Termination of the unfolded protein response is guided by ER stress-induced HAC1 mRNA nuclear retention

Cellular homeostasis is maintained by surveillance mechanisms that intervene at virtually every step of gene expression. In the nucleus, the yeast chromatin remodeler Isw1 holds back maturing mRNA ribonucleoparticles to prevent their untimely export, but whether this activity operates beyond quality control of mRNA biogenesis to regulate gene expression is unknown. Here, we identify the mRNA encoding the central effector of the unfolded protein response (UPR) HAC1, as an Isw1 RNA target. The direct binding of Isw1 to the 3' untranslated region of HAC1 mRNA restricts its nuclear export and is required for accurate UPR abatement. Accordingly, ISW1 inactivation sensitizes cells to endoplasmic reticulum (ER) stress while its overexpression reduces UPR induction. Our results reveal an unsuspected mechanism, in which binding of ER-stress induced Isw1 to HAC1 mRNA limits its nuclear export, providing a feedback loop that fine-tunes UPR attenuation to guarantee homeostatic adaptation to ER stress.

Novel functions for chromatin dynamics in mRNA biogenesis beyond transcription

The first step of gene expression results in the production of mRNA ribonucleoparticles (mRNPs) that are exported to the cytoplasm via the NPC for translation into the cytoplasm. During this process, the mRNA molecule synthesized by RNA polymerase II (Pol II) undergoes extensive maturation, folding and packaging events that are intimately coupled to its synthesis. All these events take place in a chromatin context and it is therefore not surprising that a growing number of studies recently reported specific contributions of chromatin dynamics to various steps of mRNP biogenesis. In this extra view, we replace our recent findings highlighting the contribution of the yeast chromatin remodeling complex ISW1 to nuclear mRNA quality control in the context of the recent literature.

Mapping ubiquitin modifications reveals new functions for the yeast nuclear pore complex

Covalent attachment of ubiquitin to target proteins, or ubiquitylation, has emerged as one of the most prevalent posttranslational modifications (PTMs), regulating nearly every cellular pathway. The diversity of functions associated with this particular PTM stems from the myriad ways in which a target protein can be modified by ubiquitin, e.g., monoubiquitin, multi-monoubiquitin and polyubiquitin linkages. In the current study, we took a systematic approach to analyze the ubiquitylation profiles of the yeast Saccharomyces cerevisiae nuclear pore complex (NPC) proteins or nucleoporins. We found the yeast NPC to be extensively modified by ubiquitin with highly variable ubiquitylation profiles, suggesting that dissection of these modifications may provide new insights into the regulation of NPC functions and reveal additional roles for nucleoporins beyond nuclear transport.

H2B ubiquitylation controls the formation of export-competent mRNP

Histone H2B ubiquitylation is a transcription-dependent modification that not only regulates nucleosome dynamics but also controls the trimethylation of histone H3 on lysine 4 by promoting ubiquitylation of Swd2, a component of both the histone methyltransferase COMPASS complex and the cleavage and polyadenylation factor(CPF). We show that preventing either H2B ubiquitylation or H2B-dependent modification of Swd2 results in nuclear accumulation of poly(A) RNA due to a defect in the integrity and stability of APT, a subcomplex of the CPF. Ubiquitin-regulated APT complex dynamics is required for the correct recruitment of the mRNA export receptor Mex67 to nuclear mRNPs. While H2B ubiquitylation controls the recruitment of the different Mex67 adaptors to mRNPs, the effect of Swd2 ubiquitylation is restricted to Yra1 and Nab2, which, in turn, controls poly(A) tail length. Modification of H2B thus participates in the crosstalk between cotranscriptional events and assembly of mRNPs linking nuclear processing and mRNA export.

Ubiquitin and assembly of export competent mRNP

The production of mature and export competent mRNP (mRNA ribonucleoprotein) complexes depends on a series of highly coordinated processing reactions. RNA polymerase II (RNAPII) plays a central role in this process by mediating the sequential recruitment of mRNA maturation and export factors to transcribing genes, thereby establishing a strong functional link between transcription and export through nuclear pore complexes (NPC). Growing evidence indicates that post-translational modifications participate in the dynamic association of processing and export factors with mRNAs ensuring that the transitions and rearrangements undergone by the mRNP occur at the right time and place. This review mainly focuses on the role of ubiquitin conjugation in controlling mRNP assembly and quality control from transcription down to export through the NPC. It emphasizes the central role of ubiquitylation in organizing the chronology of events along this highly dynamic pathway. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

Ubiquitylation of the nuclear pore complex controls nuclear migration during mitosis in S. cerevisiae

A systematic analysis revealed that the nuclear pore complex is extensively modified by ubiquitin and that ubiquitylation of the NPC component Nup159 is required for dynein light chain targeting to the NPC and proper nuclear segregation during mitosis.

