Gal4 turnover and transcription activation

The utility and caveat of split-GAL4s in the study of neurodegeneration

Parkinson's disease (PD) is the second most common neurodegenerative disorder, afflicting over 1% of the population of age 60 y and above. The loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) is the primary cause of its characteristic motor symptoms. Studies using Drosophila melanogaster and other model systems have provided much insight into the pathogenesis of PD. However, little is known why certain cell types are selectively susceptible to degeneration in PD. Here, we describe an approach to identify vulnerable subpopulations of neurons in the genetic background linked to PD in Drosophila, using the split-GAL4 drivers that enable genetic manipulation of a small number of defined cell populations. We identify split-GAL4 lines that target neurons selectively vulnerable in a model of leucine-rich repeat kinase 2 (LRRK2)-linked familial PD, demonstrating the utility of this approach. We also show an unexpected caveat of the split-GAL4 system in ageing-related research: an age-dependent increase in the number of GAL4-labelled cells.

CYCLIN-DEPENDENT KINASE 8 positively regulates oil synthesis by activating WRINKLED1 transcription

CYCLIN-DEPENDENT KINASE 8 (CDK8), a component of the kinase module of the Mediator complex in Arabidopsis, is involved in many processes, including flowering, plant defense, drought, and energy stress responses. Here, we investigated cdk8 mutants and CDK8-overexpressing lines to evaluate whether CDK8 also plays a role in regulating lipid synthesis, an energy-demanding anabolism. Quantitative lipid analysis demonstrated significant reductions in lipid synthesis rates and lipid accumulation in developing siliques and seedlings of cdk8, and conversely, elevated lipid contents in wild-type seed overexpressing CDK8. Transactivation assays show that CDK8 is necessary for maximal transactivation of the master seed oil activator WRINKLED1 (WRI1) by the seed maturation transcription factor ABSCISIC ACID INSENSITIVE3, supporting a direct regulatory role of CDK8 in oil synthesis. Thermophoretic studies show GEMINIVIRUS REP INTERACTING KINASE1, an activating kinase of KIN10 (a catalytic subunit of SUCROSE NON-FERMENTING1-RELATED KINASE1), physically interacts with CDK8, resulting in its phosphorylation and degradation in the presence of KIN10. This work defines a mechanism whereby, once activated, KIN10 downregulates WRI1 expression and suppresses lipid synthesis via promoting the degradation of CDK8. The KIN10-CDK8-dependent regulation of lipid synthesis described herein is additional to our previously reported KIN10-dependent phosphorylation and degradation of WRI1.

Cell cycle-dependent localization of the proteasome to chromatin

An integrative understanding of nuclear events including transcription in normal and cancer cells requires comprehensive and quantitative measurement of protein dynamics that underlie such events. However, the low abundance of most nuclear proteins hampers their detailed functional characterization. We have now comprehensively quantified the abundance of nuclear proteins with the use of proteomics approaches in both normal and transformed human diploid fibroblasts. We found that subunits of the 26S proteasome complex were markedly down-regulated in the nuclear fraction of the transformed cells compared with that of the wild-type cells. The intranuclear proteasome abundance appeared to be inversely related to the rate of cell cycle progression, with restraint of the cell cycle being associated with an increase in the amount of proteasome subunits in the nucleus, suggesting that the nuclear proteasome content is dependent on the cell cycle. Furthermore, chromatin enrichment for proteomics (ChEP) analysis revealed enrichment of the proteasome in the chromatin fraction of quiescent cells and its apparent dissociation from chromatin in transformed cells. Our results thus suggest that translocation of the nuclear proteasome to chromatin may play an important role in control of the cell cycle and oncogenesis through regulation of chromatin-associated transcription factors.

Live-cell imaging reveals the interplay between transcription factors, nucleosomes, and bursting

Transcription factors show rapid and reversible binding to chromatin in living cells, and transcription occurs in sporadic bursts, but how these phenomena are related is unknown. Using a combination of in vitro and in vivo single-molecule imaging approaches, we directly correlated binding of the Gal4 transcription factor with the transcriptional bursting kinetics of the Gal4 target genes GAL3 and GAL10 in living yeast cells. We find that Gal4 dwell time sets the transcriptional burst size. Gal4 dwell time depends on the affinity of the binding site and is reduced by orders of magnitude by nucleosomes. Using a novel imaging platform called orbital tracking, we simultaneously tracked transcription factor binding and transcription at one locus, revealing the timing and correlation between Gal4 binding and transcription. Collectively, our data support a model in which multiple RNA polymerases initiate transcription during one burst as long as the transcription factor is bound to DNA, and bursts terminate upon transcription factor dissociation.

