If pAtPSK4 is indeed a substrate for AtSBT1.1, then these proteins should occupy the same subcellular compartment. Both AtSBT1.1 and ppAtPSK4 have signal peptides, and both are predicted to be secreted proteins. To determine whether that is so, we constructed C-terminal YFP fusions and determined their location in root cells from Arabidopsis. Both AtPSK4-YFP (Figure 3a-c) and AtSBT1.1-YFP (Figure 3d-f) are associated with the periphery of the cell, coinciding with propidium iodide staining. Following plasmolysis (Von Groll et al., 2002) , most fluorescence from both YFP fusions remained associated with the extracellular matrix rather than the plasma membrane ( Figure S4 ).