Crosstalk of Phosphorylation and Arginine Methylation in Disordered SRGG Repeats of Saccharomycescerevisiae Fibrillarin and Its Association with Nucleolar Localization.

Crosstalk exists when two or more post-translational modifications, nearby in sequence or 3D space, affect each other or a protein's interactions. Saccharomyces cerevisiae protein Npl3p has six repeats of sequence SRGG, in a disordered domain, which can carry arginine methylation and serine phosphorylation. Crosstalk of the modifications controls Npl3p interactions with nuclear import, export, and other proteins. Here, we asked whether repeated SRGG motifs existed in other S. cerevisiae proteins and whether they serve a related function. Two other proteins had multiple SRGG motifs: Nop1p (fibrillarin) and Gar1p, both nucleolar proteins, which had nine and four motifs, respectively. For Nop1p, we first showed it to be extensively methylated in vivo. We then showed that the Nop1p SRGG motif is subjected to methylation by Hmt1p, phosphorylation by Sky1p, and Glc7p dephosphorylation and that there is crosstalk whereby phosphorylation blocks methylation. This is consistent with our recent motif analysis of Hmt1p, which revealed a negative specificity for acidic residues at -1 and -2 positions. On knockout of HMT1, Nop1p-GFP localization was not typically nucleolar. Conditional two-hybrid analysis, of Nop1p with C/D box small ribonuclear proteins Nop56p and Nop58p, suggested this may be associated with decreased protein-protein interactions on loss of arginine methylation. The effect of SRGG phosphorylation on the interactions of Nop1p remains unknown yet was predicted to cause a structural disorder-to-order transition in the Nop1p N-terminal domain. The SRGG motif is one of very few examples of modification crosstalk that has related functions in multiple proteins from the same species.

Knockout of the Hmt1p Arginine Methyltransferase in Saccharomyces cerevisiae Leads to the Dysregulation of Phosphate-associated Genes and Processes.

Hmt1p is the predominant arginine methyltransferase in Saccharomyces cerevisiae Its substrate proteins are involved in transcription, transcriptional regulation, nucleocytoplasmic transport and RNA splicing. Hmt1p-catalyzed methylation can also modulate protein-protein interactions. Hmt1p is conserved from unicellular eukaryotes through to mammals where its ortholog, PRMT1, is lethal upon knockout. In yeast, however, the effect of knockout on the transcriptome and proteome has not been described. Transcriptome analysis revealed downregulation of phosphate-responsive genes in hmt1Δ, including acid phosphatases PHO5, PHO11, and PHO12, phosphate transporters PHO84 and PHO89 and the vacuolar transporter chaperone VTC3 Analysis of the hmt1Δ proteome revealed decreased abundance of phosphate-associated proteins including phosphate transporter Pho84p, vacuolar alkaline phosphatase Pho8p, acid phosphatase Pho3p and subunits of the vacuolar transporter chaperone complex Vtc1p, Vtc3p and Vtc4p. Consistent with this, phosphate homeostasis was dysregulated in hmt1Δ cells, showing decreased extracellular phosphatase levels and decreased total Pi in phosphate-depleted medium. In vitro, we showed that transcription factor Pho4p can be methylated at Arg-241, which could explain phosphate dysregulation in hmt1Δ if interplay exists with phosphorylation at Ser-242 or Ser-243, or if Arg-241 methylation affects the capacity of Pho4p to homodimerize or interact with Pho2p. However, the Arg-241 methylation site was not validated in vivo and the localization of a Pho4p-GFP fusion in hmt1Δ was not different from wild type. To our knowledge, this is the first study to reveal an association between Hmt1p and phosphate homeostasis and one which suggests a regulatory link between S-adenosyl methionine and intracellular phosphate.

Protein substrates of the arginine methyltransferase Hmt1 identified by proteome arrays.