Nuclear pore complexes (NPCs) correspond to large protein transport complexes responsible for selective nucleocytoplasmic exchange. Although research has revealed much about the molecular architecture and roles of the NPC subcomplexes, little is known about the regulation of NPC functions by posttranslational modifications. We used a systematic approach to show that more than half of NPC proteins were conjugated to ubiquitin. In particular, Nup159, a nucleoporin exclusively located on the cytoplasmic side of the NPC, was monoubiquitylated by the Cdc34/SCF (Skp1–Cdc53–F-box E3 ligase) enzymes. Preventing this modification had no consequences on nuclear transport or NPC organization but strongly affected the ability of Nup159 to target the dynein light chain to the NPC. This led to defects in nuclear segregation at the onset of mitosis. Thus, defining ubiquitylation of the yeast NPC highlights yet-unexplored functions of this essential organelle in cell division.

A surveillance pathway monitors the fitness of the endoplasmic reticulum to control its inheritance

The endoplasmic reticulum (ER) plays an essential role in the production of lipids and secretory proteins. Because the ER cannot be generated de novo, it must be faithfully transmitted or divided at each cell division. Little is known of how cells monitor the functionality of the ER during the cell cycle or how this regulates inheritance. We report here that ER stress in S. cerevisiae activates the MAP kinase Slt2 in a new ER stress surveillance (ERSU) pathway, independent of the unfolded protein response. Upon ER stress, ERSU alters the septin complex to delay ER inheritance and cytokinesis. In the absence of Slt2 kinase, the stressed ER is transmitted to the daughter cell, causing the death of both mother and daughter cells. Furthermore, Slt2 is activated via the cell surface receptor Wsc1 by a previously undescribed mechanism. We conclude that the ERSU pathway ensures inheritance of a functional ER.

Transcription factor Foxo3 controls the magnitude of T cell immune responses by modulating the function of dendritic cells

Foxo transcription factors regulate cell cycle progression, survival, and DNA repair pathways. Here, we demonstrate that a deficiency in Foxo3 resulted in increased expansion of T cell populations after viral infection. This exaggerated expansion was not T cell intrinsic. Rather, it was caused by the enhanced capacity of Foxo3-deficient dendritic cells to sustain T cell viability by producing increased amounts of interleukin 6 (IL-6). CTLA-4-mediated stimulation of dendritic cells induced nuclear localization of Foxo3, which in turn inhibited IL-6 and tumor necrosis factor production. Thus, Foxo3 acts to constrain dendritic cell production of key inflammatory cytokines and control T cell survival.

A novel role in cytokinesis reveals a housekeeping function for the unfolded protein response

The unfolded protein response (UPR) pathway helps cells cope with endoplasmic reticulum (ER) stress by activating genes that increase the ER's functional capabilities. We have identified a novel role for the UPR pathway in facilitating budding yeast cytokinesis. Although other cell cycle events are unaffected by conditions that disrupt ER function, cytokinesis is sensitive to these conditions. Moreover, efficient cytokinesis requires the UPR pathway even during unstressed growth conditions. UPR-deficient cells are defective in cytokinesis, and cytokinesis mutants activate the UPR. The UPR likely achieves its role in cytokinesis by sensing small changes in ER load and making according changes in ER capacity. We propose that cytokinesis is one of many cellular events that require a subtle increase in ER function and that the UPR pathway has a previously uncharacterized housekeeping role in maintaining ER plasticity during normal cell growth.

Two Hrd1p homologues in the yeast Yarrowia lipolytica which act in different pathways

The endoplasmic reticulum associated degradation (ERAD) is a process widespread in eukaryotes that enable cells to get rid of unfolded or unassembled polypeptides which jam the endoplasmic reticulum compartment. In order to improve understanding of the initial steps of the secretory pathway and their relationship, we focused on components of the ERAD ubiquitylation machinery in the yeast Yarrowia lipolytica. Two Hrd1p homologues, Hrd1p and Hrh1p, were identified in Y. lipolytica. A study of the fate of the heterologous CPY* reporter protein showed that YlHrd1p is involved in the elimination of this misfolded polypeptide, while YlHrh1p is not. Moreover, the different phenotypic pattern displayed by Deltahrd1 and Deltahrh1 cells suggests that the two putative E3 enzymes function in separate ways. Our results bring some evidence of a coupling between the ERAD pathway and the co-translational translocation process and show that studies in Y. lipolytica can give new insights into events that take place in the ER.

