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J Biol Chem. 2017 Jul 28;292(30):12398-12411. doi: 10.1074/jbc.M117.776955. Epub 2017 May 23.

Two proteins for the price of one: Structural studies of the dual-destiny protein preproalbumin with sunflower trypsin inhibitor-1.

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From the School of Biomedical Sciences and.
the School of Molecular Sciences and ARC Centre of Excellence in Plant Energy Biology, University of Western Australia, Crawley, Perth, Western Australia 6009, Australia, and.
Centre for Advanced Imaging, University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.
CSIRO Agriculture and Food, St Lucia, Brisbane, Queensland 4067, Australia.
From the School of Biomedical Sciences and


Seed storage proteins are both an important source of nutrition for humans and essential for seedling establishment. Interestingly, unusual napin-type 2S seed storage albumin precursors in sunflowers contain a sequence that is released as a macrocyclic peptide during post-translational processing. The mechanism by which such peptides emerge from linear precursor proteins has received increased attention; however, the structural characterization of intact precursor proteins has been limited. Here, we report the 3D NMR structure of the Helianthus annuus PawS1 (preproalbumin with sunflower trypsin inhibitor-1) and provide new insights into the processing of this remarkable dual-destiny protein. In seeds, PawS1 is matured by asparaginyl endopeptidases (AEPs) into the cyclic peptide SFTI-1 (sunflower trypsin inhibitor-1) and a heterodimeric 2S albumin. The structure of PawS1 revealed that SFTI-1 and the albumin are independently folded into well-defined domains separated by a flexible linker. PawS1 was cleaved in vitro with recombinant sunflower HaAEP1 and in situ using a sunflower seed extract in a way that resembled the expected in vivo cleavages. Recombinant HaAEP1 cleaved PawS1 at multiple positions, and in situ, its flexible linker was removed, yielding fully mature heterodimeric albumin. Liberation and cyclization of SFTI-1, however, was inefficient, suggesting that specific seed conditions or components may be required for in vivo biosynthesis of SFTI-1. In summary, this study has revealed the 3D structure of a macrocyclic precursor protein and provided important mechanistic insights into the maturation of sunflower proalbumins into an albumin and a macrocyclic peptide.


asparaginyl endopeptidase (AEP); cyclic peptide; nuclear magnetic resonance (NMR); plant biochemistry; post-translational modification (PTM); preproalbumin with SFTI-1 (PawS1); protein processing; seed storage albumin; sunflower trypsin inhibitor-1 (SFTI-1)

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