclear gradient establishing thresholds for twist, snail, and short gastrulation activation along DV axis.