The Paf1 complex subunit Rtf1 buffers cells against the toxic effects of [PSI+] and defects in Rkr1-dependent protein quality control in Saccharomyces cerevisiae

The histone H4 basic patch regulates SAGA-mediated H2B deubiquitination and histone acetylation

Histone H2B monoubiquitylation (H2Bub1) has central functions in multiple DNA-templated processes, including gene transcription, DNA repair, and replication. H2Bub1 also is required for the trans-histone regulation of H3K4 and H3K79 methylation. Although previous studies have elucidated the basic mechanisms that establish and remove H2Bub1, we have only an incomplete understanding of how H2Bub1 is regulated. We report here that the histone H4 basic patch regulates H2Bub1. Yeast cells with arginine-to-alanine mutations in the H4 basic patch (H4_2RA) exhibited a significant loss of global H2Bub1. H4_2RA mutant yeast strains also displayed chemotoxin sensitivities similar to, but less severe than, strains containing a complete loss of H2Bub1. We found that the H4 basic patch regulates H2Bub1 levels independently of interactions with chromatin remodelers and separately from its regulation of H3K79 methylation. To measure H2B ubiquitylation and deubiquitination kinetics in vivo, we used a rapid and reversible optogenetic tool, the light-inducible nuclear exporter, to control the subcellular location of the H2Bub1 E3 ligase, Bre1. The ability of Bre1 to ubiquitylate H2B was unaffected in the H4_2RA mutant. In contrast, H2Bub1 deubiquitination by SAGA-associated Ubp8, but not by Ubp10, increased in the H4_2RA mutant. Consistent with a function for the H4 basic patch in regulating SAGA deubiquitinase activity, we also detected increased SAGA-mediated histone acetylation in H4 basic patch mutants. Our findings uncover that the H4 basic patch has a regulatory function in SAGA-mediated histone modifications.

Emergence and evolution of yeast prion and prion-like proteins

Background

Prions are transmissible, propagating alternative states of proteins, and are usually made from the fibrillar, beta-sheet-rich assemblies termed amyloid. Prions in the budding yeast Saccharomyces cerevisiae propagate heritable phenotypes, uncover hidden genetic variation, function in large-scale gene regulation, and can act like diseases. Almost all these amyloid prions have asparagine/glutamine-rich (N/Q–rich) domains. Other proteins, that we term here ‘prionogenic amyloid formers’ (PAFs), have been shown to form amyloid in vivo, and to have N/Q-rich domains that can propagate heritable states in yeast cells. Also, there are >200 other S.cerevisiae proteins with prion-like N/Q-rich sequence composition. Furthermore, human proteins with such N/Q-rich composition have been linked to the pathomechanisms of neurodegenerative amyloid diseases.

Results

Here, we exploit the increasing abundance of complete fungal genomes to examine the ancestry of prions/PAFs and other N/Q-rich proteins across the fungal kingdom. We find distinct evolutionary behavior for Q-rich and N-rich prions/PAFs; those of ancient ancestry (outside the budding yeasts, Saccharomycetes) are Q-rich, whereas N-rich cases arose early in Saccharomycetes evolution. This emergence of N-rich prion/PAFs is linked to a large-scale emergence of N-rich proteins during Saccharomycetes evolution, with Saccharomycetes showing a distinctive trend for population sizes of prion-like proteins that sets them apart from all the other fungi. Conversely, some clades, e.g. Eurotiales, have much fewer N/Q-rich proteins, and in some cases likely lose them en masse, perhaps due to greater amyloid intolerance, although they contain relatively more non-N/Q-rich predicted prions. We find that recent mutational tendencies arising during Saccharomycetes evolution (i.e., increased numbers of N residues and a tendency to form more poly-N tracts), contributed to the expansion/development of the prion phenomenon. Variation in these mutational tendencies in Saccharomycetes is correlated with the population sizes of prion-like proteins, thus implying that selection pressures on N/Q-rich protein sequences against amyloidogenesis are not generally maintained in budding yeasts.

Conclusions

These results help to delineate further the limits and origins of N/Q-rich prions, and provide insight as a case study of the evolution of compositionally-defined protein domains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0594-3) contains supplementary material, which is available to authorized users.

The Nucleosome Acidic Patch Regulates the H2B K123 Monoubiquitylation Cascade and Transcription Elongation in Saccharomyces cerevisiae

Eukaryotes regulate gene expression and other nuclear processes through the posttranslational modification of histones. In S. cerevisiae, the mono-ubiquitylation of histone H2B on lysine 123 (H2B K123ub) affects nucleosome stability, broadly influences gene expression and other DNA-templated processes, and is a prerequisite for additional conserved histone modifications that are associated with active transcription, namely the methylation of lysine residues in H3. While the enzymes that promote these chromatin marks are known, regions of the nucleosome required for the recruitment of these enzymes are undefined. To identify histone residues required for H2B K123ub, we exploited a functional interaction between the ubiquitin-protein ligase, Rkr1/Ltn1, and H2B K123ub in S. cerevisiae. Specifically, we performed a synthetic lethal screen with cells lacking RKR1 and a comprehensive library of H2A and H2B residue substitutions, and identified H2A residues that are required for H2B K123ub. Many of these residues map to the nucleosome acidic patch. The substitutions in the acidic patch confer varying histone modification defects downstream of H2B K123ub, indicating that this region contributes differentially to multiple histone modifications. Interestingly, substitutions in the acidic patch result in decreased recruitment of H2B K123ub machinery to active genes and defects in transcription elongation and termination. Together, our findings reveal a role for the nucleosome acidic patch in recruitment of histone modification machinery and maintenance of transcriptional integrity.

