Publish with Bio Med Central and every scientist can read your work free of charge  Proteolytic processing of a precursor protein for a growth-promoting peptide by a subtilisin serine protease in Arabidopsis Phytosulfokines (PSKs) are secreted, sulfated peptide hormones derived from larger prepropeptide precursors. Proteolytic processing of one of the precursors, AtPSK4, was demonstrated by cleavage of a preproAtPSK4–myc transgene product to AtPSK4–myc. Cleavage of proAtPSK4 was induced by placing root explants in tissue culture. The processing of proAtPSK4 was dependent on AtSBT1.1, a subtilisin-like serine protease, encoded by one of 56 subtilase genes in Arabidopsis. The gene encoding AtSBT1.1 was up-regulated following the transfer of root explants to tissue culture, suggesting that activation of the proteolytic machinery that cleaves proAtPSK4 is dependent on AtSBT1.1 expression. We also demonstrated that a fluorogenic peptide representing the putative subtilase recognition site in proAtPSK4 is cleaved in vitro by affinity-purified AtSBT1.1. An alanine scan through the recognition site peptide indicated that AtSBT1.1 is fairly specific for the AtPSK4 precursor. Thus, this peptide growth factor, which promotes callus formation in culture, is proteolytically cleaved from its precursor by a specific plant subtilase encoded by a gene that is up-regulated during the process of transfering root explants to tissue culture. Phytosulfokines (PSKs) are a class of plant peptides discovered through the study of growth factors that mediate density-dependent growth in cell culture . isolated and identified growth factors from conditioned medium that promoted the growth at low density of asparagus mesophyll cells in tissue culture. They identified a sulfated pentapeptide [H-Tyr(SO 3 H)-Ile-Tyr(SO 3 H)-Thr-Gln-OH, abbreviated sYIsYTQ], named PSK-a, and a sulfated tetrapeptide [H-Tyr(SO 3 H)-Ile-Tyr(SO 3 H)-Thr-OH], named PSK-b, that were active in the asparagus cell system.