specificity-analysis-protein-lysine-methyltransferases-using-spot

Specificity Analysis of Protein Lysine Methyltransferases Using SPOT Peptide Arrays

Lysine methylation is an emerging post-translation modification and it has been identified on several histone and non-histone proteins, where it plays ...

Stuttgart University

Chromatin structure is greatly influenced by histone tail post-translational modifications (PTM), which also play a central role in epigenetic processes. Antibodies against modified histone tails are central research reagents in chromatin biology and molecular epigenetics. We applied Celluspots peptide arrays for the specificity analysis of 36 commercial antibodies from different suppliers which are directed towards modified histone tails. The arrays contained 384 peptides from 8 different regions of the N-terminal tails of histones, viz. H3 1-19, 7-26, 16-35 and 26-45, H4 1-19 and 11-30, H2A 1-19 and H2B 1-19, featuring 59 post-translational modifications in many different combinations. Using various controls we document the reliability of the method. Our analysis revealed previously undocumented details in the specificity profile. Most of the antibodies bound well to the PTM they have been raised for, but some failed. In addition some antibodies showed high cross-reactivity and most antibodies were inhibited by specific additional PTMs close to the primary one. Furthermore, specificity profiles for antibodies directed towards the same modification sometimes were very different. The specificity of antibodies used in epigenetic research is an important issue. We provide a catalog of antibody specificity profiles for 36 widely used commercial histone tail PTM antibodies. Better knowledge about the specificity profiles of antibodies will enable researchers to implement necessary control experiments in biological studies and allow more reliable interpretation of biological experiments using these antibodies.

Detailed specificity analysis of antibodies binding to modified histone tails with peptide arrays.

We applied peptide array methylation to determine an optimized target sequence for the SET7/9 (KMT7) protein lysine methyltransferase. Based on this, we identified 91 new peptide substrates from human proteins, many of them better than known substrates. We confirmed methylation of corresponding protein domains in vitro and in vivo with a high success rate for strongly methylated peptides and showed methylation of nine nonhistone proteins (AKA6, CENPC1, MeCP2, MINT, PPARBP, ZDH8, Cullin1, IRF1, and [weakly] TTK) and of H2A and H2B, which more than doubles the number of known SET7/9 targets. SET7/9 is inhibited by phosphorylation of histone and nonhistone substrate proteins. One lysine in the MINT protein is dimethylated in vitro and in vivo demonstrating that the product pattern created by SET7/9 depends on the amino acid sequence context of the target site.

Specificity analysis-based identification of new methylation targets of the SET7/9 protein lysine methyltransferase.

Mutations in the ATRX protein are associated with the alpha-thalassemia and mental retardation X-linked syndrome (ATR-X). Almost half of the disease-causing mutations occur in its ATRX-Dnmt3-Dnmt3L (ADD) domain. By employing peptide arrays, chromatin pull-down and peptide binding assays, we show specific binding of the ADD domain to H3 histone tail peptides containing H3K9me3. Peptide binding was disrupted by the presence of the H3K4me3 and H3K4me2 modification marks indicating that the ATRX-ADD domain has a combined readout of these two important marks (absence of H3K4me2 and H3K4me3 and presence of H3K9me3). Disease-causing mutations reduced ATRX-ADD binding to H3 tail peptides. ATRX variants, which fail in the H3K9me3 interaction, show a loss of heterochromatic localization in cells, which indicates the chromatin targeting function of the ADD domain of ATRX. Disruption of H3K9me3 binding may be a general pathogenicity pathway of ATRX mutations in the ADD domain which may explain the clustering of disease mutations in this part of the ATRX protein.

The ATRX-ADD domain binds to H3 tail peptides and reads the combined methylation state of K4 and K9.

Epigenetic reading domains are involved in the regulation of gene expression and chromatin state by interacting with histones in a post-translational modification specific manner. A detailed knowledge of the target modifications of reading domains, including enhancing and inhibiting secondary modifications, will lead to a better understanding of the biological signaling processes mediated by reading domains.

Application of Celluspots peptide arrays for the analysis of the binding specificity of epigenetic reading domains to modified histone tails.

