To confirm that AtSBT1.1 is indeed involved in the processing of pAtPSK4-myc, we tested whether pAtPSK4 can be cleaved by AtSBT1.1 in vitro. To do so, we developed a pull-down assay using a C-terminal myc-tagged version of AtSBT1.1 (AtSBT1.1-myc) and a fluorogenic peptide substrate. We employed a similar assay in a previous study to test the activity of another Arabidopsis subtilase, AtS1P (Liu et al., 2007a) . In both cases, the tagged subtilases were synthesized in transgenic Arabidopsis because we were not able to produce active enzyme at high levels in heterologous systems.