血管生成素与tRNA片段化

tRNA主要功能是转运氨基酸参与蛋白质合成,在蛋白质生物合成过程中起着关键性的作用．近年来发现,tRNA是细胞内小RNA分子的重要来源,具有其它重要的生物学功能．来源于成熟tRNA分子的tRNA片段根据切割位置及生成机制的不同,主要分为两类：一类是tRNA半分子（tRNA halves）;另一类是较小的tRNA片段,称为tRFs( tRNA fragments)．在哺乳动物细胞中,tRNA半分子由血管生成素在tRNA分子反密码环处切割生成．本文主要针对tRNA半分子的加工机制、功能及在临床上的潜在应用进行综述．     

Altered expression of plant lysyl tRNA synthetase promotes tRNA misacylation and translational recoding of lysine

The Arabidopsis thaliana lysyl tRNA synthetase (AtKRS) structurally and functionally resembles the well-characterized prokaryotic class IIb KRS, including the propensity to aminoacylate tRNA(Lys) with suboptimal identity elements, as well as non-cognate tRNAs. Transient expression of AtKRS in carrot cells promotes aminoacylation of such tRNAs in vivo and translational recoding of lysine at nonsense codons. Stable expression of AtKRS in Zea mays causes translational recoding of lysine into zeins, significantly enriching the lysine content of grain.     

Cytoplasmic HYL1 modulates miRNA-mediated translational repression

MicroRNAs (miRNAs) control various biological processes by repressing target mRNAs. In plants, miRNAs mediate target gene repression via both mRNA cleavage and translational repression. However, the mechanism underlying this translational repression is poorly understood. Here, we found that Arabidopsis thaliana HYPONASTIC LEAVES1 (HYL1), a core component of the miRNA processing machinery, regulates miRNA-mediated mRNA translation but not miRNA biogenesis when it localized in the cytoplasm. Cytoplasmic HYL1 localizes to the endoplasmic reticulum and associates with ARGONAUTE1 (AGO1) and ALTERED MERISTEM PROGRAM1. In the cytoplasm, HYL1 monitors the distribution of AGO1 onto polysomes, binds to the mRNAs of target genes, represses their translation, and partially rescues the phenotype of the hyl1 null mutant. This study uncovered another function of HYL1 and provides insight into the mechanism of plant gene regulation.

The nuclear miRNA biogenesis factor HYL1 also localizes to the cytoplasm to modulate miRNA-mediated translational repression.     

Role of tRNA modification in translational fidelity

In transfer RNA many different modified nucleosides are found, especially in the anticodon region. In this region, pseudouridine (psi) is found in positions 38, 39 or 40 in a subset of tRNA species, 2-methylthio-6-hydroxyisopentenyladenosine (ms2io6A) is found in position 37 in tRNAs that read codons starting with U and 1-methylguanosine (m1G) is found in position 37 in tRNAs reading codons of the UCCNG type. We have used the mutants hisT, miaA and miaB and trmD, which are deficient in the biosynthesis of psi, ms2io6A, and m1G, respectively, to study the functional aspects of the respective modified nucleosides. We have shown: (1) Presence of psi improved the cellular growth rate, the polypeptide step-time, and the efficiency of an amber suppressor, but did not appreciably sense the codon context. (2) Presence of ms2io6A improved the cellular growth rate, the polypeptide step-time and the efficiency of several amber suppressor tRNAs. It also had a profound effect on the codon context sensitivity of the tRNA. (3) Presence of m1G improved the cellular growth rate and the polypeptide steptime and also prevented the tRNA from shifting the reading frame. Thus, these three modified nucleosides present in the anticodon region have apparently different functions.     

tRNA residues evolved to promote translational accuracy

The decoding properties of 22 structurally conservative base-pair and base-triple mutations in the anticodon hairpin and tertiary core of Escherichia coli tRNA^Ala_GGC were determined under single turnover conditions using E. coli ribosomes. While all of the mutations were able to efficiently decode the cognate GCC codon, many showed substantial misreading of near-cognate GUC or ACC codons. Although all the misreading mutations were present in the sequences of other E. coli tRNAs, they were never found among bacterial tRNA^Ala_GGC sequences. This suggests that the sequences of bacterial tRNA^Ala_GGC have evolved to avoid reading incorrect codons.     

Control of translational repression by protein-protein interactions.

The coat protein of the RNA bacteriophage MS2 is a translational repressor and interacts with a specific RNA stem-loop to inhibit translation of the viral replicase gene. As part of an effort to dissect genetically its RNA binding function, mutations were identified in the coat protein sequence that suppress mutational defects in the translational operator. Each of the mutants displayed a super-repressor phenotype, repressing translation from the wild-type and a variety of mutant operators better than did the wild-type coat protein. At least one mutant probably binds RNA more tightly than wild-type. The other mutants, however, were defective for assembly of virus-like particles, and self-associated predominantly as dimers. It is proposed that this assembly defect accounts for their super-repressor characteristics, since failure to assemble into virus-like particles elevates the effective concentration of repressor dimers. This hypothesis is supported by the observation that deletion of thirteen amino acids known to be important for assembly of dimers into capsids also resulted in the same assembly defect and in super-repressor activity. A second class of assembly defects is also described. Deletion of two amino acids from the C-terminus of coat protein resulted in failure to form capsids, most of the coat protein having the apparent molecular weight expected of trimers. This mutant (dl-8) was completely defective for repressor activity, probably because of an inability to form dimers. These results point out the inter-dependence of the structural and regulatory functions of coat protein.     

