Stanĕk David, Rader Stephen D, Klingauf Mirko, Neugebauer Karla M
Max Planck Institute of Molecular Cell Biology and Genetics, 01307 Dresden, Germany.
J Cell Biol. 2003 Feb 17;160(4):505-16. doi: 10.1083/jcb.200210087. Epub 2003 Feb 10.
The spliceosomal small nuclear RNAs (snRNAs) are distributed throughout the nucleoplasm and concentrated in nuclear inclusions termed Cajal bodies (CBs). A role for CBs in the metabolism of snRNPs has been proposed but is not well understood. The SART3/p110 protein interacts transiently with the U6 and U4/U6 snRNPs and promotes the reassembly of U4/U6 snRNPs after splicing in vitro. Here we report that SART3/p110 is enriched in CBs but not in gems or residual CBs lacking coilin. The U6 snRNP Sm-like (LSm) proteins, also involved in U4/U6 snRNP assembly, were localized to CBs as well. The levels of SART3/p110 and LSm proteins in CBs were reduced upon treatment with the transcription inhibitor alpha-amanitin, suggesting that CB localization reflects active processes dependent on transcription/splicing. The NH2-terminal HAT domain of SART3/p110 was necessary and sufficient for specific protein targeting to CBs. Overexpression of truncation mutants containing the HAT domain had dominant negative effects on U6 snRNP localization to CBs, indicating that endogenous SART3/p110 plays a role in targeting the U6 snRNP to CBs. We propose that U4 and U6 snRNPs accumulate in CBs for the purpose of assembly into U4/U6 snRNPs by SART3/p110.
剪接体小核RNA（snRNAs）分布于整个核质中，并集中在称为卡哈尔体（CBs）的核内包涵体中。有人提出CBs在snRNPs的代谢中起作用，但目前尚不清楚。SART3/p110蛋白与U6和U4/U6 snRNPs短暂相互作用，并在体外剪接后促进U4/U6 snRNPs的重新组装。在此我们报告，SART3/p110在CBs中富集，但在宝石样结构或缺乏卷曲螺旋蛋白的残留CBs中不富集。同样参与U4/U6 snRNP组装的U6 snRNP Sm样（LSm）蛋白也定位于CBs。用转录抑制剂α-鹅膏蕈碱处理后，CBs中SART3/p110和LSm蛋白的水平降低，这表明CBs定位反映了依赖于转录/剪接的活跃过程。SART3/p110的NH2末端HAT结构域对于将特定蛋白质靶向到CBs是必需且足够的。含有HAT结构域的截短突变体的过表达对U6 snRNP定位于CBs具有显性负效应，表明内源性SART3/p110在将U6 snRNP靶向到CBs中起作用。我们提出，U4和U6 snRNPs在CBs中积累，目的是通过SART3/p110组装成U4/U6 snRNPs。
