TRIM21

三分模體包含蛋白質21（Tripartite motif-containing protein 21，縮寫爲TRIM21），亦作E3泛素化蛋白質連接酶TRIM21（E3 ubiquitin-protein ligase TRIM21），係一種由人類TRIM21基因編碼的蛋白質。人們已經發現了該蛋白質的可變剪切變體，但目前僅查明了其中一種在完整狀態下的性質。大部分人類組織都表達此種蛋白。 TRIM21係TRIM蛋白質家族（英语：tripartite motif family）的成員。TRIM模體包含了三個鋅結合結構域、一個環指結構域（英语：RING finger domain）、一個1型B箱、一個2型B箱鋅指，以及一個捲曲螺旋區域。 TRIM21爲一種細胞內抗體介導蛋白水解（英语：Intracellular antibody-mediated degradation）途徑的細胞內抗體效應子。它會與標記感染細胞的無包膜病毒體的抗體上的免疫球蛋白G和免疫球蛋白M相連。隨後，其自身發生泛素化或一個輔助因子發生泛素化。其後，TRIM21會負責引導病毒體到有蛋白酶的位置。抗體和病毒的衣殼會被蛋白酶水解，但TRIM21卻不會。 TRIM21爲RoSSA核糖核蛋白的一個組成成分。RoSSA還含有一條單多肽鏈以及一條小分子RNA（這條RNA有4種）。細胞質和細胞核中均有RoSSA粒子。 RoSSA會與乾燥綜合徵和全身性紅斑狼瘡患者體內的自身抗原反應。亦有研究者以TRIM21爲基礎研發新藥。 PDB 2IWG; James LC, Keeble AH, Khan Z, Rhodes DA, Trowsdale J. Structural basis for PRYSPRY-mediated tripartite motif (TRIM) protein function. Proc. Natl. Acad. Sci. U.S.A. April 2007, 104 (15): 6200–5. PMC 1851072 . PMID 17400754. doi:10.1073/pnas.0609174104.  Frank MB, Itoh K, Fujisaku A, Pontarotti P, Mattei MG, Neas BR. 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