Genome evolution in yeasts

Identifying the mechanisms of eukaryotic genome evolution by comparative genomics is often complicated by the multiplicity of events that have taken place throughout the history of individual lineages, leaving only distorted and superimposed traces in the genome of each living organism. The hemiascomycete yeasts, with their compact genomes, similar lifestyle and distinct sexual and physiological properties, provide a unique opportunity to explore such mechanisms. We present here the complete, assembled genome sequences of four yeast species, selected to represent a broad evolutionary range within a single eukaryotic phylum, that after analysis proved to be molecularly as diverse as the entire phylum of chordates. A total of approximately 24,200 novel genes were identified, the translation products of which were classified together with Saccharomyces cerevisiae proteins into about 4,700 families, forming the basis for interspecific comparisons. Analysis of chromosome maps and genome redundancies reveal that the different yeast lineages have evolved through a marked interplay between several distinct molecular mechanisms, including tandem gene repeat formation, segmental duplication, a massive genome duplication and extensive gene loss.

Identification of an UDP-Glc:glycoprotein glucosyltransferase in the yeast Yarrowia lipolytica

The UDP-Glc:glycoprotein glucosyltransferase (UGT) is a soluble protein of the endoplasmic reticulum (ER) that plays a determining part in the mechanism by which unfolded, partially folded or misfolded glycoproteins are retained into the ER. We have identified an UGT in the yeast Yarrowia lipolytica. This protein, of a predicted molecular weight of 165.7 kDa, is encoded by a 5054 bp coding sequence containing a 643 bp intron at position 682-1323. The N-terminal part of the protein displays a signal sequence whereas its C-terminal part carries an ER retrieval signal HDEL. An interruption of the gene that removes the 1075 last nucleotides of its sequence did not lead to any evident phenotype except for a slight increased sensitivity to tunicamycin. YlUGT1 mRNA levels respond to tunicamycin treatment by an induction factor of 2-4, which indicates that the gene product participates in the quality control mechanism in this yeast. Finally, an immunofluorescence study of the protein localization, shows that the protein distribution is different from that of previously studied ER resident proteins. This could indicate that UGT distribution in the secretory pathway is not confined to the ER.

Sbh1p, a subunit of the Sec61 translocon, interacts with the chaperone calnexin in the yeast Yarrowia lipolytica

The core component of the translocation apparatus, Sec61p or alpha, was previously cloned in Yarrowia lipolytica. Using anti-Sec61p antibodies, we showed that most of the translocation sites are devoted to co-translational translocation in this yeast, which is similar to the situation in mammalian cells but in contrast to the situation in Saccharomyces cerevisiae, where post-translational translocation is predominant. In order to characterize further the minimal translocation apparatus in Y. lipolytica, the beta Sec61 complex subunit, Sbh1p, was cloned by functional complementation of a Deltasbh1, Deltasbh2 S. cerevisiae mutant. The secretion of the reporter protein is not impaired in the Y. lipolytica sbh1 inactivated strain. We screened the Y. lipolytica two-hybrid library to look for partners of this translocon component. The ER-membrane chaperone protein, calnexin, was identified as an interacting protein. By a co-immunoprecipitation approach, we confirmed this association in Yarrowia and then showed that the S. cerevisiae Sbh2p protein was a functional homologue of YlSbh1p. The interaction of Sbh1p with calnexin was shown to occur between the lumenal domain of both proteins. These results suggest that the beta subunit of the Sec61 translocon may relay folding of nascent proteins to their translocation.

[The effect of a dimenhydrinate combination of the cardiovascular system and cerebral blood circulation in the anaesthetized dog]

The influence of Vertigo-Vomex (VV) and Vertigo-Vomex Retard (VVR) on the perfusion rate of the a. carotis interna and on the peripheral circulation was tested in 30 anaesthetized dogs. These drugs represent beta-dimethylamino-ethyl-benzhydrylether-1,3-dimethyl-8-chloroxanthine (dimenhydrinate, Vomex A) with the addition of pyridine-3-carbonic acid (nicotinic acid, niacin) and pyridoxine-HCl (vitamin B6) and its retard preparation. A) Angiographic researches of the brain vessels after i.v. injection of VV. B) Perfusion rate in the a. carotis int. by electromagnetic flowmeter; measurements of blood pressure, heart rate, and acid-base balance; reactions after injection of niacin, dimenhydrinate, and pyridoxine-HCl as well as after a combination of both. C) Same measurements as in B but after oral application. In one out of four cases there was a dilation of the brain vessels, in two cases there was a small one, in one case there was none. Niacin induces a short-lasting augmentation of the perfusion rate in the a. carotis int. simultaneously with rising aortic blood pressure. Dimenhydrinate with pyridoxine-HCl slightly increases the heart rate. Combined with niacin the perfusion rate shortly increases immediately after injection of niacin without any rise of blood pressure but combined with a fall in blood pH and standard bicarbonate. After oral application of VVR there was a slight but not significant augmentation of the perfusion rate in the a. carotis int. combined with rising blood pressure but with no change of the blood pH. Therefore the certain intivertigineous effect of VV and VVR cannot be explained by a rise of the perfusion rate in the inner ear or in the brain. In view of our results the central effect of dimenhydrinate combined with niacin and pyridoxine-HCl hypothetically may be improved by metabolic influences on the cells and their membranes.