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1.  Human Mediator Subunit Med26 Functions As A Docking Site For Transcription Elongation Factors 
Cell  2011;146(1):92-104.
Promoter proximal pausing by initiated RNA polymerase II (Pol II) and regulated release of paused polymerase into productive elongation has emerged as a major mechanism of transcription activation. Reactivation of paused Pol II correlates with recruitment of SuperElongationComplexes (SECs) containing ELL/EAF family members, P-TEFb, and other proteins, but the mechanism of their recruitment is currently a major unanswered question. Here, we present evidence for a role of human Mediator subunit Med26 in this process. We identify in the conserved N-terminal domain of Med26 overlapping docking sites for SEC and a second ELL/EAF-containing complex, as well as general initiation factor TFIID. In addition, we present evidence consistent with the model that Med26 can function as a molecular switch that interacts first with TFIID in the Pol II initiation complex and then exchanges TFIID for complexes containing ELL/EAF and P-TEFb to facilitate transition of Pol II into the elongation stage of transcription.
PMCID: PMC3145325  PMID: 21729782
2.  Distinct Modes of Regulation of the Uch37 Deubiquitinating Enzyme in the Proteasome and in the Ino80 Chromatin Remodeling Complex 
Molecular cell  2008;31(6):909-917.
Deubiquitinating enzymes (DUBs) are proteases that can antagonize ubiquitin-mediated signalling by disassembling ubiquitin-protein conjugates. How DUBs are regulated in vivo and how their substrate specificities are achieved are largely unknown. The conserved DUB Uch37 is found on proteasomes in organisms ranging from fission yeast to humans. Deubiquitination by Uch37 is activated by proteasomal binding, which enables Uch37 to process polyubiquitin chains. Here we show that in the nucleus Uch37 is also associated with the human Ino80 chromatin-remodeling complex (hINO80). In hINO80, Uch37 is held in an inactive state; however, it can be activated by transient interaction of the Ino80 complex with the proteasome. Thus, DUB activities can be modulated both positively and negatively via dynamic interactions with partner proteins. In addition, our findings suggest that the proteasome and the hINO80 chromatin-remodeling complex may cooperate to regulate transcription or DNA repair, processes in which both complexes have been implicated.
PMCID: PMC2577292  PMID: 18922472
3.  Immunoaffinity Purification of Protein Complexes from Mammalian Cells 
In this chapter, we describe a purification scheme designed to isolate multisubunit protein complexes gently and quickly from crude extracts of mammalian cells using immunoaffinity purification of epitope tagged proteins and the multisubunit complexes with which they associate. As an example we describe isolation of the mammalian Mediator complex from HeLa S3 cells.
PMCID: PMC3693849  PMID: 23436370
Multisubunit protein complex; Immunoaffinity purification; Epitope tag; Stable cell line
4.  Crosstalk between NSL Histone Acetyltransferase and MLL/SET Complexes: NSL Complex Functions in Promoting Histone H3K4 Di-Methylation Activity by MLL/SET Complexes 
PLoS Genetics  2013;9(11):e1003940.
hMOF (MYST1), a histone acetyltransferase (HAT), forms at least two distinct multiprotein complexes in human cells. The male specific lethal (MSL) HAT complex plays a key role in dosage compensation in Drosophila and is responsible for histone H4K16ac in vivo. We and others previously described a second hMOF-containing HAT complex, the non-specific lethal (NSL) HAT complex. The NSL complex has a broader substrate specificity, can acetylate H4 on K16, K5, and K8. The WD (tryptophan-aspartate) repeat domain 5 (WDR5) and host cell factor 1 (HCF1) are shared among members of the MLL/SET (mixed-lineage leukemia/set-domain containing) family of histone H3K4 methyltransferase complexes. The presence of these shared subunits raises the possibility that there are functional links between these complexes and the histone modifications they catalyze; however, the degree to which NSL and MLL/SET influence one another's activities remains unclear. Here, we present evidence from biochemical assays and knockdown/overexpression approaches arguing that the NSL HAT promotes histone H3K4me2 by MLL/SET complexes by an acetylation-dependent mechanism. In genomic experiments, we identified a set of genes including ANKRD2, that are affected by knockdown of both NSL and MLL/SET subunits, suggested they are co-regulated by NSL and MLL/SET complexes. In ChIP assays, we observe that depletion of the NSL subunits hMOF or NSL1 resulted in a significant reduction of both H4K16ac and H3K4me2 in the vicinity of the ANKRD2 transcriptional start site proximal region. However, depletion of RbBP5 (a core component of MLL/SET complexes) only reduced H3K4me2 marks, but not H4K16ac in the same region of ANKRD2, consistent with the idea that NSL acts upstream of MLL/SET to regulate H3K4me2 at certain promoters, suggesting coordination between NSL and MLL/SET complexes is involved in transcriptional regulation of certain genes. Taken together, our results suggest a crosstalk between the NSL and MLL/SET complexes in cells.
