2. Modulation of Chromatin Structure Chromatin structure is dynamic rather static Practically we can consider modulation of chromatin structure at two levels: effects on nucleosome and effects on higher order structure
Transcription factor binding to DNA is inhibited within nucleosomes • • • • •
Chromatin assembly inhibits transcription by all three RNA polymerases in vitro. In vivo genetic evidence links histones to repression (and activation as well). Affinity of transcription factor for its binding site on DNA is decreased when the DNA is reconstituted into nucleosomes. The binding of TBP to the TATA box is very sensitive to chromatin assembly in vitro. Extent of inhibition is dependent on: – Location of the binding site within the nucleosome. • binding sites at the edge are more accessible than the center – The type of DNA binding domain. • Zn fingers bind more easily than bHLH domains.
Stimulate binding of transcription factors to nucleosomes • Cooperative binding of multiple factors. • The presence of histone chaperone proteins which can compete H2A/H2B dimers from the octamer. • Acetylation of the Nterminal tails of the core histones • Nucleosome disruption by ATPdependent remodeling complexes.
Mechanisms for chromatin remodeling –Modulation by incorporation of histone variants –Modulation by ATPdriven chromatin remodeling complexes –Modulation by enzymes that posttranslationally modify histones Acetylation Methylation Ubiqitination Phosphorylation ADPribosylation
There are a large number of Histone H2A variants. The function of these variants are just begun to be revealed
Modulation by Histone Variants • In addition to the major histones H2A, H2B, H3 and H4, many organisms also have distinct batteries of histone variants
– H2AZ and H2AX H2AZ has been shown to associate with actively transcribed chromatin regions H2AX has been shown to be crucial for chromatin decompaction during DNA repair. Phosphorylation on its SQE/DØ sequence is one of the earliest events in response to doublestrand DNA breaks – H3.3 and CenH3s (CenpA in human) H3.3 is a replacement H3 variant CenH3s are centromerespecific H3 variants
• Histones H2B and H4 have very few variants
Histone H3 variants
CenpA is required for the specific structure and function in centromere
Conclusions The chromatin structure and function can be different dependent on the presence or absence of histone variants because their differences in amino acid sequences and conformations. We are still at the very early phase in term of understanding the roles of histone variants.
Remodeling by ATPdriven chromatin remodeling complexes
• Yeast SWI/SNF
– 10 proteins – Needed for expression of genes involved in matingtype switching and sucrose metabolism (sucrose non fermenting). – Some suppressors of swi or snf mutants are mutations in genes encoding histones. – SWI/SNF complex interacts with chromatin to activate a subset of yeast genes. – Is an ATPase
• Mammalian homologs: hSWI/SNF
– ATPase is BRG1, related to Drosophila Brahma • Other remodeling ATPase have been discovered.
Partial list of chromatin remodeling complexes
Complex SWI/SNF RCS NRUF CHRAC ACF BRM BRG1 hBRM associated complexes
Organism Yeast Yeast Drosophila Drosophila Drosophila Drosophila Mammals Mammals
Factor swi/snf sth1 ISWI ISWI ISWI BRM BRG BRM
Chromatin Remodeling Factors ATPase
Species Yeast
subunit SWI/SNF complex SWI/SNF2 RSC complex STH1
Drosophila
NURF CHRAC ACF BRM complex
ISWI ISWI ISWI BRM
Human
BRG1 complex hbrm complex
BRG1 hbrm
•Remodeling activity is dependent on or associated with ATP hydrolysis. •NTP binding subunit possesses DNAstimulated ATPase activity. •Postulated that the NTP binding subunit acts as a processive, ATPdriven DNA translocating motor that disrupts histoneDNA interactions.
How do chromatin remodeling complexes work? • Structural alteration • Nucleosome sliding • Nucleosome eviction
The consequence of chromatin remodeling is dependent on the type of chromatin remodeling complexes involved
Remodeling by SWI/SNF SWI/SNF + ATP Nucleosome
Based on in vitro studies using reconstituted nucleosome and purified SWI/SNF complexes
1. 2. 3.
