Molecular mechanisms that integrate 'cell death and cell cycle progression.

我们最近鉴定出Polycomb - like 2 （Pc12）基因产物是Polycomb - suppression Complex-1 （PRC -1）的一个组成部分，尽管Pc12之前被鉴定为PRC2的一个组成部分。我们培育了Pc12缺陷小鼠，并揭示了其连接PRC1和PRC2的新作用。尽管PRC1突变mef（小鼠胚胎成纤维细胞）被证明会经历过早衰老，但pc12缺陷mef会消除衰老并随后转化。这表明Pc12作为肿瘤抑制因子。事实上，生化分析表明，Pc12缺乏通过稳定PRC1导致Ink4a基因的稳定抑制。与此同时，Phc2/Pc12双缺陷mef最终消除了衰老，这伴随着Ink4a基因的基因组丢失。由于Pcl2缺陷mef中PRC1的稳定与编码PRC1成分的mRNA转录的增加无关，因此这种稳定可能是由翻译调节或蛋白质稳定性介导的。为了解决这个问题，我们首先在293T细胞中通过拉下tap标记的Pcl2来确定Pc12的结合蛋白，其中涉及翻译调节的蛋白如核糖体蛋白、RNA解旋酶和RNA结合蛋白被过度表达。此外，编码PRC1成分的mRNA在Pc12缺陷mef的多体的未翻译部分中被过度表达。再加上Tudor结构域的存在，可能有助于RNA结合，Pc12可能参与了PRC1组分的翻译调控。

We have newly identified Polycomblike 2 (Pc12) gene product as a component of Polycomb Repressive Complex-1 (PRC 1) although Pc12 had been identified as a PRC2 component previously. We have generated Pc12 deficient mice and revealed its novel role to link PRC1 and PRC2. Although PRC1 mutant MEFs (mouse embryonic fibroblasts) are shown to undergo premature senescence, Pc12-deficient MEFs cancelled senescence and subsequently transformed. This suggests Pc12 acts as a tumor suppressor. Indeed, biochemical analysis revealed that Pc12 deficiency results in stable repression of Ink4a gene through the stabilization of PRC1. Concordantly, Phc2/Pc12 doubly deficient MEFs eventually cancelled senescence, which was accompanied by genomic loss of Ink4a gene. Since stabilization of PRC1 in Pcl2 deficient MEFs was not associated with increase of transcription of mRNA encoding PRC1 components, this stabilization was expected to be mediated either by translational regulation or protein stability. To address this issue, we have first identified binding proteins for Pc12 by pull-down of TAP-tagged Pcl2 upon overexpression in 293T cells, where proteins involved in translational regulation was overrepresented such as Ribosomal proteins, RNA helicases, and RNA binding proteins. In addition, mRNA encoding PRC1 components were overrepresented within untranslated fractions of polysomes in Pc12 deficient MEFs. Together with presence of Tudor domain, which may contributes to RNA binding, Pc12 is likely involved in translational regulation of PRC1 components.