The demand for practical and convenient enzyme assays for histone lysine methyltransferases (HKMTs) emerges along with the rapid development of this young class of enzymes. A supramolecular reporter pair composed of p-sulfonatocalix[4]arene (CX4) and the fluorescent dye lucigenin (LCG) has been used to monitor enzymatic trimethylation of lysine residues in peptide substrates. The assay affords a switch-ON fluorescence response and operates in a continuous, real-time, and label-free fashion. The underlying working principle relies on the higher affinity of the macrocycle towards the trimethylated product of the enzymatic reaction as compared to the substrate, which allows the assay to be carried out in the product-selective mode. The final product incorporates a trimethylammonium moiety, a known high-affinity binding motif for CX4. Two substrates corresponding to the H3 N-terminal tail, namely, S2 (RTKQTARKSTGGKAP) and S6 (QTARKSTGGS), were selected as model compounds for methylation with the Neurospora crassa Dim-5 enzyme and investigated by the newly developed supramolecular tandem HKMTs assay. Only the longer substrate S2 underwent methylation in solution. The potential of the assay for inhibitor screening was demonstrated by means of inhibition studies with 1,10-phenanthroline to afford an inhibition constant of (70±20) μM.

A fluorescence-based supramolecular tandem assay for monitoring lysine methyltransferase activity in homogeneous solution.

The SET8 histone lysine methyltransferase, which monomethylates the histone 4 lysine 20 residue plays important roles in cell cycle control and genomic stability. By employing peptide arrays we have shown that it has a long recognition sequence motif covering seven amino acid residues, viz. R(17)-H(18)-(R(19)KY)-K(20)-(V(21)ILFY)-(L(22)FY)-R(23). Celluspots peptide array methylation studies confirmed specific monomethylation of H4K20 and revealed that the symmetric and asymmetric methylation on R(17) of the H4 tail inhibits methylation on H4K20. Similarly, dimethylation of the R located at the -3 position also reduced methylation of p53 K382 which had been shown previously to be methylated by SET8. Based on the derived specificity profile, we identified 4 potential non-histone substrate proteins. After relaxing the specificity profile, we identified several more candidate substrates and showed efficient methylation of 20 novel non-histone peptides by SET8. However, apart from H4 and p53 none of the identified novel peptide targets was methylated at the protein level. Since H4 and p53 both contain the target lysine in an unstructured part of the protein, we conclude that the long recognition sequence of SET8 makes it difficult to methylate a lysine in a folded region of a protein, because amino acid side chains essential for recognition will be buried.

The SET8 H4K20 protein lysine methyltransferase has a long recognition sequence covering seven amino acid residues.

New protein lysine methyltransferase (PKMT) assays are needed to facilitate screening for improved PKMT inhibitors, because PKMTs are mutated or overexpressed in several cancers. In cells, methylated lysine residues are recognized by reading domains such as the chromodomain of HP1β, which bind to target proteins in a lysine-methylation-specific manner. Herein we describe a sensitive, robust, and non-radioactive high-throughput PKMT assay that employs the HP1β chromodomain to detect the methylation of peptide substrates by the human SUV39H1 and SUV39H2 PKMTs. The assay has a very good dynamic range and high signal-to-noise ratio. It can be used to screen for PKMT inhibitors, as illustrated by analyzing the inhibition of SUV39H1 by chaetocin. The IC50 value of this inhibition was found to be 480 nM, which is close to its published value. Our data indicate that natural reading domains can be used as alternates to methyl-specific antibodies in PKMT assays. Reading domains can be produced recombinantly in E. coli at low cost and consistent quality, and they are accessible to protein design.

Non-radioactive protein lysine methyltransferase microplate assay based on reading domains.

PHF1 associates with the Polycomb repressive complex 2 and it was demonstrated to stimulate its H3K27-trimethylation activity. We studied the interaction of the PHF1 Tudor domain with modified histone peptides and found that it recognizes H3K36me3 and H3tK27me3 (on the histone variant H3t) and that it uses the same trimethyllysine binding pocket for the interaction with both peptides. Since both peptide sequences are very different, this result indicates that reading domains can have dual specificities. Sub-nuclear localization studies of full-length PHF1 in human HEK293 cells revealed that it co-localizes with K27me3, but not with K36me3, and that this co-localization depends on the trimethyllysine binding pocket indicating that K27me3 is an in vivo target for the PHF1 Tudor domain. Our data suggest that PHF1 binds to H3tK27me3 in human chromatin, and H3t has a more general role in Polycomb regulation.

The Tudor domain of the PHD finger protein 1 is a dual reader of lysine trimethylation at lysine 36 of histone H3 and lysine 27 of histone variant H3t.

