Translational partitioning of the SL RNA

描述（由申请人提供）：着丝胞体目含有几种具有医学重要性的病原体。这些生物采用一种不寻常的基因表达机制，需要将每个核mRNA与39-nt剪接的先导RNA （SL）进行反式剪接。这种RNA和剪接模式在人类中没有发现，代表了化疗的独特靶点。SL RNA接受一个m7G共转录帽，随后在前四个核苷酸的糖和/或碱基部分添加7个甲基化，构成独特的“cap 4”。在布鲁氏锥虫和利什曼原虫中，在反式剪接之前，低甲基化的底物SL RNA在细胞内被转运，将潜在的竞争对手引入细胞质中以获得翻译起始因子。该应用的基础是对我们关于SL RNA成熟的假设的新补充，该假设涉及一种“屏蔽”蛋白，该蛋白可阻止未剪接底物上eIF4F翻译起始复合物的形成。对于L. tarentolae中通常表现为cap 1表型的SL RNA突变体的低甲基化和多体形成缺失，一个统一的解释是，它们被翻译掩蔽因子识别，但不能被揭开。这些SL突变体无法去除翻译掩膜的原因是：1)它们的5'端无法进行随后的cap - 2-4甲基化；2)一旦它们被反式剪接，它们仍然是翻译掩膜的，要么不能从细胞核输出，要么不能装载到细胞质溶胶中的多聚体上。已知帽结合蛋白之间存在的歧视水平促使我们在eIF4F因子家族中寻找这种SL掩膜。这些实验背后的假设是，未成熟的SL RNA必须从翻译机制中被掩盖，这样不完整的5'帽结构就不会干扰蛋白质合成。布鲁氏t细胞质帽结合蛋白eIF4E和相关翻译起始因子eIF4G的6个潜在同源物的存在为研究提供了候选物；初步数据表明，thebeif4e -3符合SL掩码要求。该申请的具体目的是：1)验证beif4e -3（利什曼原虫主要帽0结合蛋白的同源物）和/或其他候选物的帽结合特性；2)测定theif4e -3 RNA底物及相关蛋白。这些研究将有助于深入了解动质体SL的生物发生以及RNA加工和翻译之间的相互作用。公共卫生相关性：着丝胞体目含有几种具有医学重要性的病原体。这些生物采用一种不寻常的基因表达机制，需要将每个核mRNA与39-nt剪接的先导RNA （SL）进行转剪接。这种RNA和剪接模式在人类中没有发现，代表了化疗的独特靶点。

DESCRIPTION (provided by applicant): The Order Kinetoplastida contains several pathogens of medical importance. These organisms employ an unusual mechanism of gene expression that requires the trans-splicing of every nuclear mRNA with the 39-nt spliced leader (SL) RNA. This RNA and mode of splicing is not found in humans and represents a unique target for chemotherapy. The SL RNA receives a m7G cap co-transcriptionally, followed by the addition of seven methylations on the sugar and/or base moieties of the first four nucleotides, constituting the unique 'cap 4'. Undermethylated substrate SL RNA is trafficked intracellularly prior to trans-splicing in Trypanosoma brucei and Leishmania tarentolae, introducing potential competitors into the cytosol for translation initiation factors. The basis of this application is a new addition to our hypothesis on the maturation of SL RNA involving a 'mask' protein that prevents eIF4F translation-initiation complex formation on the unspliced substrate. A unifying explanation for the undermethylation and loss of polysome formation for SL RNA mutants in L. tarentolae, which generally exhibit the cap 1 phenotype, is that they are recognized by the translational masking factor, but cannot be unmasked. The inability of these SL mutants to remove the translational mask is that 1) their 5' ends are not accessable for subsequent cap 2-4 methylations and 2) once they are trans-spliced, they are still translationally masked and either not exported from the nucleus or not loaded onto polysomes in the cytosol. The level of discrimination that exists among known cap-binding proteins has led us to search for this SL mask in the families of eIF4F factors. The hypothesis underlying these experiments is that immature SL RNA must be masked from the translation machinery so that the incomplete 5'-cap structure does not interfere with protein synthesis. The presence of six potential homologs of the cytosolic cap-binding protein eIF4E and six of associated translation initiation factor eIF4G in T. brucei provided candidates for examination; preliminary data indicates that TbeIF4E-3 fits the SL mask requirements. The specific aims of this application are: 1) To validate the cap-binding properties of TbeIF4E-3, a homolog of the Leishmania major cap 0-binding protein, and/or other candidates; and 2) To determine the TbeIF4E-3 RNA substrate(s) and any associated proteins. These studies will lead to a thorough understanding of kinetoplastid SL biogenesis and the interplay between RNA processing and translation. PUBLIC HEALTH RELEVANCE: The Order Kinetoplastida contains several pathogens of medical importance. These organisms employ an unusual mechanism of gene expression that requires the transsplicing of every nuclear mRNA with the 39-nt spliced leader (SL) RNA. This RNA and mode of splicing is not found in humans and represents a unique target for chemotherapy.