AtPSK precursor may be important in determining the site of cleavage, but are not found in the peptide substrates that we used to characterize the cleavage reaction. We have shown that YFP fusions of AtSBT1.1 and PSK4 accumulate in the extracellular matrix, and it is probable that cleavage occurs there simply because AtSBT1.1 has a slightly acid pH optimum and the apoplast is acidic (Bibikova et al., 1998) . PSK4 is tyrosine-sulfated , and the protein is likely to be sulfated in the trans-Golgi as are other sulfated proteins in animal cells (Baeuerle and Huttner, 1987) . Therefore, it seems reasonable that the precursor is sulfated before it is cleaved. However, the precursor does not have to be sulfated to be cleaved, because AtSBT1.1-myc can cleave the unsulfated peptide, fpAtPSK4. Sulfation, however, is important for the function of the peptide, because the sulfated peptide binds to the PSK receptor (Matsubayashi et al., 2002) .