Author Summary
Covalent modification of N-terminal tails of histone proteins is accomplished by a variety of chromatin modifying complexes. These complexes catalyze at least eight distinct types of histone modifications including acetylation, methylation, phosphorylation, and ubiquitination. Histone modifications may act alone or in a coordinated manner to activate or repress chromosomal processes. For example, a particular histone modification may recruit or activate chromatin modifying complexes that generate a different histone modification. Coordination between hMOF-mediated histone H4K16 acetylation and other histone modifications has been reported by several research groups. The presence of subunits shared between the hMOF-containing NSL and MLL/SET family complexes suggests there may be functional links between two complexes. Consistent with this idea, we identified a set of genes that are co-regulated by the NSL and MLL/SET complexes. Both in vitro and in vivo experimental approaches provide evidence that the NSL HAT functions in promoting histone H3K4 di-methylation activity by MLL/SET complexes. Interestingly crosstalk between hMOF/NSL HAT and MLL/SET HMT activity seems to be unidirectional since there we detected no effect of MLL/SET activity on NSL HAT, either in vitro or in cells.
PMCID: PMC3828133  PMID: 24244196
5.  A conserved Mediator–CDK8 kinase module association regulates Mediator–RNA polymerase II interaction 
The CDK8 kinase module (CKM) is a conserved, dissociable Mediator subcomplex whose component subunits were genetically linked to the RNA polymerase II (RNAPII) carboxy-terminal domain (CTD) and individually recognized as transcriptional repressors before Mediator was identified as a preeminent complex in eukaryotic transcription regulation. We used macromolecular electron microscopy and biochemistry to investigate the subunit organization, structure, and Mediator interaction of the Saccharomyces cerevisiae CKM. We found that interaction of the CKM with Mediator’s Middle module interferes with CTD-dependent RNAPII binding to a previously unknown Middle module CTD-binding site targeted early on in a multi-step holoenzyme formation process. Taken together, our results reveal the basis for CKM repression, clarify the origin of the connection between CKM subunits and the CTD, and suggest that a combination of competitive interactions and conformational changes that facilitate holoenzyme formation underlie the Mediator mechanism.
PMCID: PMC3648612  PMID: 23563140
6.  The Human EKC/KEOPS Complex Is Recruited to Cullin2 Ubiquitin Ligases by the Human Tumour Antigen PRAME 
PLoS ONE  2012;7(8):e42822.
The human tumour antigen PRAME (preferentially expressed antigen in melanoma) is frequently overexpressed during oncogenesis, and high PRAME levels are associated with poor clinical outcome in a variety of cancers. However, the molecular pathways in which PRAME is implicated are not well understood. We recently characterized PRAME as a BC-box subunit of a Cullin2-based E3 ubiquitin ligase. In this study, we mined the PRAME interactome to a deeper level and identified specific interactions with OSGEP and LAGE3, which are human orthologues of the ancient EKC/KEOPS complex. By characterizing biochemically the human EKC complex and its interactions with PRAME, we show that PRAME recruits a Cul2 ubiquitin ligase to EKC. Moreover, EKC subunits associate with PRAME target sites on chromatin. Our data reveal a novel link between the oncoprotein PRAME and the conserved EKC complex and support a role for both complexes in the same pathways.
PMCID: PMC3418287  PMID: 22912744
7.  Purification and Assay of the Human INO80 and SRCAP Chromatin Remodeling Complexes 
Methods (San Diego, Calif.)  2006;40(4):312-317.
Chromatin modifying and remodeling enzymes play critical roles in many aspects of chromosome biology including transcription, replication, recombination, and repair. Our laboratory recently identified and characterized two multisubunit human chromatin remodeling enzymes designated the INO80 and SRCAP complexes. Mechanistic studies revealed that the human INO80 complex catalyzes nucleosome sliding and the SRCAP complex catalyzes ATP-dependent exchange of histone H2A/H2B dimers containing the histone variant H2A.Z into nucleosomes. Here we describe methods for purification and assay of the INO80 and SRCAP chromatin remodeling complexes.
