多聚核糖體，SGs和P小體：此三者對細胞質mRNA命運及功能的動態調控

這篇文章於2017年11月22日在線發表於Plant Physiology，原題目為：

Polysomes, stress granules and processing bodies: a dynamic triumvirate controlling cytoplasmic mRNA fate and function

圖2. 植物細胞質mRNA 翻譯，儲存和降解概覽

細胞質mRNA經歷活性翻譯形式的多聚核糖體，在受損或翻譯抑制前會保持翻譯活性狀態（A）。抑制可以由多種原因引起（比如，改變特定翻譯因子或RNA結合蛋白的丰度或磷酸化，以及核糖體停滯），從而限制翻譯起始或核糖體轉移，促進轉變成翻譯抑制的mRNP，這些mRNA可以被RNA 結合蛋白（例如，UBP1，G3BP和RBP45/47）封存，或是通過不同的途徑降解（B）。通常通過細胞應激觸發mRNA的封存，經伴侶分子（例如熱應激恢復期間的HSP101）促進，進入翻譯過程。一些從SGs釋放的mRNA可能會在逆境脅迫後被降解（D）。在一般的mRNA降解中，通過不同類別的脫醯基酶（Nocturnin，Poly（A）-特異性核糖核酸酶（PARN）和CCR-NOT複合物）去除3保護性多聚腺苷酸尾部，然後通過多聚體SKI-外泌體複合體和/或RRP44B/SOV核酸外切酶在3至5』的方向降解。去腺苷酸化之後，也可以從5至3方向發生mRNA降解。通過這種模式降解的mRNA可以定位在胞質溶膠和/或PBs中，這裡多蛋白質mRNA脫帽複合物去除保護性5-帽子，隨後5-3 XRN4核糖核酸外切酶催化核苷酸水解（E）。除了脫腺苷依賴性的降解途徑外，翻譯抑制的mRNA可以在5至3方向上直接降解，而與通過XRN4介導的共翻譯降解的延長核糖體相關聯。這種降解模式繞過了脫腺苷酸化，但需要mRNA 脫帽。通過XRN4進行的mRNA的逐步5-3核酸外切，產生密碼子-密碼子 3-核苷酸標籤，其被認為反映了大多數5核糖體沿著轉錄本的運動（F）。縮寫：AUG，翻譯起始密碼子; STOP，終止密碼子

Figure 2. Overview of cytoplasmic mRNA translation, storage and decay in plants.

Cytoplasmic mRNAs undergoing active translation form polysomes and remain in a translationally active state until damaged or translationally repressed (A). Repression can result from multiple causes (i.e., altered abundance or phosphorylation of specific translation factors or RNA binding proteins and ribosome stalling) that limits translational initiation or ribosome translocation, promoting transition into a translationally repressed mRNP, where the mRNA is either sequestered by RNA-binding proteins (e.g., UBP1, G3BP, and RBP45/47) or degraded via different pathways (B). mRNA sequestration is typically triggered by cellular stress, serving as a enter translation via a process facilitated by a chaperone (e.g. HSP101 during heat stress recovery). Some mRNAs released from SGs may be targeted for degradation after stress (D). In general mRNA decay, the 3』 protective poly(A) tail is removed by different classes of deadenylases (Nocturnin, Poly(A)-specific ribonuclease (PARN) and CCR-NOT complex), then degradation proceeds in the 3』 to 5』 direction by the multimeric SKI-exosome complex and/or the RRP44B/SOV exonuclease. After deadenylation, mRNA degradation can also occur from the 5』 to 3』 direction. mRNAs degraded via this mode could be localized in the cytosol and/or PBs where the multi-protein mRNA decapping complex removes the protective 5』-cap, and subsequently the 5』-3』 XRN4 exoribonuclease catalyzes nucleotide hydrolysis (E). In addition to the deadenylation dependent decay pathway, translationally repressed mRNAs can be degraded directly in the 5』 to 3』 direction while in association with elongating ribosomes via XRN4-mediated co-translational decay. This mode of degradation bypasses deadenylation but requires mRNA decapping. The progressive 5』-3』 exonucleolytic destruction of the mRNA by XRN4 yields a codon-by-codon 3-nucleotide signature that is thought to reflect the movement of the most 5』 ribosome along the transcript (F). Abbreviations: AUG, translation initiation codon; STOP, termination codon.

每日一詞

sequestration

英[?si:kw?stre??n]

美[?sikw??stre??n, ?s?kw?-]

n. 扣押，沒收;

[例句]It also revealed which farming methods are best for carbon sequestration.

研究還顯示了哪一種農業方式對於碳匯最有利

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