Arginine methylation on nonhistone proteins is associated with a number of cellular processes including RNA splicing, protein localization, and the formation of protein complexes. In this manuscript, Saccharomyces cerevisiae proteome arrays carrying 4228 proteins were used with an antimethylarginine antibody to first identify 88 putatively arginine-methylated proteins. By treating the arrays with recombinant arginine methyltransferase Hmt1, 42 proteins were found to be possible substrates of this enzyme. Analysis of the putative arginine-methylated proteins revealed that they were predominantly nuclear or nucleolar in localization, consistent with the localization of Hmt1. Many are involved in known methylarginine-associated functions, such as RNA processing and ribonucleoprotein complex biogenesis, yet others are of newer classes, namely RNA/DNA helicases and tRNA-associated proteins. Using ex vivo methylation and MS/MS, a set of 12 proteins (Brr1, Dia4, Hts1, Mpp10, Mrd1, Nug1, Prp43, Rpa43, Rrp43, Spp381, Utp4, and Npl3), including the RNA helicase Prp43 and tRNA ligases Dia4 and Hts1, were all validated as Hmt1 substrates. Interestingly, the majority of these also had human orthologs, or family members, that have been documented elsewhere to carry arginine methylation. These results confirm arginine methylation as a widespread modification and Hmt1 as the major arginine methyltransferase in the S. cerevisiae cell.

The activity of a yeast Family 16 methyltransferase, Efm2, is affected by a conserved tryptophan and its N-terminal region.

The Family 16 methyltransferases are a group of eukaryotic nonhistone protein methyltransferases. Sixteen of these have recently been described in yeast and human, but little is known about their sequence and structural features. Here we investigate one of these methyltransferases, Saccharomyces cerevisiae elongation factor methyltransferase 2 (Efm2), by site-directed mutagenesis and truncation. We show that an active site-associated tryptophan, invariant in Family 16 methyltransferases and at position 222 in Efm2, is important for methyltransferase activity. A second highly conserved tryptophan, at position 318 in Efm2, is likely involved in S-adenosyl methionine binding but is of lesser consequence for catalysis. By truncation analysis, we show that the N-terminal 50-200 amino acids of Efm2 are critical for its methyltransferase activity. As N-terminal regions are variable among Family 16 methyltransferases, this suggests a possible role in determining substrate specificity. This is consistent with recently solved structures that show the core of Family 16 methyltransferases to be near-identical but the N termini to be structurally quite different. Finally, we show that Efm2 can exist as an oligomer but that its N terminus is not necessary for oligomerisation to occur.

Yeast proteins Gar1p, Nop1p, Npl3p, Nsr1p, and Rps2p are natively methylated and are substrates of the arginine methyltransferase Hmt1p.

The Hmt1 methyltransferase is the predominant arginine methyltransferase in Saccharomyces cerevisiae. There are 18 substrate proteins described for this methyltransferase, however native sites of methylation have only been identified on two of these proteins. Here we used peptide immunoaffinity enrichment, followed by LC-ETD-MS/MS, to discover 21 native sites of arginine methylation on five putative Hmt1 substrate proteins, namely Gar1p (H/ACA ribonucleoprotein complex subunit 1), Nop1p (rRNA 2'-O-methyltransferase fibrillarin), Npl3p (nucleolar protein 3), Nsr1p (nuclear localization sequence-binding protein), and Rps2p (40S ribosomal protein S2). The sites, many of which were found to be mono- or di-methylated, were predominantly found in RGG (Arg-Gly-Gly) motifs. Heavy methyl-SILAC validated the majority of these peptides. The above proteins, and relevant sites of methylation, were subsequently validated by in vitro methylation with recombinant Hmt1. This brings the total of Hmt1 substrate proteins for which native methylation sites have been identified to five.

Lysine methylation modulates the protein-protein interactions of yeast cytochrome C Cyc1p.

In recent years, protein methylation has been established as a major intracellular PTM. It has also been proposed to modulate protein-protein interactions (PPIs) in the interactome. To investigate the effect of PTMs on PPIs, we recently developed the conditional two-hybrid (C2H) system. With this, we demonstrated that arginine methylation can modulate PPIs in the yeast interactome. Here, we used the C2H system to investigate the effect of lysine methylation. Specifically, we asked whether Ctm1p-mediated trimethylation of yeast cytochrome c Cyc1p, on lysine 78, modulates its interactions with Erv1p, Ccp1p, Cyc2p and Cyc3p. We show that the interactions between Cyc1p and Erv1p, and between Cyc1p and Cyc3p, are significantly increased upon trimethylation of lysine 78. This increase of interaction helps explain the reported facilitation of Cyc1p import into the mitochondrial intermembrane space upon methylation. This first application of the C2H system to the study of methyllysine-modulated interactions further confirms its robustness and flexibility.

A web of possibilities: network-based discovery of protein interaction codes.

