Identification of a 50-kDa Systemin-Binding Protein in Tomato Plasma Membranes Having Kex-2p-Like Properties

A protein of 50-kDa (SBP50) was identified in plasma membranes of tomato leaves which resembles proteases of the family of Kex2p-like prohormone convertases. To our knowledge, proteases of this class have not been reported in plants previously. A biotinylated derivative of systemin, the 18-aa polypeptide inducer of proteinase inhibitors in tomato and potato leaves, was bound by SBP50 with high specificity. When a systemin derivative was labeled with biotin at residue 8 and with [35S]methionine at position 15, the biotin moiety but not the radioactive label was bound by SBP50. At least 4 aa from the C terminus that included [35S]methionine were missing, indicating that proteolytic cleavage had occurred. Whereas residues in systemin most important for binding SBP50 appear to be located in the N-terminal half of the molecule, amino acids crucial for proteinase inhibitor induction are located within the C terminus. The residues important for binding include a cleavage site for furin, a member of the family of Kex2p-like prohormone-processing enzymes. Processing of systemin at the predicted furin cleavage site was confirmed in vitro. An antiserum against a Kex2p-like protease from Drosophila inhibited binding of biotinylsystemin to SBP50 and recognized a protein of about 60 kDa in Western blot analyses of tomato plasma membrane proteins. The data suggest a possible role for a membrane bound, furin-like protease in the mechanism of defense gene signaling by systemin.