Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, Scotland, UK.
Mol Cell Biol. 2010 Feb;30(4):976-84. doi: 10.1128/MCB.01359-09. Epub 2009 Dec 14.
The 18S rRNA component of yeast (Saccharomyces cerevisiae) 40S ribosomes undergoes cytoplasmic 3' cleavage following nuclear export, whereas exported pre-60S subunits were believed to contain only mature 5.8S and 25S rRNAs. However, in situ hybridization detected 3'-extended forms of 5.8S rRNA in the cytoplasm, which were lost when Crm1-dependent preribosome export was blocked by treatment with leptomycin B (LMB). LMB treatment rapidly blocked processing of 6S pre-rRNA to 5.8S rRNA, leading to TRAMP-dependent pre-rRNA degradation. The 6S pre-rRNA was coprecipitated with the 60S export adapter Nmd3 and cytoplasmic 60S synthesis factor Lsg1. The longer 5.8S+30 pre-rRNA (a form of 5.8S rRNA 3' extended by approximately 30 nucleotides) is processed to 6S by the nuclear exonuclease Rrp6, and nuclear pre-rRNA accumulated in the absence of Rrp6. In contrast, 6S to 5.8S processing requires the cytoplasmic exonuclease Ngl2, and cytoplasmic pre-rRNA accumulated in strains lacking Ngl2. We conclude that nuclear pre-60S particles containing the 6S pre-rRNA bind Nmd3 and Crm1 and are exported to the cytoplasm prior to final maturation by Ngl2.
酵母（酿酒酵母）40S 核糖体的 18S rRNA 成分在核输出后经历细胞质 3' 切割，而出口的前 60S 亚基被认为仅含有成熟的 5.8S 和 25S rRNA。然而，原位杂交在细胞质中检测到 5.8S rRNA 的 3' 延伸形式，当用莱普霉素 B（LMB）阻断 Crm1 依赖性前核糖体出口时，这些形式会丢失。LMB 处理迅速阻断了 6S 前 rRNA 向 5.8S rRNA 的加工，导致 TRAMP 依赖性前 rRNA 降解。6S 前 rRNA 与 60S 出口适配器 Nmd3 和细胞质 60S 合成因子 Lsg1 共沉淀。较长的 5.8S+30 前 rRNA（5.8S rRNA 的 3' 延伸约 30 个核苷酸的一种形式）被核外切酶 Rrp6 加工成 6S，并且在没有 Rrp6 的情况下核前 rRNA 积累。相比之下，6S 到 5.8S 的加工需要细胞质外切酶 Ngl2，并且在缺乏 Ngl2 的菌株中细胞质前 rRNA 积累。我们得出结论，含有 6S 前 rRNA 的核前 60S 颗粒与 Nmd3 和 Crm1 结合，并在 Ngl2 进行最终成熟之前被出口到细胞质。