Many proteins, including p53, the FoxO transcription factors, RNA polymerase II, pRb, and the chaperones, have extensive post-translational modifications (PTMs). Many of these modifications modulate protein-protein interactions, controlling interaction presence/absence and specificity. Here we propose the notion of the interaction code, a widespread means by which modifications are used to control interactions in the proteome. Minimal interaction codes are likely to exist on proteins that have two modifications and two or more interaction partners. By contrast, complex interaction codes are likely to be found on "date hub" proteins that have many interactions, many PTMs, or are targeted by many modifying and demodifying enzymes. Proteins with new interaction codes should be discoverable by examining protein interaction networks, annotated with PTMs and protein-modifying enzyme-substrate links. Multiple instances or combinations of phosphorylation, acetylation, methylation, O-GlcNAc, or ubiquitination will likely form interaction codes, especially when colocated on a protein's single interaction interface. A network-based example of code discovery is given, predicting the yeast protein Npl3p to have a methylation/phosphorylation-dependent interaction code.

Elongation factor methyltransferase 3--a novel eukaryotic lysine methyltransferase.

Here we describe the discovery of Saccharomycescerevisiae protein YJR129Cp as a new eukaryotic seven-beta-strand lysine methyltransferase. An immunoblotting screen of 21 putative methyltransferases showed a loss in the methylation of elongation factor 2 (EF2) on knockout of YJR129C. Mass spectrometric analysis of EF2 tryptic peptides localised this loss of methylation to lysine 509, in peptide LVEGLKR. In vitro methylation, using recombinant methyltransferases and purified EF2, validated YJR129Cp as responsible for methylation of lysine 509 and Efm2p as responsible for methylation at lysine 613. Contextualised on previously described protein structures, both sites of methylation were found at the interaction interface between EF2 and the 40S ribosomal subunit. In line with the recently discovered Efm1 and Efm2 we propose that YJR129C be named elongation factor methyltransferase 3 (Efm3). The human homolog of Efm3 is likely to be the putative methyltransferase FAM86A, according to sequence homology and multiple lines of literature evidence.

The Saccharomyces cerevisiae poly(A)-binding protein is subject to multiple post-translational modifications, including the methylation of glutamic acid.

Poly(A)-binding protein in mouse and man was recently found to be highly post-translationally modified. Here we analysed an ortholog of this protein, Pab1 from Saccharomyces cerevisiae, to assess the conservation and thus likely importance of these modifications. Pab1 showed the presence of six sites of methylated glutamate, five sites of lysine acetylation, and one phosphorylation of serine. Many modifications on Pab1 showed either complete conservation with those on human or mouse PABPC1, were present on nearby residues and/or were present in the same domain(s). The conservation of methylated glutamate, an unusual modification, was of particular note and suggests a conserved function. Comparison of methylated glutamate sites in human, mouse and yeast poly(A)-binding protein, along with methylation sites catalysed by CheR L-glutamyl protein methyltransferase from Salmonella typhimurium, revealed that the methylation of glutamate preferentially occurs in EE and DE motifs or other small regions of acidic amino acids. The conservation of methylated glutamate in the same protein between mouse, man and yeast suggests the presence of a eukaryotic l-glutamyl protein methyltransferase and that the modification is of functional significance.

Interactions affected by arginine methylation in the yeast protein-protein interaction network.

Protein-protein interactions can be modulated by the methylation of arginine residues. As a means of testing this, we recently described a conditional two-hybrid system, based on the bacterial adenylate cyclase (BACTH) system. Here, we have used this conditional two-hybrid system to explore the effect of arginine methylation in modulating protein-protein interactions in a subset of the Saccharomyces cerevisiae arginine methylproteome network. Interactions between the yeast hub protein Npl3 and yeast proteins Air2, Ded1, Gbp2, Snp1, and Yra1 were first validated in the absence of methylation. The major yeast arginine methyltransferase Hmt1 was subsequently included in the conditional two-hybrid assay, initially to determine the degree of methylation that occurs. Proteins Snp1 and Yra1 were confirmed as Hmt1 substrates, with five and two novel arginine methylation sites mapped by ETD LC-MS/MS on these proteins, respectively. Proteins Ded1 and Gbp2, previously predicted but not confirmed as substrates of Hmt1, were also found to be methylated with five and seven sites mapped respectively. Air2 was found to be a novel substrate of Hmt1 with two sites mapped. Finally, we investigated the interactions of Npl3 with the five interaction partners in the presence of active Hmt1 and in the presence of Hmt1 with a G68R inactivation mutation. We found that the interaction between Npl3 and Air2, and Npl3 and Ded1, were significantly increased in the presence of active Hmt1; the interaction of Npl3 and Snp1 showed a similar degree of increase in interaction but this was not statistically significant. The interactions of Npl3 and Gbp2, along with Npl3 and Yra1, were not significantly increased or decreased by methylation. We conclude that methylarginine may be a widespread means by which the interactions of proteins are modulated.