Atrogin-1 Increases Smooth Muscle Contractility Through Myocardin Degradation

Atrogin-1, an E3 ligase present in skeletal, cardiac and smooth muscle, down-regulates myocardin protein during skeletal muscle differentiation. Myocardin, the master regulator of smooth muscle cell (SMC) differentiation, induces expression of smooth muscle marker genes through its association with serum response factor (SRF), which binds to the CArG box in the promoter. Myocardin undergoes ubiquitylation and proteasomal degradation. Evidence suggests that proteasomal degradation of myocardin is critical for myocardin to exert its transcriptional activity, but there is no report about the E3 ligase responsible for myocardin ubiquitylation and subsequent transactivation. Here, we showed that overexpression of atrogin-1 increased contractility of cultured SMCs and mouse aortic tissues in organ culture. Overexpression of dominant-negative myocardin attenuated the increase in SMC contractility induced by atrogin-1. Atrogin-1 overexpression increased expression of the SM contractile markers while downregulated expression of myocardin protein but not mRNA. Atrogin-1 also ubiquitylated myocardin for proteasomal degradation in vascular SMCs. Deletion studies showed that atrogin-1 directly interacted with myocardin through its amino acids 284-345. Immunostaining studies showed nuclear localization of atrogin-1, myocardin, and the Rpt6 subunit of the 26S proteasome. Atrogin-1 overexpression not only resulted in degradation of myocardin but also increased recruitment of RNA Polymerase II onto the promoters of myocardin target genes. In summary, our results have revealed the roles for atrogin-1 in the regulation of smooth muscle contractility through enhancement of myocardin ubiquitylation/degradation and its transcriptional activity. J. Cell. Physiol. 232: 806-817, 2017. © 2016 Wiley Periodicals, Inc.

Interaction of Gcn4 with target gene chromatin is modulated by proteasome function

The yeast transcription factor Gcn4 requires a ubiquitin ligase and the proteasome in order to function. Inhibiting proteasome function prevents the interaction of Gcn4 with target gene chromatin, and this activity is suppressed by inactivation of the ubiquitin-selective chaperone Cdc48. Thus proteolysis of Gcn4 is not required for its function.

The ubiquitin–proteasome system (UPS) influences gene transcription in multiple ways. One way in which the UPS affects transcription centers on transcriptional activators, the function of which can be stimulated by components of the UPS that also trigger their destruction. Activation of transcription by the yeast activator Gcn4, for example, is attenuated by mutations in the ubiquitin ligase that mediates Gcn4 ubiquitylation or by inhibition of the proteasome, leading to the idea that ubiquitin-mediated proteolysis of Gcn4 is required for its activity. Here we probe the steps in Gcn4 activity that are perturbed by disruption of the UPS. We show that the ubiquitylation machinery and the proteasome control different steps in Gcn4 function and that proteasome activity is required for the ability of Gcn4 to bind to its target genes in the context of chromatin. Curiously, the effect of proteasome inhibition on Gcn4 activity is suppressed by mutations in the ubiquitin-selective chaperone Cdc48, revealing that proteolysis per se is not required for Gcn4 activity. Our data highlight the role of Cdc48 in controlling promoter occupancy by Gcn4 and support a model in which ubiquitylation of activators—not their destruction—is important for function.

Functions of the proteasome on chromatin

The proteasome is a large self-compartmentalized protease complex that recognizes, unfolds, and destroys ubiquitylated substrates. Proteasome activities are required for a host of cellular functions, and it has become clear in recent years that one set of critical actions of the proteasome occur on chromatin. In this review, we discuss some of the ways in which proteasomes directly regulate the structure and function of chromatin and chromatin regulatory proteins, and how this influences gene transcription. We discuss lingering controversies in the field, the relative importance of proteolytic versus non-proteolytic proteasome activities in this process, and highlight areas that require further investigation. Our intention is to show that proteasomes are involved in major steps controlling the expression of the genetic information, that proteasomes use both proteolytic mechanisms and ATP-dependent protein remodeling to accomplish this task, and that much is yet to be learned about the full spectrum of ways that proteasomes influence the genome.

