Regulation of Easter activity is required for shaping the Dorsal gradient in the Drosophila embryo

Dorsoventral polarity of the Drosophila embryo requires maternal spatzle-Toll signaling to establish a nuclear gradient of Dorsal protein. The shape of this gradient is altered in embryos produced by females carrying dominant alleles of easter (ea(D)). The easter gene encodes a serine protease that generates processed Spatzle, which is proposed to act as the Toll ligand. By examining the expression domains of the zygotic genes zen, sog, rho and twist, which are targets of nuclear Dorsal, we show that the slope of the Dorsal gradient is progressively flattened in stronger eaD alleles. In the wild-type embryo, activated Easter is found in a high M, complex called Ea-X, which is hypothesized to contain a protease inhibitor. In eaD embryo extracts, we detect an Easter form corresponding to the free catalytic domain, which is never observed in wild type. These mutant eaD proteins retain protease activity, as determined by the production of processed Spatzle both in the embryo and in cultured Drosophila cells. These experiments suggest that the eaD mutations interfere with inactivation of catalytic Easter, and imply that this negative regulation is essential for generating the wild-type shape of the Dorsal gradient.