Analysis of the proteome of Saccharomyces cerevisiae for methylarginine.

Arginine methylation is a post-translational modification that has been implicated in a plethora of cellular processes. In the present manuscript, using two antimethylarginine antibodies and combinatorial deletion mutants of arginine methyltransferases, we found evidence of widespread arginine methylation in the Saccharomyces cerevisiae proteome. Immunoprecipitation was used for enrichment of methylarginine-containing proteins, which were identified via tandem mass spectrometry. From this, we identified a total of 90 proteins, of which 5 were previously known to be methylated. The proteins identified were involved in known methylarginine-associated biological functions such as RNA processing, nuclear transport, carbohydrate metabolic process, GMP biosynthetic process and protein folding. Through in vivo methylation by the incorporation of [3H]-methyl groups, we validated the methylation of 7 proteins (Ded1, Imd4, Lhp1, Nop1, Cdc11, Gus1, Pob3). By LC-MS/MS, we then confirmed a total of 15 novel methylarginine sites on 5 proteins (Ded1, Lhp1, Nop1, Pab1, and Ugp1). By examination of methylation on proteins from the triple knockout of methyltransferases Hmt1, Hsl7, Rmt2, we present evidence for the existence of additional unidentified arginine methyltransferases in the Saccharomyces cerevisiae proteome.

A conditional two-hybrid (C2H) system for the detection of protein-protein interactions that are mediated by post-translational modification.

The original bacterial two-hybrid system is widely used but does not permit the study of interactions regulated by PTMs. Here, we have built a conditional two-hybrid (C2H) system, in which bait and prey proteins can be co-expressed in the presence of a modifying enzyme such as a methyltransferase, acetyltransferase, or kinase. Any increase or decrease in interaction due to the modification of the proteins can be measured by an increased or decreased level of reporter gene expression. The C2H system is comprised of eight new vectors based on the Novagen Duet co-expression plasmids. These vectors include two multiple cloning sites per vector as well as a hexahistidine tag or S-tag to aid in purification, if desired. We demonstrate the use of the C2H system to study the dimerization of the yeast protein Npl3, which is increased when methylated by the methyltransferase Hmt1.

Enhanced methylarginine characterization by post-translational modification-specific targeted data acquisition and electron-transfer dissociation mass spectrometry.

When localizing protein post-translational modifications (PTMs) using liquid-chromatography (LC)-tandem mass spectrometry (MS/MS), existing implementations are limited by inefficient selection of PTM-carrying peptides for MS/MS, particularly when PTM site occupancy is sub-stoichiometric. The present contribution describes a method by which peptides carrying specific PTMs of interest-in this study, methylarginines-may be selectively targeted for MS/MS: peptide features are extracted from high mass accuracy single-stage MS data, searched against theoretical PTM-carrying peptide masses, and matching features are subjected to targeted data acquisition LC-MS/MS. Using trypsin digested Saccharomyces cerevisiae Npl3, in which evidence is presented for 18 methylarginine sites-17 of which fall within a glycine-arginine-rich (GAR) domain spanning <120 amino acids-it is shown that this approach outperforms conventional data dependent acquisition (DDA): when applied to a complex protein mixture featuring in vivo methylated Npl3, 95% more (P=0.030) methylarginine-carrying peptides are selected for MS/MS than DDA, leading to an 86% increase (P=0.044) in the number of methylated peptides producing Mascot ion scores ≥20 following electron-transfer dissociation (ETD). Notably, significantly more low abundance arginine methylated peptides (maximum ion intensities <6×10(4) cps) are selected for MS/MS using this approach relative to DDA (50% more in a digest of purified in vitro methylated Npl3). It is also demonstrated that relative to collision-induced dissociation (CID), ETD facilitates a 586% increase (P=0.016) in average Mascot ion scores of methylarginine-carrying peptides. The present PTM-specific targeted data acquisition approach, though described using methylarginine, is applicable to any ionizable PTM of known mass.

The methylproteome and the intracellular methylation network.