Independent RNA polymerase II preinitiation complex dynamics and nucleosome turnover at promoter sites in vivo

Transcription by all three eukaryotic RNA polymerases involves the assembly of a large preinitiation complex (PIC) at gene promoters. The PIC comprises several general transcription factors (GTFs), including TBP, and the respective RNA polymerase. It has been suggested that some GTFs remain stably bound at active promoters to facilitate multiple transcription events. Here we used two complementary approaches to show that, in G1-arrested yeast cells, TBP exchanges very rapidly even at the most highly active RNA Pol II promoters. A similar situation is observed at RNA Pol III promoters. In contrast, TBP remains stably bound at RNA Pol I promoters. We also provide evidence that, unexpectedly, PIC dynamics are neither the cause nor the consequence of nucleosome exchange at most of the RNA Pol II promoters we analyzed. These results point to a stable reinitiation complex at RNA Pol I promoters and suggest independent PIC and nucleosome turnover at many RNA Pol II promoters.

Measuring chromatin interaction dynamics on the second time scale at single-copy genes

The chromatin immunoprecipitation (ChIP) assay is widely used to capture interactions between chromatin and regulatory proteins, but it is unknown how stable most native interactions are. Although live-cell imaging suggests short-lived interactions at tandem gene arrays, current methods cannot measure rapid binding dynamics at single-copy genes. We show, by using a modified ChIP assay with subsecond temporal resolution, that the time dependence of formaldehyde cross-linking can be used to extract in vivo on and off rates for site-specific chromatin interactions varying over a ~100-fold dynamic range. By using the method, we show that a regulatory process can shift weakly bound TATA-binding protein to stable promoter interactions, thereby facilitating transcription complex formation. This assay provides an approach for systematic, quantitative analyses of chromatin binding dynamics in vivo.

Physical and genetic associations of the Irc20 ubiquitin ligase with Cdc48 and SUMO

A considerable percentage of the genome is dedicated to the ubiquitin-proteasome system, with the yeast genome predicted to encode approximately 100 ubiquitin ligases (or E3s), and the human genome predicted to encode more than 600 E3s. The most abundant class of E3s consists of RING finger-containing proteins. Although many insights have been obtained regarding the structure and catalytic mechanism of the E3s, much remains to be learned about the function of the individual E3s. Here we characterize IRC20, which encodes a dual RING- and Snf/Swi family ATPase domain-containing protein in yeast that has been implicated in DNA repair. We found that overexpression of IRC20 causes two transcription-associated phenotypes and demonstrate that the Irc20 RING domain possesses ubiquitin E3 activity in vitro. Two mass spectrometry approaches were undertaken to identify Irc20-associated proteins. Wild-type Irc20 associated with Cdc48, a AAA-ATPase that serves as an intermediary in the ubiquitin-proteasome system. A second approach using a RING mutant derivative of Irc20 detected increased association of the Irc20 mutant with SUMO. These findings provide a foundation for understanding the roles of Irc20 in transcription and DNA repair.

Quantifying transcription factor kinetics: at work or at play?

Transcription factors (TFs) interact dynamically in vivo with chromatin binding sites. Here we summarize and compare the four different techniques that are currently used to measure these kinetics in live cells, namely fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), single molecule tracking (SMT) and competition ChIP (CC). We highlight the principles underlying each of these approaches as well as their advantages and disadvantages. A comparison of data from each of these techniques raises an important question: do measured transcription kinetics reflect biologically functional interactions at specific sites (i.e. working TFs) or do they reflect non-specific interactions (i.e. playing TFs)? To help resolve this dilemma we discuss five key unresolved biological questions related to the functionality of transient and prolonged binding events at both specific promoter response elements as well as non-specific sites. In support of functionality, we review data suggesting that TF residence times are tightly regulated, and that this regulation modulates transcriptional output at single genes. We argue that in addition to this site-specific regulatory role, TF residence times also determine the fraction of promoter targets occupied within a cell thereby impacting the functional status of cellular gene networks. Thus, TF residence times are key parameters that could influence transcription in multiple ways.