The nuclear receptor binding SET [su(var) 3-9, enhancer of zeste, trithorax] domain-containing protein 1 (NSD1) protein lysine methyltransferase (PKMT) was known to methylate histone H3 lysine 36 (H3K36). We show here that NSD1 prefers aromatic, hydrophobic, and basic residues at the -2, -1 and +2, and +1 sites of its substrate peptide, respectively. We show methylation of 25 nonhistone peptide substrates by NSD1, two of which were (weakly) methylated at the protein level, suggesting that unstructured protein regions are preferred NSD1 substrates. Methylation of H4K20 and p65 was not observed. We discovered strong methylation of H1.5 K168, which represents the best NSD1 substrate protein identified so far, and methylation of H4K44 which was weaker than H3K36. Furthermore, we show that Sotos mutations in the SET domain of NSD1 inactivate the enzyme. Our results illustrate the importance of specificity analyses of PKMTs for understanding protein lysine methylation signaling pathways.

Substrate specificity analysis and novel substrates of the protein lysine methyltransferase NSD1.

Methylation of lysine residues is an important post-translational modification of histone and non-histone proteins, which is introduced by protein lysine methyltransferases (PKMTs). An increasing number of reports demonstrate that aberrant lysine methylation plays a central role in carcinogenesis that is often correlated with abnormal expression of PKMTs. Recent whole genome and whole transcriptome sequencing projects have also discovered several somatic mutations in PKMTs that frequently appear in various tumors. These include chromosomal translocations that lead to aberrant expression or mistargeting of PKMTs and nonsense or frameshift mutations, which cause the loss of the protein function. Another type of mutations are missense mutations which may lead to the loss of enzyme activity, but may also alter the properties of PKMTs either by changing the product or substrate specificity or by increasing the enzymatic activity finally leading to a gain-of-function phenotype. In this review, we provide an overview of the roles of EZH2, SETD2, NSD family, SMYD family, MLL family and DOT1L PKMTs in cancer focusing on the effects of somatic cancer mutations in these enzymes. Investigation of the effect of somatic cancer mutations in PKMTs is pivotal to understand the general role of this important class of enzymes in carcinogenesis and to improve and develop more individualized cancer therapies.

Role of somatic cancer mutations in human protein lysine methyltransferases.

Post-translational modifications (PTMs) of histones constitute a major chromatin indexing mechanism, and their proper characterization is of highest biological importance. So far, PTM-specific antibodies have been the standard reagent for studying histone PTMs despite caveats such as lot-to-lot variability of specificity and binding affinity. Herein, we successfully employed naturally occurring and engineered histone modification interacting domains for detection and identification of histone PTMs and ChIP-like enrichment of different types of chromatin. Our results demonstrate that histone interacting domains are robust and highly specific reagents that can replace or complement histone modification antibodies. These domains can be produced recombinantly in Escherichia coli at low cost and constant quality. Protein design of reading domains allows for generation of novel specificities, addition of affinity tags, and preparation of PTM binding pocket variants as matching negative controls, which is not possible with antibodies.

Article	Year
Detailed specificity analysis of antibodies binding to modified histone tails with peptide arrays. Epigenetics : official journal of the DNA Methylation Society\| PubMed ID: 20962581	2011
Specificity analysis-based identification of new methylation targets of the SET7/9 protein lysine methyltransferase. Chemistry & biology\| PubMed ID: 21276944	2011
The ATRX-ADD domain binds to H3 tail peptides and reads the combined methylation state of K4 and K9. Human molecular genetics\| PubMed ID: 21421568	2011
Application of Celluspots peptide arrays for the analysis of the binding specificity of epigenetic reading domains to modified histone tails. BMC biochemistry\| PubMed ID: 21884582	2011
A fluorescence-based supramolecular tandem assay for monitoring lysine methyltransferase activity in homogeneous solution. Chemistry (Weinheim an der Bergstrasse, Germany)\| PubMed ID: 22367964	2012
The SET8 H4K20 protein lysine methyltransferase has a long recognition sequence covering seven amino acid residues. Biochimie\| PubMed ID: 22583696	2012
Non-radioactive protein lysine methyltransferase microplate assay based on reading domains. ChemMedChem\| PubMed ID: 23671032	2014
The Tudor domain of the PHD finger protein 1 is a dual reader of lysine trimethylation at lysine 36 of histone H3 and lysine 27 of histone variant H3t. Journal of molecular biology\| PubMed ID: 23954330	2014
Substrate specificity analysis and novel substrates of the protein lysine methyltransferase NSD1. Chemistry & biology\| PubMed ID: 24412544	2014
Role of somatic cancer mutations in human protein lysine methyltransferases. Biochimica et biophysica acta\| PubMed ID: 25123655	2014
Application of histone modification-specific interaction domains as an alternative to antibodies. Genome research\| PubMed ID: 25301795	2014

Srikanth Kudithipudi

Biography

JoVE Journal Publications

Other Publications