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• W73512526 abstract "The major contribution this thesis makes is the discovery of a new and structurally diverse class of seed peptides in the daisy family, named the PawS-Derived Peptides (PDPs). Background studies (Mylne et al. 2011) revealed that in sunflower (Helianthus annuus) a pair of small cyclic peptides emerge as a surprising addition from a precursor protein of completely different nature; namely one which produces napin-type seed storage albumins, which are heterodimeric proteins. Of particular relevance to this thesis was the structural mimicry between one of these cyclic peptides called Sunflower Trypsin Inhibitor 1 (SFTI-1) and the inhibitory loop of a class of much larger proteins called Bowman-Birk Inhibitors (Chapter 1). Cloning of PawS genes from a range of daisy species, conducted by Zaiyang Phua (Masters 2010) and Christina Delay (Honours 2011), revealed that PDPs might be more ancient and widespread than we could have anticipated.A large part of this thesis is dedicated to proving that predicted PDPs are found in vivo and that they have evolved rapidly, explaining their structural diversity (Chapter 2). The physicochemical properties of each PDP differs significantly and therefore, particular care was required in NMR studies to optimise the experimental parameters on a case-by-case basis. Although structurally similar, with the same basic scaffold stabilised by a single disulfide bond, and a network of hydrogen bonds that make them well ordered, I have found that PDPs have different conformations as a result of different loop lengths and amino acid sequences. I have also found that in the case of PDPs the structure calculations and evaluation statistics, that are routinely employed, require careful consideration. Potentially this was due to expectations built into the various structure calculation programs not accounting for the highly constrained nature of some peptides, especially small, tightly looped and disulfide-bonded peptides such as the PDPs from daisy seeds (Chapter 3).In addition to structural studies, I have also investigated the physiological function of the discovered PDPs. Cyclic peptides often are biocidal; therefore, PDPs were tested against bacteria, fungi, insects and proteases, but no activities were found to be significant (Chapter 4). Although disappointing, this provoked consideration of other, less-biased approaches to understanding PDP function. NMR studies show that PDPs largely differ in size, physicochemical properties and flexibility; therefore, it should not be assumed that all PDPs have the same function. Nevertheless, it is clear that in the sunflower lineage (Heliantheae) that there has been convergence upon the structural motif Bowman Birk Inhibitors use to inhibit proteases, specifically trypsin.The enzyme asparaginyl endo-peptidase (AEP) is well known for its role in maturing seed storage proteins. In a recent study, Mylne et al. (2011), it was shown that AEP is required to cleave SFTI-1 at both proto-termini within PawS1 and a model in which AEP ligates SFTI-1 was proposed. This hypothesis is further supported by more recent findings in which a similar mechanism was identified for a non-related class of cyclic peptides that arise from within very different precursor proteins (Mylne et al. 2012). The recruitment of AEP for the simultaneous cleavage/ligation in vivo of non-related cyclic peptide families suggests “biosynthetic parallelism”. This biological evidence (i.e. evolutionary) is indirect. My thesis describes the use of one approach, which could have provided more direct evidence. I attempted to reconcile why cyclisation of SFTI-1 was so inefficient in Arabidopsis with the assumption that the endogenous AEPs were not suited to ligating sunflower SFTI-1 (Chapter 5). With a transgenic construct expressing sunflower AEP, I have shown that the seed storage proteins in Arabidopsis can be efficiently cleaved in the absence of endogenous Arabidopsis AEP but not cyclised. These studies suggest that other factors might be involved.This thesis provides new knowledge of the seed peptide family, PDPs, and sheds light on a series of important topics that can now be more deeply investigated (Chapter 6). In particular: (i) the use of non-model de novo transcriptomics to accelerate PDP discovery, (ii) what approaches could be used to study the biological function of PDPs, distinct from the ones commonly identified in cyclic peptides, and (iii) provoke the question: is AEP really acting alone in peptide cyclisation?" @default.
• W73512526 created "2016-06-24" @default.
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• W73512526 date "2013-01-01" @default.
• W73512526 modified "2023-09-27" @default.
• W73512526 title "A new class of daisy seed peptides" @default.
• W73512526 hasPublicationYear "2013" @default.
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