Eukaryotic elongation factor-2 (eEF2): its regulation and peptide chain elongation

Regulation at the level of translation in eukaryotes is feasible because of the longer lifetime of eukaryotic mRNAs in the cell. The elongation stage of mRNA translation requires a substantial amount of energy and also eukaryotic elongation factors (eEFs). The important component of eEFs, i.e. eEF2 promotes the GTP-dependent translocation of the nascent protein chain from the A-site to the P-site of the ribosome. Mostly the eEF2 is regulated by phosphorylation and dephosphorylation by a specific kinase known as eEF2 kinase, which itself is up-regulated by various mechanisms in the eukaryotic cell. The activity of this kinase is dependent on calcium ions and calmodulin. Recently it has been shown that the activity of eEF2 kinase is regulated by MAP kinase signalling and mTOR signalling pathway. There are also various stimuli that control the peptide chain elongation in eukaryotic cell; some stimuli inhibit and some activate eEF2. These reports provide the mechanisms by which cells likely serve to slow down protein synthesis and conserve energy under nutrient deprived conditions via regulation of eEF2. The regulation via eEF2 has also been seen in mammary tissue of lactating cows, suggesting that eEF2 may be a limiting factor in milk protein synthesis. Regulation at this level provides the molecular understanding about the control of protein translocation reactions in eukaryotes, which is critical for numerous biological phenomenons. Further the elongation factors could be potential targets for regulation of protein synthesis like milk protein synthesis and hence probably its foreseeable application to synthetic biology.     

Y-box Binding Protein 1: Providing a New Angle on Translational Regulation

Current models of translational regulation are mostly focused on how translational factors engage a messenger mRNA to the ribosome to initiate translation. Since the majority of mRNAs in eukaryotes are translated in a cap-dependent manner, the mRNA 5’ cap-binding protein eIF4E was characterized as a key player responsible for the recruitment of mRNAs to the initiation complex. The availability of eIF4E is believed to be especially critical for translational activation of mRNAs with extensive secondary structures in their 5’UTRs, many of which code for labile regulatory proteins essential for cell growth or viability. Surprisingly, little attention is paid to the other side of translational control, e.g., to define mechanisms responsible for translational silencing and storage of the above messages. In this review, we discuss the possibility that eIF4E per se may not be sufficient to release mRNAs from translational block. We found that many growth- and stress-related mRNAs are associated with the translational repressor YB-1, which can compete with the eIF4E-driven translation initiation complex for binding to the capped 5’ mRNA terminus. Moreover, the cap-dependent repressor activity of YB-1 appears to be negatively regulated via Akt-mediated phosphorylation of the Ser-102 residue of YB-1. Taken together with recent evidence suggesting that translational activation of growth-related messages is a primary cellular response to activation of Ras-Erk and PI3K-Akt signaling pathways, our data suggest that differential expression of specific mRNA subsets is regulated by the PI3K-Akt pathway and achieved via coordinated activation of the components of translational machinery and inactivation of general translational repressors such as YB-1.     

植物叶绿体分裂的分子机制

叶绿体是植物细胞内一种重要的细胞器．它不仅是光合作用的场所,还是其它多种中间代谢的场所．叶绿体起源于蓝细菌,与其原核祖先类似,通过二分裂方式进行增殖．最近的研究表明,叶绿体的分裂装置包含原核起源和真核起源的蛋白质,它们在叶绿体的内膜内侧和外膜外侧协同作用以完成叶绿体的分裂．在过去十几年里,包括丝状温度敏感蛋白Z（FtsZ）、Min系统蛋白、质体分裂蛋白（PDV）和ARC蛋白等在内的多个叶绿体分裂相关组分被分离鉴定．本文简要介绍了叶绿体分裂装置各成员的发现、叶绿体被膜的收缩和叶绿体分裂位点的选择机制．另外,植物发育过程中叶绿体分裂可能受到细胞的控制,但目前对细胞如何调控叶绿体分裂知之甚少．本文对该领域的最新研究进展也进行了综述．     

Mechanisms through which sulfur amino acids control protein metabolism and oxidative status

Amino acids regulate protein synthesis and breakdown (i.e., protein turnover) and consequently protein deposition, which corresponds to the balance between the two processes. Elucidating the mechanisms involved in such regulation is important from fundamental and applied points of view since it can provide a basis to optimize amino acid requirements and to control protein mass, body composition and so forth. Amino acids, which have long been considered simply as precursors of protein synthesis, are now recognized to exert other significant influences; that is, they are precursors of essential molecules, act as mediators or signal molecules and affect numerous functions. For example, amino acids act as mediators of metabolic pathways in the same manner as certain hormones. Thus, they modulate the activity of intracellular protein kinases involved in the regulation of metabolic pathways such as mRNA translation. We provide here an overview of the roles of amino acids as regulators of protein metabolism, by focusing particularly on sulfur amino acids. The potential importance of methionine as a "nutrient signal" is discussed in the light of recent findings. Emphasis is also placed on mechanisms controlling oxidative status since sulfur amino acids are involved in the synthesis of intracellular antioxidants (glutathione, taurine etc.) and in the methionine sulfoxide reductase antioxidant system.     

