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Piwi
The piwi (sometimes also PIWI; originally P-element induced wimpy testis in ''Drosophila''〔Saito K, Nishida KM, Mori T, Kawamura Y, Miyoshi K, Nagami T, Siomi H, Siomi MC. (2006). Specific association of Piwi with rasiRNAs derived from retrotransposon and heterochromatic regions in the Drosophila genome. ''Genes Dev'' 20(16):2214-22. PMID 16882972〕) class of genes was originally identified as encoding regulatory proteins responsible for maintaining incomplete differentiation in stem cells and maintaining the stability of cell division rates in germ line cells.〔Cox DN, Chao A, Lin H. (2000). piwi encodes a nucleoplasmic factor whose activity modulates the number and division rate of germline stem cells. ''Development'' 127(3):503-14. PMID 10631171〕 Piwi proteins are highly conserved across evolutionary lineages and are present in both plants and animals.〔Cox DN, Chao A, Baker J, Chang L, Qiao D, Lin H. (1998). A novel class of evolutionarily conserved genes defined by piwi are essential for stem cell self-renewal. ''Genes Dev'' 12(23):3715-27. PMID 9851978〕 One of the major human homologues, whose upregulation is implicated in the formation of tumours such as seminomas, is called ''hiwi'';〔Qiao D, Zeeman AM, Deng W, Looijenga LH, Lin H. (2002). Molecular characterization of hiwi, a human member of the piwi gene family whose overexpression is correlated to seminomas. ''Oncogene'' 21(25):3988-99. PMID 12037681〕 other variants on the theme include the ''miwi'' protein in mice.〔Deng W, Lin H. (2002). miwi, a murine homolog of piwi, encodes a cytoplasmic protein essential for spermatogenesis. ''Dev Cell'' 2(6):819-30. PMID 12062093〕
==Role in RNA interference==
The ''piwi domain'' is a protein domain found in piwi proteins and a large number of related nucleic acid-binding proteins, especially those that bind and cleave RNA. The function of the domain is double stranded-RNA-guided hydrolysis of single stranded-RNA that has been determined in the argonaute family of related proteins.〔 Argonautes, the most well-studied family of nucleic-acid binding proteins, are RNase H-like enzymes that carry out the catalytic functions of the RNA-induced silencing complex (RISC). In the well-known cellular process of RNA interference, the argonaute protein in the RISC complex can bind both small interfering RNA (siRNA) generated from exogenous double-stranded RNA and microRNA (miRNA) generated from endogenous non-coding RNA, both produced by the ribonuclease Dicer, to form an RNA-RISC complex. This complex binds and cleaves complementary base pairing messenger RNA, destroying it and preventing its translation into protein. Crystallised piwi domains have a conserved basic binding site for the 5' end of bound RNA; in the case of argonaute proteins binding siRNA strands, the last unpaired nucleotide base of the siRNA is also stabilised by base stacking-interactions between the base and neighbouring tyrosine residues.〔Ma J, Yuan Y, Meister G, Pei Y, Tuschl T, Patel D (2005). "Structural basis for 5'-end-specific recognition of guide RNA by the A. fulgidus Piwi protein". ''Nature'' 434 (7033): 666-70. PMID 15800629〕
Recent evidence suggests that the functional role of piwi proteins in germ-line determination is due to their capacity to interact with miRNAs. Components of the miRNA pathway appear to be present in pole plasm and to play a key role in early development and morphogenesis of ''Drosophila melanogaster'' embryos, in which germ-line maintenance has been extensively studied.〔Megosh HB, Cox DN, Campbell C, Lin H. (2006). The role of PIWI and the miRNA machinery in Drosophila germline determination. ''Curr Biol'' 16(19):1884-94. PMID 16949822〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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