Genome-wide measurement of protein-DNA binding dynamics using competition ChIP

Competition chromatin immunoprecipitation (competition ChIP) enables experimenters to measure protein-DNA dynamics at a single locus or across the entire genome, depending on the detection method. Competition ChIP relies on a cell containing two copies of a single DNA-associated factor, with each copy of the factor differentially epitope tagged. One of the copies is expressed constitutively and the second is induced as a competitor. The ratio of isoforms associated with discrete genomic locations is detected by ChIP-on-chip (ChIP-chip) or ChIP-sequencing (ChIP-seq). The rate at which the resident isoform of the protein is replaced by the competitor at each binding location enables the calculation of residence time for that factor at each site of interaction genome wide. Here we provide a detailed protocol for designing and performing competition ChIP experiments in Saccharomyces cerevisiae, which takes ∼5 d to complete (not including strain production and characterizations, which may take as long as 6 months). Included in this protocol are guidelines for downstream bioinformatic analysis to extract residence times throughout the genome.

The ubiquitin-proteasome system of Saccharomyces cerevisiae

Protein modifications provide cells with exquisite temporal and spatial control of protein function. Ubiquitin is among the most important modifiers, serving both to target hundreds of proteins for rapid degradation by the proteasome, and as a dynamic signaling agent that regulates the function of covalently bound proteins. The diverse effects of ubiquitylation reflect the assembly of structurally distinct ubiquitin chains on target proteins. The resulting ubiquitin code is interpreted by an extensive family of ubiquitin receptors. Here we review the components of this regulatory network and its effects throughout the cell.

How the ubiquitin proteasome system regulates the regulators of transcription

The ubiquitin proteasome system plays an important role in transcription. Monoubiquitination of activators is believed to aid their function, while the 26S proteasomal degradation of repressors is believed to restrict their function. What remains controversial is the question of whether the degradation of activators aids or restricts their function.

Sumoylation of transcription factor Gcn4 facilitates its Srb10-mediated clearance from promoters in yeast

The small ubiquitin-related modifier (SUMO) is a conserved factor that post-translationally regulates proteins involved in many cellular processes, including gene transcription. We previously demonstrated that promoter-bound factors become sumoylated during activation of inducible genes in yeast, but the identity of these factors, and the role of sumoylation in their function, was unknown. Here we show that the transcriptional activator Gcn4 is sumoylated on two specific lysine residues and in a manner that depends on its ability to bind DNA, indicating that sumoylation occurs after Gcn4 binding to target promoters. Importantly, this functions to facilitate the subsequent removal of the activator from these promoters after recruitment of RNA polymerase II, which can prevent inappropriate transcription of target genes. Furthermore, we show that clearance of sumoylated Gcn4 requires the protein kinase and Mediator complex subunit Srb10, linking activator removal with target gene transcription. Our study demonstrates an unexpected role for protein sumoylation in the process of transcriptional activation.

Mediator acts upstream of the transcriptional activator Gal4

We show that Mediator, a protein originally isolated on the basis of its ability to respond to transcriptional activators, and thought to be regulated by an activator, can also be the master that controls the activator.

The proteasome inhibitor MG132 had been shown to prevent galactose induction of the S. cerevisiae GAL1 gene, demonstrating that ubiquitin proteasome-dependent degradation of transcription factors plays an important role in the regulation of gene expression. The deletion of the gene encoding the F-box protein Mdm30 had been reported to stabilize the transcriptional activator Gal4 under inducing conditions and to lead to defects in galactose utilization, suggesting that recycling of Gal4 is required for its function. Subsequently, however, it was argued that Gal4 remains stably bound to the enhancer under inducing conditions, suggesting that proteolytic turnover of Gal4 might not be required for its function. We have performed an alanine-scanning mutagenesis of ubiquitin and isolated a galactose utilization-defective ubiquitin mutant. We have used it for an unbiased suppressor screen and identified the inhibitor Gal80 as a suppressor of the transcriptional defects of the ubiquitin mutant, indicating that the protein degradation of the inhibitor Gal80, and not of the activator Gal4, is required for galactose induction of the GAL genes. We also show that in the absence of Gal80, Mdm30 is not required for Gal4 function, strongly supporting this hypothesis. Furthermore, we have found that Mediator controls the galactose-induced protein degradation of Gal80, which places Mediator genetically upstream of the activator Gal4. Mediator had originally been isolated by its ability to respond to transcriptional activators, and here we have discovered a leading role for Mediator in the process of transcription. The protein kinase Snf1 senses the inducing conditions and transduces the signal to Mediator, which initiates the degradation of the inhibitor Gal80 with the help of the E3 ubiquitin ligase SCF^Mdm30. The ability of Mediator to control the protein degradation of transcriptional inhibitors indicates that Mediator is actually able to direct its own recruitment to gene promoters.

