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Defensins are small cysteine-rich cationic proteins found in both vertebrates and invertebrates. They have also been reported in plants.〔(Structure–activity studies of AtPep1, a plant peptide signal involved in the innate immune response ) Peptides Volume 29, Issue 12, December 2008, Pages 2083-2089〕〔(Planta. 2002 Dec;216(2):193-202. Epub 2002 Oct 8. Plant defensins. )〕 They are, and function as, host defense peptides. They are active against bacteria, fungi and many enveloped and nonenveloped viruses. They consist of 18-45 amino acids including six (in vertebrates) to eight conserved cysteine residues. Cells of the immune system contain these peptides to assist in killing phagocytosed bacteria, for example in neutrophil granulocytes and almost all epithelial cells. Most defensins function by binding to the microbial cell membrane, and, once embedded, forming pore-like membrane defects that allow efflux of essential ions and nutrients. ==Varieties== The name 'defensin' was coined in the early 1990s, though the proteins had been studied as 'Cationic Antimicrobial Proteins'.〔(''Nature'' ) timeline of discovery of definsins〕 The underlying genes responsible for defensin production are highly polymorphic. Some aspects are conserved, however; the hallmarks of a β-defensin are its small size, high density of cationic charge, and six-cysteine-residue motif. In general, they are encoded by two-exon genes, wherein the first exon encodes for a hydrophobic leader sequence and the second for a peptide containing the cysteine motif. All defensins have disulfide linkages. But the role of these highly conserved cysteines is not known. Many reports reveal that disulfide bonds are not necessary for antimicrobial activity of arthropod defensins.()〔Jobin Varkey, Shashi Singh, Ramakrishnan Nagaraj. Antibacterial activity of linear peptides spanning the carboxy-terminal β-sheet domain of arthropod defensins 〕 Similarly, mammalian defensins also do not require disulfide bonds to exhibit antimicrobial activity. ()〔Jobin Varkey, Ramakrishnan Nagaraj. Antibacterial Activity of Human Neutrophil Defensin HNP-1 Analogs without Cysteines 〕 The disulfide linkages have been suggested to be essential for activities related to innate immunity in mammals. Insect defensins as a group occur in various species and in their three-dimensional structure some are very similar to proteins from scorpion toxins. The venom gland of a Chinese Buthus species yielded complementary DNA encoding a peptide resembling an insect defensin-like peptide. The precursor has an organization similar in various respects to that of insect defensins, for example, the positions of several cysteines and a conserved glycine are common, suggesting that these peptides should share a cysteine-stabilized motif. Phylogenetic analysis suggests closer resemblances between the newly discovered scorpion peptide and ancient insect defensins in the scorpion haemolymph, than to toxins in scorpion venoms.〔Shunyi Zhu, Wenxin Li, Dahe Jiang, and Xianchun Zeng. Evidence for the Existence of Insect defensin-like Peptide in Scorpion Venom" ''IUBMB Life'' 50: 57–61, 2000〕 Subsequent investigations confirmed relationships between channel toxins and insect defensins in their conserved three-dimensional structure and their disruption of membrane functions of invasive microbes. Experimental deletion of a small loop of a defensin molecule from a Hymenopteran parasitoid venom that shares attributes of scorpion toxin, removed steric hindrance of interactions between peptides and channels. The resulting peptide was neurotoxin that selectively inhibited potassium channels, binding to the channels in the same manner as scorpion toxins. The results presented structural and functional evidence for the basis of toxin evolution.〔Shunyi Zhu, Steve Peigneur, Bin Gao, Yoshitaka Umetsu, Shinya Ohki, Jan Tytgat. Experimental Conversion of a Defensin into a Neurotoxin: Implications for Origin of Toxic Function. Mol Biol Evol (2014) 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「defensin」の詳細全文を読む スポンサード リンク
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