Fig. 8. Biochemical model of the dorsal–ventral protease cascade. Based upon our co-expression results, we propose a model in which GD, Snk, Ea and Spz sequentially activate each other. GD, SNK and EA indicate active forms of the proteases. SPZ indicates the ligand form of SPZ. GD, which can become active in the absence of a specific activator, must be kept inactivated via an inhibitory factor. Activation of the cascade would therefore require relief from inhibition by inactivation of this inhibitory factor. Upon complexing with and activating Snk, GD undergoes auto proteolysis to generate cleaved and proteolytically inactive forms represented as GD. An active form of Ea feeds back upon either the proform or the active form of GD to process it and down-regulate the signal.