The expression levels of proteins are tightly regulated, not only via their production but also via their degradation. Genes are transcribed only if their encoded proteins are required by the environmental or developmental conditions of a cell, and once a certain protein is no longer needed, it is rapidly degraded by the ubiquitin proteasome system (UPS). Transcriptional activators appeared to contradict this simple economic principle, as it had been claimed that they had to be degraded in order to function. The claim was based upon a correlation: if the degradation of an activator was prevented by drugs or mutations in the UPS, the activator became stable but also nonfunctional. We have now shown that it is not the activator itself but its inhibitor that is the functionally relevant target of the UPS. Furthermore, we have found that the degradation of the inhibitor is controlled by a protein complex called Mediator. The activator is known to recruit Mediator to gene promoters, where Mediator assists RNA polymerase in initiating transcription. Mediator was always considered to be completely under the control of the activator; however, we observe that by regulating the degradation of the inhibitor, Mediator is also able to control the activator and thereby to orchestrate its own recruitment to gene promoters.

Regulation of gene expression by the ubiquitin-proteasome system

Transcription is the foremost regulatory point during the process of producing a functional protein. Not only specific genes need to be turned on and off according to growth and environmental conditions, the amounts and quality of transcripts produced are fine-tuned to offer optimal responses. As a result, numerous regulatory mechanisms converge to provide temporal and spatial specificity for this process. In the past decade, the ubiquitin-proteasome system (UPS), which is best known as a pathway for intracellular proteolysis, has emerged as another pivotal player in the control of gene expression. There is increasing evidence that the UPS has both proteolytic and non-proteolytic functions in multiple aspects of the transcription process, including initiation, elongation, mRNA processing as well as chromatin dynamics. In this review, we introduce the many interfaces between the UPS and transcription with focuses on the mechanistic understanding of UPS function in each process.

Three decades of studies to understand the functions of the ubiquitin family

Many intracellular proteins are metabolically unstable or can become unstable during their lifetime in a cell. The in vivo half-lives of specific proteins range from less than a minute to many days. Among the functions of intracellular proteolysis are the elimination of misfolded or otherwise abnormal proteins; maintenance of amino acid pools in cells affected by stresses such as starvation; and generation of protein fragments that act as hormones, antigens, or other effectors. One major function of proteolytic pathways is the selective destruction of proteins whose concentrations must vary with time and alterations in the state of a cell. Short in vivo half-lives of such proteins provide a way to generate their spatial gradients and to rapidly adjust their concentration or subunit composition through changes in the rate of their degradation. The regulated (and processive) degradation of intracellular proteins is carried out largely by the ubiquitin-proteasome system (Ub system), in conjunction with autophagy-lysosome pathways. Other contributors to intracellular proteolysis include cytosolic and nuclear proteases, such as caspases, calpains, and separases. They often function as "upstream" components of the Ub system, which destroys protein fragments that had been produced by these (nonprocessive) proteases. Ub, a 76-residue protein, mediates selective proteolysis through its enzymatic conjugation to proteins that contain primary degradation signals (degrons (1)), thereby marking such proteins for degradation by the 26S proteasome, an ATP-dependent multisubunit protease. Ub conjugation involves the formation of a poly-Ub chain that is linked (in most cases) to the ε-amino group of an internal Lys residue in a substrate protein. Ub is a "secondary" degron, in that Ub is conjugated to proteins that contain primary degradation signals.

Galactose induction of the GAL1 gene requires conditional degradation of the Mig2 repressor

Skp1 an essential component of the SCF (Skp1/cullin/F-box) E3 ubiquitin ligases, which target proteins for degradation by the 26S proteasome. We generated a skp1dM mutant strain that is defective for galactose induction of the GAL1 gene and we have found that galactose-induced protein degradation of the repressor Mig2 is defective in this strain. Mig2 degradation was also abolished in cells lacking the protein kinase Snf1 and the F-box protein Das1, suggesting that Snf1 triggers galactose-induced protein degradation of Mig2 by SCFDas1. Chromatin immunoprecipitation showed that Mig2 associates with the GAL1 promoter upon the galactose-induced exit of Mig1 in skp1dM cells, but not in wild-type cells, suggesting that the conditional degradation of Mig2 is required to prevent it from binding to the GAL1 promoter under inducing conditions. A galactose-stable deletion derivative of Mig2 caused a strong Mig (multi-copy inhibition of GAL gene expression) phenotype, confirming that galactose induction of the GAL1 gene requires the degradation of the repressor Mig2. Our results shed new light on the conflicting reports about the functional role of the degradation of transcriptional activators and indicate that gene expression studies interfering with proteasome degradation should take the stabilization of potential repressors into account.