Structural alteration Generation of stable dimers Octamer transfer
Chromatin remodeling ATPases catalyze stable alteration of the nucleosome
II: form a stably remodeled dimer, altered DNAse digestion pattern III: transfer a histone octamer to a different DNA fragment
Involvement of SWI/SNF in transcription •
The SWI/SNF complex is required for transcriptional activation of 5% yeast genes.
•
Can be recruited directly through interaction with DNA binding transcription factors.
•
Can be recruited indirectly by interaction with other transcriptional coactivators or along with the RNA polymerase holoenzyme.
•
Other unidentified chromatin-remodeling activities might be recruited by pathways as yet undefined.
•
SWI/SNF complex exerts its major effect in transcriptional activation at a step subsequent to transcriptional activator-promoter recognition.
•
In the yeast, the SWI/SNF complex has been proposed to antagonize the repressive effects of chromatin by disrupting nucleosomes.
•
Dependent on the chromatin organization as well as the transcription factors involved, the SWI/SNF also contributes to transcriptional repression.
Nucleosome sliding induced by ISWI containing complexes NURF +ATP
NURF +ATP
TF
Stable, low energy state
NURF +ATP
Evenly spaced nucleosomes
Allow TF to bind
• Make nucleosome mobile in the presence of ATP • Also involve in nucleosome/chromatin assembly • Have roles in both transcriptional activation and repression
Recent evidence: nucleosome eviction as a result of chromatin remodeling by SWI/SNF Yeast Pho5 gene Induction with low phosphate
Swi4/ Pol II swi6
• Promoter region is DNase I hypersensitive upon induction • Promoter region contains less histones after induction
Remodeling by covalent modification of histones in chromatin
Multiple modifications and enormous potential combinations
Two types of Histone Acetyltransferases (HATs). • Type A nuclear HATs: acetylate histones in chromatin. • Type B cytoplasmic HATs: acetylate free histones prior to their assembly into chromatin. – Acetylate K5 and K12 in histone H4
Acetylation by nuclear HATs is associated with transcriptional activation • Highly acetylated histones are associated with actively transcribed chromatin
– Increasing histone acetylation can turn on some genes. – Immunoprecipitation of DNA crosslinked to chromatin with antibodies against Achistones enriches for actively transcribed genes.
• Acetylation of histone Nterminal tails affects the ability of nucleosomes to associate in higherorder structures – The acetylated chromatin is more “open”
• DNase sensitive • accessible to transcription factors and polymerases
• HATs are implicated as coactivators of genes in chromatin, and HDACs (histone deacetylases) are implicated as corepressors
Nuclear HAT As are coactivators • Gcn5p is a transcriptional activator of many genes in yeast. It is also a HAT. • PCAF (P300/CBP associated factor) is a HAT and is homologous to yeast Gcn5p. • P300 and CBP are similar proteins that interact with many transcription factors (e.g. CREB, AP1 and MyoD). • P300/CBP are needed for activation by these factors, and thus are considered coactivators. • P300/CBP has intrinsic HAT activity as well as binding to the HAT PCAF.
Table 1. HAT families and their transcription-related functions (adapted from Marmorstein and Roth, 2001)
HAT family GNAT
Members Gcn5, PCAF, Ada, SAGA Hat1 Elp3, Hpa2
Function Coactivator Replication dependent chromatin assembly
MYST
Sas2, Ybf2/Sas3, Esa1 MOF, Tip60 MOZ HBO1
Silencing, Cell cycle progression Dosage compensation Leukemogenesis Origin recognition interaction
TAFII250
TAFII250
TBP-associated factor
CBP/p300
CBP, p300
Global coactivator
SRC
SRC-1, ACTR, SRC-3 TIF-2, GRIP1
Steroid receptor coactivators
ATF-2
ATF-2
Sequence specific DNA binding activator
TFIIIC
TFIIIC
RNA pol III initiation
HAT complexes often contain several transcription regulatory proteins. • Example of the SAGA complex components: • Gcn5: catalytic subunit, histone acetyl transferase • Ada proteins – transcription adaptor proteins required for function of some activators in yeast. • Spt proteins (TBPgroup) – regulate function of the TATAbinding protein. • TAF proteins – associate with TBP and also regulate its function. • Tra1
– homologue of a human protein involved in cellular transformation. – May be direct target of activator proteins.
