Cloning, expression and characterization of insulin-degrading enzyme from tomato (Solanum lycopersicum)

Yeast Ste23p shares functional similarities with mammalian insulin-degrading enzymes

The S. cerevisiae genome encodes two M16A enzymes: Axl1p and Ste23p. Of the two, Ste23p shares significantly higher sequence identity with M16A enzymes from other species, including mammalian insulin-degrading enzymes (IDEs). In this study, recombinant Ste23p and R. norvegicus IDE (RnIDE) were isolated from E. coli, and their enzymatic properties compared. Ste23p was found to cleave established RnIDE substrates, including the amyloid-beta peptide (Abeta1-40) and insulin B-chain. A novel internally quenched fluorogenic substrate (Abz-SEKKDNYIIKGV-nitroY-OH) based on the polypeptide sequence of the yeast P2 a-factor mating propheromone was determined to be a suitable substrate for both Ste23p and RnIDE, and was used to conduct comparative enzymological studies. Both enzymes were most active at 37 degrees C, in alkaline buffers and in high salt environments. In addition, the proteolytic activities of both enzymes towards the fluorogenic substrate were inhibited by metal chelators, thiol modifiers, inhibitors of cysteine protease activity and insulin. Characteristics of STE23 expression were also evaluated. Our analysis indicates that the 5' terminus of the STE23 gene has been mischaracterized, with the physiologically relevant initiator corresponding to residue M53 of the publicly annotated protein sequence. Finally, we demonstrate that, unlike haploid-specific Axl1p, Ste23p is expressed in both haploid and diploid cell types. Our study presents the first comprehensive biochemical analysis of a yeast M16A enzyme, and provides evidence that S. cerevisiae Ste23p has enzymatic properties that are highly consistent with mammalian IDEs and other M16A enzymes.

The protease-associated domain and C-terminal extension are required for zymogen processing, sorting within the secretory pathway, and activity of tomato subtilase 3 (SlSBT3)

A transgenic plant cell suspension culture was established as a versatile and efficient expression system for the subtilase SlSBT3 from tomato. The recombinant protease was purified to homogeneity from culture supernatants by fractionated ammonium sulfate precipitation, batch adsorption to cation exchange material, and anion exchange chromatography. Purified SlSBT3 was identified as a 79-kDa glycoprotein with both complex and paucimannosidic type glycan chains at Asn(177), Asn(203), Asn(376), Asn(697), and Asn(745). SlSBT3 was found to be a very stable enzyme, being fully active at 60 degrees C and showing highest activity at alkaline conditions with a maximum between pH 7.5 and 8.0. Substrate specificity of SlSBT3 was analyzed in detail, revealing a preference for Gln and Lys in the P(1) and P(2) positions of oligopeptide substrates, respectively. Similar to bacterial, yeast, and mammalian subtilases, SlSBT3 is synthesized as a preproenzyme, and processing of the prodomain in the endoplasmic reticulum is a prerequisite for passage through the secretory pathway. SlSBT3 S538A and S538C active site mutants accumulated intracellularly as unprocessed zymogens, indicating that prodomain cleavage occurs autocatalytically. The wild-type SlSBT3 protein failed to cleave the prodomain of the S538A mutant in trans, demonstrating that zymogen maturation is an intramolecular process. Distinguishing features of plant as compared with mammalian subtilases include the insertion of a large protease-associated domain between the His and Ser residues of the catalytic triad and the C-terminal extension to the catalytic domain. Both features were found to be required for SlSBT3 activity and, consequently, for prodomain processing and secretion.