Chromatin, a complex of DNA wrapped around histone proteins, impacts all DNA-templated processes, including gene expression. Cells employ various strategies to alter chromatin structure and control access to the genetic material. Nucleosomes, the building blocks of chromatin, are subject to a myriad of modifications on their constituent histone proteins. One highly conserved modification with important connections to human health is the addition of ubiquitin to histone H2B. H2B ubiquitylation modulates chromatin structure during gene transcription and acts as a master regulator for downstream histone modifications. The proteins that promote H2B ubiquitylation have been identified; however, little is known about how these proteins interface with the nucleosome. Here, we exploited the genetic tools of budding yeast to reveal features of the nucleosome that are required for H2B ubiquitylation. Our genetic screen identified amino acids on the nucleosome acidic patch, a negatively charged region on the nucleosome surface, as being important for this process. The acidic patch is critical for regulating chromatin transactions, and, in our study, we identified roles for the acidic patch throughout transcription. Our data reveal that the acidic patch recruits histone modifiers, regulates histone modifications within the H2B ubiquitylation cascade, and maintains transcriptional fidelity.

Rkr1/Ltn1 Ubiquitin Ligase-mediated Degradation of Translationally Stalled Endoplasmic Reticulum Proteins

Aberrant nonstop proteins arise from translation of mRNA molecules beyond the coding sequence into the 3'-untranslated region. If a stop codon is not encountered, translation continues into the poly(A) tail, resulting in C-terminal appendage of a polylysine tract and a terminally stalled ribosome. In Saccharomyces cerevisiae, the ubiquitin ligase Rkr1/Ltn1 has been implicated in the proteasomal degradation of soluble cytosolic nonstop and translationally stalled proteins. Rkr1 is essential for cellular fitness under conditions associated with increased prevalence of nonstop proteins. Mutation of the mammalian homolog causes significant neurological pathology, suggesting broad physiological significance of ribosome-associated quality control. It is not known whether and how soluble or transmembrane nonstop and translationally stalled proteins targeted to the endoplasmic reticulum (ER) are detected and degraded. We generated and characterized model soluble and transmembrane ER-targeted nonstop and translationally stalled proteins. We found that these proteins are indeed subject to proteasomal degradation. We tested three candidate ubiquitin ligases (Rkr1 and ER-associated Doa10 and Hrd1) for roles in regulating abundance of these proteins. Our results indicate that Rkr1 plays the primary role in targeting the tested model ER-targeted nonstop and translationally stalled proteins for degradation. These data expand the catalog of Rkr1 substrates and highlight a previously unappreciated role for this ubiquitin ligase at the ER membrane.

Interaction networks of prion, prionogenic and prion-like proteins in budding yeast, and their role in gene regulation

Prions are transmissible, propagating alternative states of proteins. Prions in budding yeast propagate heritable phenotypes and can function in large-scale gene regulation, or in some cases occur as diseases of yeast. Other ‘prionogenic’ proteins are likely prions that have been determined experimentally to form amyloid in vivo, and to have prion-like domains that are able to propagate heritable states. Furthermore, there are over 300 additional ‘prion-like’ yeast proteins that have similar amino-acid composition to prions (primarily a bias for asparagines and glutamines). Here, we examine the protein functional and interaction networks that involve prion, prionogenic and prion-like proteins. Set against a marked overall preference for N/Q-rich prion-like proteins not to interact with each other, we observe a significant tendency of prion/prionogenic proteins to interact with other, N/Q-rich prion-like proteins. This tendency is mostly due to a small number of networks involving the proteins NUP100p, LSM4p and PUB1p. In general, different data analyses of functional and interaction networks converge to indicate a strong linkage of prionogenic and prion-like proteins, to stress-granule assembly and related biological processes. These results further elucidate how prions may impact gene regulation, and reveal a broader horizon for the functional relevance of N/Q-rich prion-like domains.

Amyloids and yeast prion biology

The prions (infectious proteins) of Saccharomyces cerevisiae are proteins acting as genes, by templating their conformation from one molecule to another in analogy to DNA templating its sequence. Most yeast prions are amyloid forms of normally soluble proteins, and a single protein sequence can have any of several self-propagating forms (called prion strains or variants), analogous to the different possible alleles of a DNA gene. A central issue in prion biology is the structural basis of this conformational templating process. The in-register parallel β sheet structure found for several infectious yeast prion amyloids naturally suggests an explanation for this conformational templating. While most prions are plainly diseases, the [Het-s] prion of Podospora anserina may be a functional amyloid, with important structural implications. Yeast prions are important models for human amyloid diseases in general, particularly because new evidence is showing infectious aspects of several human amyloidoses not previously classified as prions. We also review studies of the roles of chaperones, aggregate-collecting proteins, and other cellular components using yeast that have led the way in improving the understanding of similar processes that must be operating in many human amyloidoses.