Quality control systems for aberrant mRNAs induced by aberrant translation elongation and termination

RNA processing is an essential gene expression step and plays a crucial role to achieve diversity of gene products in eukaryotes. Various aberrant mRNAs transiently produced during RNA processing reactions are recognized and eliminated by specific quality control systems. It has been demonstrated that these mRNA quality control systems stimulate the degradation of aberrant mRNA to prevent the potentially harmful products derived from aberrant mRNAs. Recent studies on quality control systems induced by abnormal translation elongation and termination have revealed that both aberrant mRNAs and proteins are subjected to rapid degradation. In NonStop Decay (NSD) quality control system, a poly(A) tail of nonstop mRNA is translated and the synthesis of poly-lysine sequence results in translation arrest followed by co-translational degradation of aberrant nonstop protein. In No-Go Decay (NGD) quality control system, the specific amino acid sequences of the nascent polypeptide induce ribosome stalling, and the arrest products are ubiquitinated and rapidly degraded by the proteasome. In Nonfunctional rRNA Decay (NRD) quality control system, aberrant ribosomes composed of nonfunctional ribosomal RNAs are also eliminated when aberrant translation elongation complexes are formed on mRNA. I describe recent progresses on the mechanisms of quality control systems and the relationships between quality control systems. This article is part of a Special issue entitled: RNA Decay mechanisms.     

Evolution of macromolecular import pathways in mitochondria,hydrogenosomes and mitosomes

All eukaryotes require mitochondria for survival and growth. The origin of mitochondria can be traced down to a single endosymbiotic event between two probably prokaryotic organisms. Subsequent evolution has left mitochondria a collection of heterogeneous organelle variants. Most of these variants have retained their own genome and translation system. In hydrogenosomes and mitosomes, however, the entire genome was lost. All types of mitochondria import most of their proteome from the cytosol, irrespective of whether they have a genome or not. Moreover, in most eukaryotes, a variable number of tRNAs that are required for mitochondrial translation are also imported. Thus, import of macromolecules, both proteins and tRNA, is essential for mitochondrial biogenesis. Here, we review what is known about the evolutionary history of the two processes using a recently revised eukaryotic phylogeny as a framework. We discuss how the processes of protein import and tRNA import relate to each other in an evolutionary context.     

Regulation of mammalian mitochondrial translation by post-translational modifications

Mitochondria are responsible for the production of over 90% of the energy in eukaryotes through oxidative phosphorylation performed by electron transfer and ATP synthase complexes. Mitochondrial translation machinery is responsible for the synthesis of 13 essential proteins of these complexes encoded by the mitochondrial genome. Emerging data suggest that acetyl-CoA, NAD(+), and ATP are involved in regulation of this machinery through post-translational modifications of its protein components. Recent high-throughput proteomics analyses and mapping studies have provided further evidence for phosphorylation and acetylation of ribosomal proteins and translation factors. Here, we will review our current knowledge related to these modifications and their possible role(s) in the regulation of mitochondrial protein synthesis using the homology between mitochondrial and bacterial translation machineries. However, we have yet to determine the effects of phosphorylation and acetylation of translation components in mammalian mitochondrial biogenesis. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.     

Structure and function of MYST1 histone acetyltransferase in the interactome of animal cells

The major function of protein MYST1 is acetylation of histone H4 at the K16 residue. This modification is essential for chromatin remodeling and is used for regulation of gene expression in eukaryotes. MYST1 is a part of multiprotein complexes that accomplish functions of male X-chromosome activation and thereby functions of dosage compensation in drosophila and, in mammals, global acetylation of histone H4 K16. Recently, novel functional links between MYST1 and proteins ATM and p53 have been observed, and it is recognized that MYST1 plays a role in tumor suppression mechanisms. In the present review, we examine novel data about functional composition and mechanisms of MYST1-containing complexes. Interplay between MYST1 and other components of the animal cell interactome is also discussed.     

Lessons (not) learned from mistakes about translation

Some popular ideas about translational regulation in eukaryotes have been recognized recently as mistakes. One example is the rejection of a long-standing idea about involvement of S6 kinase in translation of ribosomal proteins. Unfortunately, new proposals about how S6 kinase might regulate translation are based on evidence that is no better than the old. Recent findings have also forced rejection of some popular ideas about the function of sequences at the 3' end of viral mRNAs and rejection of some ideas about internal ribosome entry sequences (IRESs). One long-held belief was that tissue-specific translation via an IRES underlies the neurotropism of poliovirus and the attenuation of Sabin vaccine strains. Older experiments that appeared to support this belief and recent experiments that refute it are discussed. The hypothesis that dyskeratosis congenita is caused by a defect in IRES-mediated translation is probably another mistaken idea. The supporting evidence, such as it is, comes from a mouse model of the disease and is contradicted by studies carried out with cells from affected patients. The growing use of IRESs as tools to study other questions about translation is discussed and lamented. The inefficient function of IRESs (if they are IRESs) promotes misunderstandings. I explain again why it is not valid to invoke a special mechanism of initiation based on the finding that edeine (at very low concentrations) does not inhibit the translation of a putative IRES from cricket paralysis virus. I explain why new assays, devised to rule out splicing in tests with dicistronic vectors, are not valid and why experiments with IRESs are not a good way to investigate the mechanism whereby microRNAs inhibit translation.     

Are Anaphase Events Really Irreversible? The Endmost Stages of Cell Division and the Paradox of the DNA Double-Strand Break Repair

It has been recently demonstrated that yeast cells are able to partially regress chromosome segregation in telophase as a response to DNA double-strand breaks (DSBs), likely to find a donor sequence for homology-directed repair (HDR). This regression challenges the traditional concept that establishes anaphase events as irreversible, hence opening a new field of research in cell biology. Here, the nature of this new behavior in yeast is summarized and the underlying mechanisms are speculated about. It is also discussed whether it can be reproduced in other eukaryotes. Overall, this work brings forwards the need of understanding how cells attempt to repair DSBs when transiting the latest stages of mitosis, i.e., anaphase and telophase.