Fig. 3. Co-expression of GD, Snk, Ea and Spz leads to activation of a protease cascade. Immunoblot of Spz as in Figure 2. Expression of either Spz 8.19 alone or in combination with Ea zymogen and Snk zymogen led to no processing of Spz. However, co-expression of GD plus the other three proteins led to highly efficient processing of the Spz precursor to generate a 12 kDa fragment. Mutation of the active site serine of GD eliminated processing and GD does not bypass Snk to activate Ea.