Activation of a protease cascade involved in patterning the Drosophila embryo

Dorsoventral patterning of the Drosophila embryo is initiated by a ventrali zing signal. Production of this signal requires the serine proteases Gastru lation Defective (GD), Snake, and Easter, which genetic studies suggest act sequentially in a cascade that is activated locally in response to a ventr al cue provided by the pipe gene. Here, we demonstrate biochemically that G D activates Snake, which in turn activates Easter. We also provide evidence that GD zymogen cleavage is important for triggering this cascade but is n ot spatially localized by pipe. Our results suggest that a broadly, rather than locally, activated protease cascade produces the ventralizing signal, so a distinct downstream step in this cascade must be spatially regulated t o restrict signaling to the ventral side of the embryo.