Since its discovery more than 50 years ago, post-translational modification (PTM) of proteins via methylation has grown in prominence, its involvement having been recognised in a number of central processes in the cell. Of these, the best characterised is its role in the epigenetic code. However, there is increasing evidence that its role extends far beyond this and we propose that it is a key regulator in interactome dynamics. In this review, we focus on the role of methylation in regulating protein-protein interactions and illustrate, by providing a broad-scale summary of our current knowledge of methylation and its impact on systems biology, how this can ultimately affect interactome dynamics. We describe the variety of analytical techniques available for the study of the methylproteome, comment on their advantages and limitations, and consider how these tools can help elucidate how methylation regulates the dynamics of the interactome. The insights gained from methyltransferase-substrate networks will be summarised and the ability of protein methylation to facilitate or block protein-protein interactions as well as their interplay with other post-translational modifications, in particular phosphorylation, is highlighted. Finally, the importance of methylation in pathology-associated protein interaction networks will be discussed using examples involving human diseases and the p53 protein.

Monitoring cytoplasmic protein complexes with blue native gel electrophoresis and stable isotope labelling with amino acids in cell culture: analysis of changes in the 20S proteasome.

Analysis of protein complexes is of increasing interest in the field of proteomics. A challenge is to develop methods for monitoring changes in the quantity and subunit composition of protein complexes on a proteome-wide scale. Here, we describe the combination of 1-D blue native polyacrylamide gel electrophoresis (BN-PAGE) with stable isotope labelling of amino acids in cell culture (SILAC) and tandem mass spectrometry (MS/MS). Cleared lysates from normal and perturbed samples, one incorporating heavy stable isotopes and the other light isotopes, are co-separated by blue native PAGE and then analysed and quantitated with MS/MS and appropriate software. This permits the analysis of cytoplasmic complexes. To demonstrate this technique, we explored how the 20S proteasome changes when the Pre9/α3 subunit, the only non-essential subunit of this complex, was deleted. Our results showed that ΔPre9/α3 cells can form the 20S proteasome complex, although with reduced efficiency. This involves an increase in expression of the α4 subunit. Our findings suggest this technique as an approach for the study of quantitative and qualitative differences in protein complexes, from cleared cell lysates.

Analysis of the RNA degradosome complex in Vibrio angustum S14.

The RNA degradosome is built on the C-terminal half of ribonuclease E (RNase E) which shows high sequence variation, even amongst closely related species. This is intriguing given its central role in RNA processing and mRNA decay. Previously, we have identified RhlB (ATP-dependent DEAD-box RNA helicase)-binding, PNPase (polynucleotide phosphorylase)-binding and enolase-binding microdomains in the C-terminal half of Vibrio angustum S14 RNase E, and have shown through two-hybrid analysis that the PNPase and enolase-binding microdomains have protein-binding function. We suggest that the RhlB-binding, enolase-binding and PNPase-binding microdomains may be interchangeable between Escherichia coli and V. angustum S14 RNase E. In this study, we used two-hybrid techniques to show that the putative RhlB-binding microdomain can bind RhlB. We then used Blue Native-PAGE, a technique commonly employed in the separation of membrane protein complexes, in a study of the first of its kind to purify and analyse the RNA degradosome. We showed that the V. angustum S14 RNA degradosome comprises at least RNase E, RhlB, enolase and PNPase. Based on the results obtained from sequence analyses, two-hybrid assays, immunoprecipitation experiments and Blue Native-PAGE separation, we present a model for the V. angustum S14 RNA degradosome. We discuss the benefits of using Blue Native-PAGE as a tool to analyse the RNA degradosome, and the implications of microdomain-mediated RNase E interaction specificity.

Identification and functional analysis of RNase E of Vibrio angustum S14 and two-hybrid analysis of its interaction partners.

RNase E is an essential enzyme that catalyses RNA processing. Microdomains which mediate interactions between RNase E and other members of the degradosome have been defined. To further elucidate the role of these microdomains in molecular interactions, we studied RNase E from Vibrio angustum S14. Protein sequence analysis revealed that its C-terminal half is less conserved and structured than its N-terminal half. Within this structural disorder, however, exist five small regions of predicted structural propensity. Four are similar to interaction-mediating microdomains identified in other RNase E proteins; the fifth did not correspond to any known functional motif. The function of the V. angustum S14 enolase-binding microdomain was confirmed using bacterial two-hybrid analysis, demonstrating the conserved function of this microdomain for the first time in a species other than Escherichia coli. Further, PNPase in V. angustum S14 was shown to interact with the last 80 amino acids of the C-terminal region of RNase E. This raises the possibility that PNPase interacts with the small ordered region at residues 1026-1041. The role of RNase E as a hub protein and the implications of microdomain-mediated interactions in relation to specificity and function are discussed.