Stalder Lukas, Mühlemann Oliver
Institute of Cell Biology, University of Berne, 3012 Berne, Switzerland.
RNA. 2009 Jul;15(7):1265-73. doi: 10.1261/rna.1672509. Epub 2009 May 27.
Nonsense-mediated mRNA decay (NMD) is a eukaryotic quality-control mechanism that recognizes and degrades mRNAs with premature termination codons (PTCs). In yeast, PTC-containing mRNAs are targeted to processing bodies (P-bodies), and yeast strains expressing an ATPase defective Upf1p mutant accumulate P-bodies. Here we show that in human cells, an ATPase-deficient UPF1 mutant and a fraction of UPF2 and UPF3b accumulate in cytoplasmic foci that co-localize with P-bodies. Depletion of the P-body component Ge-1, which prevents formation of microscopically detectable P-bodies, also impairs the localization of mutant UPF1, UPF2, and UPF3b in cytoplasmic foci. However, the accumulation of the ATPase-deficient UPF1 mutant in P-bodies is independent of UPF2, UPF3b, or SMG1, and the ATPase-deficient UPF1 mutant can localize into the P-bodies independent of its phosphorylation status. Most importantly, disruption of P-bodies by depletion of Ge-1 affects neither the mRNA levels of PTC-containing reporter genes nor endogenous NMD substrates. Consistent with the recently reported decapping-independent SMG6-mediated endonucleolytic decay of human nonsense mRNAs, our results imply that microscopically detectable P-bodies are not required for mammalian NMD.
无义介导的mRNA降解（NMD）是一种真核生物质量控制机制，可识别并降解带有提前终止密码子（PTC）的mRNA。在酵母中，含有PTC的mRNA被靶向加工小体（P小体），表达ATP酶缺陷型Upf1p突变体的酵母菌株会积累P小体。在此我们表明，在人类细胞中，ATP酶缺陷型UPF1突变体以及一部分UPF2和UPF3b会在与P小体共定位的细胞质焦点中积累。P小体成分Ge-1的缺失会阻止显微镜下可检测到的P小体形成，同时也会损害突变型UPF1、UPF2和UPF3b在细胞质焦点中的定位。然而，ATP酶缺陷型UPF1突变体在P小体中的积累独立于UPF2、UPF3b或SMG1，并且ATP酶缺陷型UPF1突变体可独立于其磷酸化状态定位于P小体中。最重要的是，通过缺失Ge-1破坏P小体既不影响含PTC报告基因的mRNA水平，也不影响内源性NMD底物。与最近报道的人无义mRNA的脱帽非依赖性SMG6介导的内切核酸酶降解一致，我们的结果表明，哺乳动物NMD不需要显微镜下可检测到的P小体。