General translational repression by activators of mRNA decapping

Translation and mRNA degradation are affected by a key transition where eukaryotic mRNAs exit translation and assemble an mRNP state that accumulates into processing bodies (P bodies), cytoplasmic sites of mRNA degradation containing non-translating mRNAs, and mRNA degradation machinery. We identify the decapping activators Dhh1p and Pat1p as functioning as translational repressors and facilitators of P body formation. Strains lacking both Dhh1p and Pat1p show strong defects in mRNA decapping and P body formation and are blocked in translational repression. Contrastingly, overexpression of Dhh1p or Pat1p causes translational repression, P body formation, and arrests cell growth. Dhh1p, and its human homolog, RCK/p54, repress translation in vitro, and Dhh1p function is bypassed in vivo by inhibition of translational initiation. These results identify a broadly acting mechanism of translational repression that targets mRNAs for decapping and functions in translational control. We propose this mechanism is competitively balanced with translation, and shifting this balance is an important basis of translational control.     

Membrane-type 1 matrix metalloproteinase cleaves CD44 and promotes cell migration

Migratory cells including invasive tumor cells frequently express CD44, a major receptor for hyaluronan and membrane-type 1 matrix metalloproteinase (MT1-MMP) that degrades extracellular matrix at the pericellular region. In this study, we demonstrate that MT1-MMP acts as a processing enzyme for CD44H, releasing it into the medium as a soluble 70-kD fragment. Furthermore, this processing event stimulates cell motility; however, expression of either CD44H or MT1-MMP alone did not stimulate cell motility. Coexpression of MT1-MMP and mutant CD44H lacking the MT1-MMP-processing site did not result in shedding and did not promote cell migration, suggesting that the processing of CD44H by MT1-MMP is critical in the migratory stimulation. Moreover, expression of the mutant CD44H inhibited the cell migration promoted by CD44H and MT1-MMP in a dominant-negative manner. The pancreatic tumor cell line, MIA PaCa-2, was found to shed the 70-kD CD44H fragment in a MT1-MMP-dependent manner. Expression of the mutant CD44H in the cells as well as MMP inhibitor treatment effectively inhibited the migration, suggesting that MIA PaCa-2 cells indeed use the CD44H and MT1-MMP as migratory devices. These findings revealed a novel interaction of the two molecules that have each been implicated in tumor cell migration and invasion.     

RNA-binding protein-mediated translational repression of transgene expression in plants

We have demonstrated that RNA-binding proteins from coliphages and yeast can function as translational repressors in plants. RNA sequences called translational operators were inserted at a cap-proximal position in the 5-UTR of mRNAs of two reporter genes, gusor aroA:CP4. Translation of the reporter mRNAs was efficiently repressed when the RNA binding protein that specifically binds to its cognate operator was co-expressed. The efficiency of translational repression by RNA-binding protein positively correlated with the amount of binding protein in transformed plant cells. Detailed studies on coliphage MS2 coat protein-mediated translational repression also suggested that the efficiency of translational repression was position-dependent. A translational operator situated at the cap-proximal position was more efficient in conferring repression than one that was placed cap-distal. Translational repression can be an efficient means for regulation of transgene expression, thereby broadening current approaches for transgene regulation in plants.these authors contributed equally to this workthese authors contributed equally to this work     

Scanning for a unified model for translational repression by microRNAs

MicroRNAs (miRNAs) silence target mRNAs by inhibiting translation and subsequently initiating mRNA decay. The mechanism by which miRNAs silence translation is still poorly understood, with a number of competing models proposed. In this issue of The EMBO Journal, Kuzuo?lu‐Öztürk et al ( 2016 ) investigated miRNA silencing in human and insect cells. Their data support a model whereby miRNAs inhibit translation initiation. However, in contrast to several recent reports, their data suggest that translational inhibition is independent of 43S ribosomal subunit scanning, eIF4A translation factor activity, and 5′UTR secondary structure.     

Gene silencing by microRNAs: contributions of translational repression and mRNA decay

Despite their widespread roles as regulators of gene expression, important questions remain about target regulation by microRNAs. Animal microRNAs were originally thought to repress target translation, with little or no influence on mRNA abundance, whereas the reverse was thought to be true in plants. Now, however, it is clear that microRNAs can induce mRNA degradation in animals and, conversely, translational repression in plants. Recent studies have made important advances in elucidating the relative contributions of these two different modes of target regulation by microRNAs. They have also shed light on the specific mechanisms of target silencing, which, although it differs fundamentally between plants and animals, shares some common features between the two kingdoms.