Rech J E, Huang M H, LeStourgeon W M, Flicker P F
Department of Molecular Biology, Vanderbilt University, Nashville, Tennessee 37235, USA.
J Struct Biol. 1995 Mar-Apr;114(2):84-92. doi: 10.1006/jsbi.1995.1008.
In mammalian cells approximately 700 nucleotide lengths of pre-mRNA are packaged during transcription by a unique group of abundant nuclear proteins to form a repeating array of regular ribonucleoprotein complexes termed 30-40S heterogeneous nuclear ribonucleoprotein particles (hnRNP particles). We have used electron microscopy to examine complexes that form when in vitro-transcribed RNA is bound by one of the purified native core-particle proteins which comprise the 40S monoparticle (the C protein tetramer). Negatively stained images of the C protein tetramer bound to particle-length RNA (700 nt) demonstrate that three tetramers bind each RNA molecule to form a stable closed triangular complex. The triangular complexes have an isosceles shape with a base of 18.0 nm and sides of 23.0 nm. When RNA molecules of 230 nt are used as substrates single tetramers bind to form complexes that appear as small rounded structures with an average diameter of 9.7 nm. Twice this length of RNA (456 nt) supports the assembly of mostly bilobed complexes that are 20.4 nm long and 11.8 nm wide. Images of the C protein-RNA complexes which assemble on 1400-nucleotide lengths of RNA (two particle lengths of RNA) clearly show complexes composed of two triangles while three-triangle complexes are seen when 2100-nt lengths of RNA are used as the assembly substrate. These ultrastructural results demonstrate that groups of three C protein tetramers combine with the length of RNA packaged in monoparticles to form a discreet triad structure.(ABSTRACT TRUNCATED AT 250 WORDS)
在哺乳动物细胞中，大约700个核苷酸长度的前体mRNA在转录过程中被一组独特的丰富核蛋白包装，形成一系列重复排列的规则核糖核蛋白复合物，称为30 - 40S异质性核核糖核蛋白颗粒（hnRNP颗粒）。我们利用电子显微镜检查了体外转录的RNA与构成40S单颗粒（C蛋白四聚体）的一种纯化天然核心颗粒蛋白结合时形成的复合物。C蛋白四聚体与颗粒长度的RNA（700 nt）结合的负染图像表明，三个四聚体结合每个RNA分子，形成一个稳定的封闭三角形复合物。三角形复合物呈等腰形状，底边为18.0 nm，侧边为23.0 nm。当使用230 nt的RNA分子作为底物时，单个四聚体结合形成平均直径为9.7 nm的小圆形结构复合物。两倍长度的RNA（456 nt）支持形成大多为双叶状的复合物，其长20.4 nm，宽11.8 nm。在1400个核苷酸长度的RNA（两个颗粒长度的RNA）上组装的C蛋白 - RNA复合物图像清楚地显示出由两个三角形组成的复合物，而当使用2100 nt长度的RNA作为组装底物时，可以看到三个三角形的复合物。这些超微结构结果表明，三个C蛋白四聚体与单颗粒中包装的RNA长度结合，形成一个离散的三联体结构。（摘要截短于250字）