Proteasomal degradation of myocardin is required for its transcriptional activity in vascular smooth muscle cells

Myocardin is a transcriptional co-activator of serum response factor (SRF) and can be degraded through ubiquitin-proteasome system. Our preliminary studies unexpectedly revealed that accumulation of myocardin in response to proteasome inhibition by MG132 or lactacystin resulted in decrease of transcriptional activity of myocardin as indicated by reduced expression of SMC contractile marker genes (SM α-actin, SM22, and calponin) and muscle-enriched microRNAs (miR-143/145 and miR-1/133a), and reduced contractility of human vascular smooth muscle cells (SMCs) embedded in collagen gel lattices, suggesting that myocardin degradation is required for its transcriptional activity. Further studies using chromatin immunoprecipitation assay revealed that proteasome inhibition, although increased the occupancy of myocardin and SRF on the promoter of SM α-actin gene, abolished myocardin-dependent recruitment of RNA polymerase II. We further examined the degradation of myocardin in epithelioid and spindle-shaped SMCs and revealed that myocardin in more differentiated spindle-shaped SMCs was more quickly degraded and had shorter half-life than in epithelioid SMCs. In neointimal lesions, we found that stabilization of myocardin protein was companied by downregulation of transcripts of ubiquitin and proteasome subunits, further illustrating the mechanism underlying reduction of myocardin transcriptional activity. In summary, our results have suggested that proteasomal degradation of myocardin is required for its transcriptional activity.

Combined chemical and genetic approach to inhibit proteolysis by the proteasome

Regulated protein destruction by the proteasome is crucial for the maintenance of normal cellular homeostasis. Much of our understanding of proteasome function stems from the use of drugs that inhibit its activity. Curiously, despite the importance of proteasomal proteolysis, previous studies have found that proliferation of the yeast Saccharomyces cerevisiae is relatively resistant to the effects of proteasome inhibitors such as MG132, even in the presence of mutations that increase inhibitor levels in cells. We reasoned that part of the resistance of S. cerevisiae to proteasome inhibitors stems from the fact that most proteasome inhibitors preferentially target the chymotryptic activity of the proteasome, and that the caspase-like and tryptic sites within the 20S core could compensate for proteasome function under these conditions. To test this hypothesis, we generated a strain of yeast in which the gene encoding the drug efflux pump Pdr5 is deleted, and the tryptic and caspase-like proteasome activities are inactivated by mutation. We find that this strain has dramatically increased sensitivity to the proteasome inhibitor MG132. Under these conditions, treatment of yeast with MG132 blocks progression through the cell cycle, increases the accumulation of polyubiquitylated proteins and decreases the ability to induce transcription of certain genes. These results highlight the contribution of the caspase-like and tryptic activities of the proteasome to its function, and provide a strategy to potently block proteasomal proteolysis in yeast that has practical applications.

The proteasome and its regulatory roles in gene expression

Cumulative evidence indicates that the proteasome, which is mainly known as a protein-degrading machine, is very essential for gene expression. Destructive functions of the proteasome, i.e., ubiquitin-dependent proteolytic activity, are significant for activator localization, activator destruction, co-activator/repressor destruction and PIC disassembly. Non-proteolytic functions of the proteasome are important for recruitment of activators and co-activators to promoters, ubiquitin-dependent histone modification, transcription elongation and possibly maturation of mRNA via the facilitation of mRNA export from the nucleus to the cytoplasm. In this review, we discuss how the proteasome regulates transcription at numerous stages during gene expression. This article is part of a Special Issue entitled The 26S Proteasome: When degradation is just not enough!

No Splicing, no dicing: non-proteolytic roles of the ubiquitin-proteasome system in transcription

The ubiquitin-proteasome pathway (UPP) is responsible for most programmed turnover of proteins in eukaryotic cells, and this activity has been known for some time to be involved in transcriptional regulation. More recently, intersections of the UPP and transcription have been discovered that are not proteolytic in nature and appear to revolve around the chaperonin-like activities of the ATPases in the 19 S regulatory subunit of the proteasome. Moreover, monoubiquitylation, which does not signal degradation, has been found to be a key modification of many transcription factors and histones. These various non-proteolytic roles of the UPP in transcription are reviewed here, and plausible mechanistic models are discussed.