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Exosome complex
The exosome complex (or PM/Scl complex, often just called the exosome) is a multi-protein intracellular complex capable of degrading various types of RNA (ribonucleic acid) molecules. Exosome complexes are found in both eukaryotic cells and archaea, while in bacteria a simpler complex called the degradosome carries out similar functions.
The core of the exosome contains a six-membered ring structure to which other proteins are attached. In eukaryotic cells, the exosome complex is present in the cytoplasm, nucleus and especially the nucleolus, although different proteins interact with the exosome complex in these compartments regulating the RNA degradation activity of the complex to substrates specific to these cell compartments. Substrates of the exosome include messenger RNA, ribosomal RNA, and many species of small RNAs. The exosome has an exoribonucleolytic function, meaning it degrades RNA starting at one end (the 3′ end in this case), and in eukaryotes also an endoribonucleolytic function, meaning it cleaves RNA at sites within the molecule.
Several proteins in the exosome are the target of autoantibodies in patients with specific autoimmune diseases (especially the PM/Scl overlap syndrome) and some antimetabolitic chemotherapies for cancer function by blocking the activity of the exosome. In addition, mutations in exosome component 3 cause pontocerebellar hypoplasia and spinal motor neuron disease.
==Discovery==
The exosome was first discovered as an RNase in 1997 in the budding yeast ''Saccharomyces cerevisiae'', an often-used model organism. Not long after, in 1999, it was realized that the exosome was in fact the yeast equivalent of an already described complex in human cells called the ''PM/Scl complex'', which had been identified as an autoantigen in patients with certain autoimmune diseases years earlier (see below). Purification of this "PM/Scl complex" allowed the identification of more human exosome proteins and eventually the characterization of all components in the complex. In 2001, the increasing amount of genome data that had become available allowed the prediction of exosome proteins in archaea, although it would take another 2 years before the first exosome complex from an archaeal organism was purified.
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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