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Definition
Tachykinin is one of the most well-known bioactive peptides found in vertebrates, and tachykinin-related peptides have also been isolated from various invertebrate species. Urechistachykinin I (UI) and Urechistachykinin II (UII) are tachykinins found in Urechis unicinctus.

Discovery
In 1993, Ikeda et al., isolated two novel neuropeptides, urechistachykinin I and urechistachykinin II from the ventral nerve cords of the echiuroid worm, Urechis unicinctus 1.

Structural Characteristics
The amino acid sequences of  UI and UII are found to be significantly homologous with those of the vertebrate and insect tachykinins 1. The amino acid sequence of urechistachykinins is as follows 1: UI (H-Leu-Arg-Gln-Ser-Gln-Phe-Val-Gly-Ser-Arg-NH2) and UII (H-Ala-Ala-Gly-Met-Gly-Phe-Phe-Gly-Ala-Arg-NH2).

Mode of Action
Structurally tachykinin-related peptides have been isolated from various invertebrate species and shown to exhibit their biological activities through a G-protein-coupled receptor (GPCR) for a tachykinin-related peptide. Earlier studies report the identification of a tachykinin-related peptide receptor, the urechistachykinin receptor (UTKR) from the echiuroid worm, Urechis unitinctus. The deduced UTKR precursor includes seven transmembrane domains and typical sites for mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors. A experiment was conducted to study functional analysis of the UTKR, therefore UTKR was expressed in Xenopus oocytes. It was found that UTKR, like tachykinin receptors and tachykinin-related peptide receptors, activates calcium-dependent signal transduction upon binding to its endogenous ligands, urechistachykinins (Uru-TKs) I-V and VII. UTKR responded to all Uru-TKs equivalently, showing that UTKR possesses no selective affinity with Uru-TKs. In contrast, UTKR was not activated by substance P or an Uru-TK analog containing a C-terminal Met-NH2 instead of Arg-NH2. Furthermore, the genomic analysis revealed that the UTKR gene, like mammalian tachykinin receptor genes, consists of five exons interrupted by four introns, and all the intron-inserted positions are completely compatible with those of mammalian tachykinin receptor genes. These results suggest that mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors were evolved from a common ancestral GPCR gene 2.

Functions

Antimicrobial effect and membrane-active mechanism of Urechistachykinins, neuropeptides derived from Urechis unicinctus: In a study the antimicrobial effects of UI and UII and their modes of action was investigated. It was found that UI and UII showed antimicrobial activities without a hemolytic effect. To investigate the mechanism(s) of UI and UII, cellular localization was examined. Confocal microscopy results showed that peptides were located in the cell envelope. To elucidate the physical changes of membrane induced by UI and UII in Candida albicans, flow cytometry analyses were performed by using bis-(1,3-dibutylbarbituric acid) trimethine oxonol, and changes in membrane dynamics were assessed using 1,6-diphenyl-1,3,5-hexatriene. The results suggest that UI and UII may exert their antimicrobial effect by disrupting the cell membranes3.

References 

  1. Ikeda T, Minakata H, Nomoto K, Kubota I, Muneoka Y (1993). Two novel tachykinin-related neuropeptides in the echiuroid worm, Urechis unicinctus. Biochem. Biophys. Res. Commun., 192(1):1-6.
  2. Kawada T, Furukawa Y, Shimizu Y, Minakata H, Nomoto K, Satake H (2002). A novel tachykinin-related peptide receptor. Sequence, genomic organization, and functional analysis. Eur. J. Biochem., 269(17):4238-4246.
  3. Sung WS, Park SH, Lee DG (2008). Antimicrobial effect and membrane-active mechanism of Urechistachykinins, neuropeptides derived from Urechis unicinctus. FEBS Lett., 582(16):2463-2466.

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