title: Discovery, structural determination, and putative processing of the precursor protein that produces the cyclic trypsin inhibitor sunflower trypsin inhibitor 1.
abstract: Backbone-cyclized proteins are becoming increasingly well known, although the mechanism by which they are processed from linear precursors is poorly understood. In this report the sequence and structure of the linear precursor of a cyclic trypsin inhibitor, sunflower trypsin inhibitor 1 (SFTI-1) from sunflower seeds, is described. The structure indicates that the major elements of the reactive site loop of SFTI-1 are present before processing. This may have importance for a protease-mediated cyclizing reaction as the rigidity of SFTI-1 may drive the equilibrium of the reaction catalyzed by proteolytic enzymes toward the formation of a peptide bond rather than the normal cleavage reaction. The occurrence of residues in the SFTI-1 precursor susceptible to cleavage by asparaginyl proteases strengthens theories that involve this enzyme in the processing of SFTI-1 and further implicates it in the processing of another family of plant cyclic proteins, the cyclotides. The precursor reported here also indicates that despite strong active site sequence homology, SFTI-1 has no other similarities with the Bowman-Birk trypsin inhibitors, presenting interesting evolutionary questions.
authors.affiliation: Institute for Molecular Bioscience, Australian Research Council Center for Functional and Applied Genomics, University of Queensland, Brisbane.
authors.forename: Jason P
authors.initials: JP
authors.lastname: Mulvenna
authors.forename: Fiona M
authors.initials: FM
authors.lastname: Foley
authors.forename: David J
authors.initials: DJ
authors.lastname: Craik
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D002918:Chymotrypsin / Q000737:chemistry; D030562:Databases, Protein; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D010456:Peptides, Cyclic / Q000737:chemistry* / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry; D010974:Platelet Aggregation; D016133:Polymerase Chain Reaction; D011487:Protein Conformation; D011498:Protein Precursors / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D013025:Glycine max / Q000378:metabolism; D013237:Stereoisomerism; D014357:Trypsin / Q000737:chemistry; D014358:Trypsin Inhibitor, Bowman-Birk Soybean / Q000737:chemistry / Q000494:pharmacology
chemical_list: D010455:Peptides; D010456:Peptides, Cyclic; D010940:Plant Proteins; D011498:Protein Precursors; C448501:SFTI-1 peptide, sunflower; D014358:Trypsin Inhibitor, Bowman-Birk Soybean; D002918:Chymotrypsin; D014357:Trypsin