Hormonal Regulation of Messenger RNA Stability

描述（由申请人提供）：mRNA衰变在基因表达中起着核心作用，几乎所有mRNA的半衰期都受mRNA及其同源结合蛋白内的调控序列控制。快速周转是编码生长因子、转录因子、细胞因子和细胞信号分子的mrna的特征，而对这一过程的选择性调节是控制这些蛋白数量的一种方式。大多数mRNA的衰变始于poly(A)尾的缩短，5‘帽的去除以及mRNA体的5’-3‘和3’-5'降解。除了聚(A)缩短外，这些过程作用于非翻译mrna。然而，转录组的一个子集的周转是由内切酶裂解催化的，而mrna是由翻译核糖体参与的。典型的mRNA内切酶是PMR1，这是一种酶，最初被鉴定为雌激素诱导的核糖核酸酶活性，其在多体上的出现与血清蛋白mRNA的不稳定相一致。内切酶介导的mRNA衰变的标志是其对特定mRNA的选择性。这是由含有PMR1及其翻译底物mRNA的mRNP复合物（称为复合物I）的形成决定的。要加入这个复合体，PMR1必须在蛋白质的多体靶向区域的酪氨酸残基上磷酸化，过去的资助周期确定c-Src是负责这一关键激活步骤的激酶。这是致癌酪氨酸激酶直接参与mRNA衰变的第一个例子，它提出了pmr1介导的mRNA衰变可能是癌症中c-Src的靶标的可能性。与此一致的是，PMR1与肌动蛋白细胞骨架的调节因子Ena/VASP蛋白结合，在细胞中表达具有催化活性的PMR1会增加细胞的运动性。目的1将使用串联亲和层析法恢复复合体I的mRNP，鉴定其组成蛋白，并确定其在mRNP组装和mRNA衰变中的作用。这是破译PMR1-mRNA衰变的“RNP代码”的第一步。含有SH2结构域的蛋白质是募集PMR1到mRNP的“看门人”，这将是特别有趣的，因为这些都没有已知的rna结合活性。Aim 2继续上一个周期开始的工作，使用微阵列通过复合物i恢复PMR1目标mrna来识别这些mrna，这些mrna将与通过增加PMR1表达而选择性减少和通过敲除PMR1而选择性增加的mrna进行比较。这些也将用于识别与Aim 1中的蛋白质一起定义底物mRNP的共享序列或结构特征。在表达活性PMR1的细胞中，细胞运动性增加，Aim 3的实验将使用细胞运动成像、定量PCR和免疫荧光来研究运动性、PMR1与Ena/VASP蛋白结合及其向复合体i募集之间的关系。本工作的长期目标是了解PMR1介导的mRNA衰变的分子机制，它是如何被调节的，以及它在发育过程中控制基因表达的作用。对荷尔蒙刺激的反应，以及恶性肿瘤。

DESCRIPTION (provided by applicant): mRNA decay plays a central role in gene expression, with the half-life of virtually every mRNA controlled by regulatory sequences within the mRNA and their cognate binding proteins. Rapid turnover is a characteristic feature of mRNAs encoding growth factors, transcription factors, cytokines and cell signaling molecules, and the selective modulation of this process is one way of controlling the amount of these proteins. The decay of most mRNAs begins with shortening of the poly(A) tail, removal of the 5' cap and simultaneous 5'-3' and 3'-5' degradation of the mRNA body. With the exception of poly(A) shortening these processes act on nontranslat- ing mRNAs. However, the turnover of a subset of the transcriptome is catalyzed by endonuclease cleavage while mRNAs are engaged by translating ribosomes. The prototypical mRNA endonuclease is PMR1, an en- zyme that was originally identified as an estrogen-induced ribonuclease activity whose appearance on polysomes coincides with the destabilization of serum protein mRNAs. The hallmark of endonuclease- mediated mRNA decay is its selectivity for specific mRNAs. This is determined by the formation of an mRNP complex (termed Complex I) containing PMR1 and its translating substrate mRNA. To join this complex PMR1 must be phosphorylated on a tyrosine residue in the polysome-targeting domain of the protein, and the past funding cycle identified c-Src as the kinase that is responsible for this key activation step. This is the first example of direct involvement of an oncogenic tyrosine kinase in mRNA decay, and it raises the possibility that PMR1-mediated mRNA decay may be a target of c-Src in cancer. Consistent with this, PMR1 binds to the Ena/VASP proteins, which are regulators of the actin cytoskeleton, and cell motility is increased in cells ex- pressing catalytically-active PMR1. Aim 1 will use tandem affinity chromatography to recover the Complex I mRNP, identify its constituent proteins, and determine their role in mRNP assembly and mRNA decay. This is the first step toward deciphering the 'RNP code' for PMR1-mRNA decay. The SH2 domain containing protein that is the 'gatekeeper' for recruiting PMR1 to the mRNP will be of particular interest, since none of these has known RNA-binding activity. Aim 2 continues work begun in the last cycle using microarrays to identify PMR1 target mRNAs by their recovery with Complex I. These will be compared to mRNAs that are selectively reduced by increasing expression of PMR1 and selectively increased by its knockdown. These will also be used to identify shared sequence or structural features that together with proteins in Aim 1 define the substrate mRNP. Cell motility is increased in cells expressing active PMR1, and the experiments in Aim 3 will use imag- ing of cell movement, quantitative PCR and immunofluorescence to examine the relationship between motility, PMR1 binding to the Ena/VASP proteins, and its recruitment to Complex I. The long-term goal of this work is to understand the molecular mechanisms of PMR1-mediated mRNA decay, how it is regulated and its role in con- trolling gene expression during development, in response to hormonal stimuli, and in malignancy.