AtSBT1.1 functions similarly to prohormone convertases that release peptide hormones and neuropeptides from protein precursors in animal cells (Steiner, 1998) . Prohormone convertases are subtilases in the secretory pathway that cleave substrates with mono-or di-basic amino acids in the general recognition motif R/K-Xn-R/Kfl (Seidah, 1997) . In Arabidopsis, AtSBT1.1 cleaves substrates in a fairly site-specific manner, cleaving pAtPSK4 at RRSLVLflHTDY and showing preference for cleavage of proAtPSK4 over other PSK precursors. Cleavage by AtS-BT1.1 at the preferred cleavage site of the PSK4 precursor leaves three residues on the N-terminus of the five-residue peptide. This is of concern because demonstrated that even the presence of the tripeptide GGG severely reduces the activity of PSKs in an asparagus suspension cell system However, the three residues (HTD) remaining on the amino-end of AtSBT1.1processed AtPSK4 are highly conserved among the PSKs, and it is possible that the sequence is part of the active, mature peptide or serves as a recognition signal for further processing by enzymes such as tripeptyl peptidases (Book et al., 2005) . Complete proteolytic processing of PSK4 probably involves a number of other steps, including trimming of the peptide at its C-terminus .