PMCID: PMC3092633  PMID: 17101442
Nucleosome; chromatin; histone; histone exchange; chromatin remodeler
8.  AFF4, a component of the ELL/p-TEFb elongation complex and a shared subunit of MLL chimeras can link transcription elongation to leukemia 
Molecular cell  2010;37(3):429-437.
Chromosomal translocations involving the MLL gene are associated with infant acute lymphoblastic and mixed lineage leukemia. There are a large number of translocation partners of MLL that share very little sequence or seemingly functional similarities, however, their translocations into MLL result in the pathogenesis of leukemia. To define the molecular reason why these translocations result in the pathogenesis of leukemia, we purified several of the commonly occurring MLL chimeras. We have identified a novel super elongation complex (SEC) associated with all chimeras purified. SEC includes ELL, P-TEFb, AFF4 and several other factors. AFF4 is required for SEC stability and proper transcription by poised RNA polymerase II in metazoans. Knockdown of AFF4 within SEC in leukemic cells shows reduction in MLL chimera target gene expression suggesting that AFF4/SEC could be a key regulator in the pathogenesis of leukemia through many of the MLL partners.
PMCID: PMC2872029  PMID: 20159561
9.  When transcription meets recombination: a lesson from the human RECQ protein complexes 
Since the cloning of the first human RECQ gene, RECQ1, more than 15 years ago, RECQ helicases have been a major focus in cancer research. Recent studies of human RECQ protein complexes are providing insight into their roles in various DNA metabolic pathways that protect the integrity of our genome.
PMCID: PMC2948292  PMID: 20948612
10.  Direct Inhibition of RNA Polymerase II Transcription by RECQL5* 
The Journal of Biological Chemistry  2009;284(35):23197-23203.
DNA helicases of the RECQ family are important for maintaining genome integrity, from bacteria to humans. Although progress has been made in understanding the biochemical role of some human RECQ helicases, that of RECQL5 remains elusive. We recently reported that RECQL5 interacts with RNA polymerase II (RNAPII), pointing to a role for the protein in transcription. Here, we show that RECQL5 inhibits both initiation and elongation in transcription assays reconstituted with highly purified general transcription factors and RNAPII. Such inhibition is not observed with the related, much more active RECQL1 helicase or with a version of RECQL5 that has normal helicase activity but is impaired in its ability to interact with RNAPII. Indeed, RECQL5 helicase activity is not required for inhibition. We discuss our findings in light of the fact that RECQ5−/− mice have elevated levels of DNA recombination and a higher incidence of cancer.
PMCID: PMC2749093  PMID: 19570979
11.  Identification of a transcription factor IIIA-interacting protein 
Nucleic Acids Research  2000;28(9):1986-1993.
Transcription factor IIIA (TFIIIA) activates 5S ribosomal RNA gene transcription in eukaryotes. The protein from vertebrates has nine contiguous Cys2His2 zinc fingers which function in nucleic acid binding, and a C-terminal region involved in transcription activation. In order to identify protein partners for TFIIIA, yeast two-hybrid screens were performed using the C-terminal region of Xenopus TFIIIA as an attractor and a rat cDNA library as a source of potential partners. A cDNA clone was identified which produced a protein in yeast that interacted with Xenopus TFIIIA but not with yeast TFIIIA. This rat clone was sequenced and the primary structure of the human homolog (termed TFIIIA-intP for TFIIIA-interacting protein) was determined from expressed sequence tags. In vitro interaction of recombinant human TFIIIA-intP with recombinant Xenopus TFIIIA was demonstrated by immunoprecipitation of the complex using anti-TFIIIA-intP antibody. Interaction of rat TFIIIA with rat TFIIIA-intP was indicated by co-chromatography of the two proteins on DEAE-5PW following fractionation of a rat liver extract on cation, anion and gel filtration resins. In a HeLa cell nuclear extract, recombinant TFIIIA-intP was able to stimulate TFIIIA-dependent transcription of the Xenopus 5S ribosomal RNA gene but not TFIIIA-independent transcription of the human adenovirus VA RNA gene.
PMCID: PMC103300  PMID: 10756201

Results 1-11 (11)