Collart Clara, Ramis Joana M, Down Thomas A, Smith James C
Wellcome Trust/CR-UK Gurdon Institute and Department of Zoology, University of Cambridge, Cambridge, UK.
Development. 2009 Oct;136(20):3451-61. doi: 10.1242/dev.027714.
Smicl (Smad-interacting CPSF 30-like) is an unusual protein that interacts with transcription factors as well as with the cleavage and polyadenylation specificity factor (CPSF). Previous work has shown that Smicl is expressed maternally in the Xenopus embryo and is later required for transcription of Chordin. In this paper we search for additional targets of Smicl. We identify many genes whose onset of expression at the midblastula transition (MBT) requires Smicl and is correlated with the translocation of Smicl from cytoplasm to nucleus. At least one such gene, Xiro1, is regulated via 3'-end processing. In searching for a general mechanism by which Smicl might regulate gene expression at the MBT, we have discovered that it interacts with the tail of Rpb1, the largest subunit of RNA polymerase II. Our results show that Smicl is required for the phosphorylation of the Rpb1 tail at serine 2 of the repeated heptapeptide YSPTSPS. This site becomes hyperphosphorylated at the MBT, thus allowing the docking of proteins required for elongation of transcription and RNA processing. Our work links the onset of zygotic gene expression in the Xenopus embryo with the translocation of Smicl from cytoplasm to nucleus, the phosphorylation of Rpb1 and the 3'-end processing of newly transcribed mRNAs.
Smicl（与Smad相互作用的类CPSF 30）是一种不同寻常的蛋白质，它既能与转录因子相互作用，也能与切割和聚腺苷酸化特异性因子（CPSF）相互作用。先前的研究表明，Smicl在非洲爪蟾胚胎中由母体表达，并且后来是脊索蛋白转录所必需的。在本文中，我们寻找Smicl的其他靶标。我们鉴定出许多基因，它们在囊胚中期转换（MBT）时的表达起始需要Smicl，并且与Smicl从细胞质向细胞核的转位相关。至少有一个这样的基因，即Xiro1，是通过3'端加工来调控的。在寻找Smicl可能在MBT时调控基因表达的一般机制的过程中，我们发现它与RNA聚合酶II最大亚基Rpb1的尾部相互作用。我们的结果表明，Smicl是Rpb1尾部在重复七肽YSPTSPS的丝氨酸2处磷酸化所必需的。该位点在MBT时发生超磷酸化，从而允许转录延伸和RNA加工所需的蛋白质对接。我们的工作将非洲爪蟾胚胎中合子基因表达的起始与Smicl从细胞质向细胞核的转位、Rpb1的磷酸化以及新转录mRNA的3'端加工联系起来。
