Ireland David C, Colgrave Michelle L, Craik David J
Institute for Molecular Bioscience and Australian Research Council Special Research Centre for Functional and Applied Genomics, The University of Queensland, Brisbane QLD 4072, Australia.
Biochem J. 2006 Nov 15;400(1):1-12. doi: 10.1042/BJ20060627.
Cyclotides are a fascinating family of plant-derived peptides characterized by their head-to-tail cyclized backbone and knotted arrangement of three disulfide bonds. This conserved structural architecture, termed the CCK (cyclic cystine knot), is responsible for their exceptional resistance to thermal, chemical and enzymatic degradation. Cyclotides have a variety of biological activities, but their insecticidal activities suggest that their primary function is in plant defence. In the present study, we determined the cyclotide content of the sweet violet Viola odorata, a member of the Violaceae family. We identified 30 cyclotides from the aerial parts and roots of this plant, 13 of which are novel sequences. The new sequences provide information about the natural diversity of cyclotides and the role of particular residues in defining structure and function. As many of the biological activities of cyclotides appear to be associated with membrane interactions, we used haemolytic activity as a marker of bioactivity for a selection of the new cyclotides. The new cyclotides were tested for their ability to resist proteolysis by a range of enzymes and, in common with other cyclotides, were completely resistant to trypsin, pepsin and thermolysin. The results show that while biological activity varies with the sequence, the proteolytic stability of the framework does not, and appears to be an inherent feature of the cyclotide framework. The structure of one of the new cyclotides, cycloviolacin O14, was determined and shown to contain the CCK motif. This study confirms that cyclotides may be regarded as a natural combinatorial template that displays a variety of peptide epitopes most likely targeted to a range of plant pests and pathogens.
环肽是一类迷人的植物源肽，其特征在于头尾环化的主链以及三个二硫键的打结排列。这种保守的结构架构，称为CCK（环状胱氨酸结），赋予了它们对热、化学和酶降解的非凡抗性。环肽具有多种生物活性，但其杀虫活性表明它们的主要功能是植物防御。在本研究中，我们测定了堇菜科植物香堇菜（Viola odorata）中的环肽含量。我们从该植物的地上部分和根部鉴定出30种环肽，其中13种是新序列。这些新序列提供了有关环肽自然多样性以及特定残基在定义结构和功能中作用的信息。由于环肽的许多生物活性似乎与膜相互作用有关，我们使用溶血活性作为新环肽生物活性的标志物。测试了新环肽对一系列酶的抗蛋白水解能力，与其他环肽一样，它们对胰蛋白酶、胃蛋白酶和嗜热菌蛋白酶完全抗性。结果表明，虽然生物活性随序列而异，但框架的蛋白水解稳定性并非如此，这似乎是环肽框架的固有特征。确定了一种新环肽环紫罗兰素O14的结构，显示其含有CCK基序。这项研究证实，环肽可被视为一种天然组合模板，展示了多种肽表位，最有可能针对一系列植物害虫和病原体。