Roles of histone acetylation • Increase access of transcription factors to DNA in nucleosomes. • Decondense 30nm chromatin fibers • Serve as markers for binding of nonhistone proteins (e.g. bromodomain proteins).
Effect of Histone Acetylation on Chromatin Structure and Transcription
Histone deacetylation is catalyzed by histone deacetylases and associated with transcriptional repression Histone deacetylases (HDACs): 2. Three classes, about 20 identified members. 3. can be recruited by transcriptional repressors to specific target genes and/or deacetylate histones in chromatin in a nontargeting, global fashion. 4. Acetylation and deacetylation are very dynamic events 5. Aberrant histone deacetylation has been linked to cancer
Effect of Histone Acetylation on Chromatin Structure and Transcription Repression
Activation
Information about three classes of HDACs • Class I HDACs are relatively small in size, abundant, ubiqitously expressed, mainly nuclear, sensitive to TSA and tend to associate corepressor proteins to form large corepressor complexes. • Class II HDACs are relatively larger in size, less abundant, shuffling between cytoplasm and nuclei, likely tissuespecific and sensitive to TSA. • Class III HDACs are less well known and involved in silencing of rRNA genes and telomere silencing and not sensitive to TSA.
HDAC inhibitors can induce cell differentiation possibly through induction of p21 and cyclin D1and has been tested as cancer treatment drugs in clinical trials
Class I HDACs are found in large protein complexes HDAC3 SMRT/ NCoR
HDAC1/2 RbAp46
TBL1 TBLR1
Sin3A
GPS2
RbAp48 SAP18
SMRT/NCoR complexes Sin3A complex
Comparison of Sin3, Mi2/NURD and SMRT/NCoR class I HDAC complexes Sin3
Mi2/NURD
SMRT/NCoR
HDAC1/HDAC2 HDAC1/HDAC2 HDAC3
Histone deacetylases WD40 repeat histone binding proteins
RbAp46
RbAp46
TBL1
RbAp48
RbAp48
TBLR1
SAP18
MTA2
GPS2
SAP30
MBD3
IR10
Sin3
CHD3/CHD4
SMRT/NCoR
Scaffold proteins
MeCP2
MBD2
Kaiso
Methyl CpG binding protein
Repression by deacetylation of histones by SIR2
Methylated DNA can recruit HDACs
Histone Methylation
Histone Methylation • Two types of HMTs: arginine specificHMTs and lysine specific HMTs. • Histone methylation (mono, di and trimethylation) is known for a long time, but the HMTs responsible for histone methylation have only recently been identified. • In contrast to histone acetylation, histone methylation is stable. Turnover rate of histone methylation is similar to that of histone turnover. • The first identified Argspecific HMT is Carm1/PRMT1. It can methylates Arg2, Arg17 and Arg26 in H3. It functions as a transcriptional coactivator for nulcear hormone receptor. • The first identified Lysspecific HMT is SUV391, based on its similarity to plant protein metyltransferase. The enzymatic activity resides in the highly conserved SET (Suppressor of variegation, Enhancer of Zeste and Trithorax) domain.
Nature (Article) 406, 593599. Aug. 10, 2000
Regulation of chromatin structure by sitespecific histone H3 methyltransferases Stephen Rea, Grank Eisenhaber, Donal O’Carroll, Brian D. Strahl, ZuWen Sun, Manfred Schmid, Susanne Opravil, Karl Mechtler, Chris P. Ponting, C. David Allis & Thomas Jenuwein
1. Human and murine SUV39H1 contains a SET domain resembling plant methyltransferase proteins 2. Human and murine SUV39H1 is a H3 K9 Specific HMTase 3. Mapped the catalytic motif (SET domain plus the adjacent cysteinerich regions) 4. K9 methylation interferes with S10 phosphorylation 5. Suv39h1 null cells have heterochromatin defects
Approaches for identification and characterization of HMTs 1. Biochemical purification of HMT activities using in vitro HMT assay. 2. Sequence similarity: testing the proteins containing a SET domain.
Summary of Known HMTases and Their Target Sites HMTase
Histones
Sites
Roles in transcription
CARM1
H3
R2, R17, R26
PRMT1
H4
R3
activation
Suv39H1/Clr4/ESET/
H3
K9
silencing
ySET1/mSET9/mSET7
H3
K4
Silencing and elongation
ySET2
H3
K36
elongation
G9a
H3
K9, K27
silencing
EZH1/EZH2
H3
K27
silencing
DOT1
H3
K79
Not clear
SET8
H4
K20
Silencing, cell cycle
Activation and elongation
Underlining mechanisms • H4R3 methylation facilitates acetylation of H4 by p300 (why it is associated with activation. • H3K9 methylation creates a binding site for HP1 and HP1 is known to associate with HDACs and involved in heterochromatin formation (why it is involved in repression) • The underlining mechanism for many other modifications is not clear
Histone phosphorylation • Phosphorylation on Ser10 of H3 is involved in both transcriptional activation and in chromosome condensation during mitosis. • Phosphorylation on Ser10 of H3 facilitates acetylation of histone H3 on Lys9 and Lys14. • Phosphorylation of histone H2X variant is involved in DNA repair • Many Ser and Thr sites in histone tails can be phosphorylated. In most cases the functional consequence is not clear.
Histone Ubiqitination • H2A (Lys119) and H2B (Lys123) can be monoubiqitinated. • Monoubiqitination is not associated with protein degradation by proteasome pathway. • H2B ubiqitination in yeast is catalyzed by Rad6 and is required for methylation on Lys 4 and Lys79. The underlining mechanism is unknown.
Interplay Between Different Histone Modifications
Human H3
Human H4
Me Me
Me Ac p
Ac
Me Ac
Ac
Me Me p
9
14
18
23
27
NARTKQTARKSTGGKAPRKQLATKAARKSAP... 4
Me
Me Ac
Ac
Ac
Ac
Me
3 5
8
12
16
20
AcNSGRGKGGKGLGKGGAKRHRKVLRDNIQGIT...
Multiple modifications and enormous potential combinations: histone code theory
‘Histone Code’ and How the Code be Read? Histone code hypothesis: that multiple histone modifications, acting in combinatorial or sequential fashion on one or multiple histone tails, specify unique downstream functions. How the histone code be read? Likely read by specific protein domains Code
Protein Motif
AcLys
Bromo
MeK9
HP1 Chromo
MeK27
Polycomb Chromo
PhosS10
Different combinations
?
?
Optimal transcriptional activation requires multiple chromatin remodeling factors
Functional interplay among different chromatin remodeling factors? The functions of SWI/SNF and the SAGA complex are genetically linked • Some genes require both complexes for activation. • Other genes require one or the other complex. • Many genes require neither presumably utilize different ATPdependent complexes and/or HATs
The Order of Recruitment? The yeast HO endonuclease gene requires both SWI/SNF and SAGA
• The order of recruitment at the HO promoter: – 1. SWI5 activator: sequence recognition – 2. SWI/SNF complex: remodel nucleosomes – 3. SAGA: acetylate histones – 4. SBF activator (still at specific sequences) – 5. general transcription factors
• Cosma, Tanaka and Nasmyth (1999) Cell 97:299 311.
• The order is likely to differ at different genes
A scenario for transitions from silenced to open to actively transcribed chromatin
Movement from hetero to euchromatin
Nucleosome remodelers and HATs further open chromatin
Assembly of preinitiation complex on open chromatin