Mapping targets for small nucleolar RNAs in yeast

Background: Recent analyses implicate changes in the expression of the box C/D class of small nucleolar RNAs (snoRNAs) in several human diseases. Methods: Here we report the identification of potential novel RNA targets for box C/D snoRNAs in budding yeast, using the approach of UV crosslinking and sequencing of hybrids (CLASH) with the snoRNP proteins Nop1, Nop56 and Nop58. We also developed a bioinformatics approach to filter snoRNA-target interactions for bona fide methylation guide interactions. Results: We recovered 241,420 hybrids, out of which 190,597 were classed as reproducible, high energy hybrids. As expected, the majority of snoRNA interactions were with the ribosomal RNAs (rRNAs). Following filtering, 117,047 reproducible hybrids included 51 of the 55 reported rRNA methylation sites. The majority of interactions at methylation sites were predicted to guide methylation. However, competing, potentially regulatory, binding was also identified. In marked contrast, following CLASH performed with the RNA helicase Mtr4 only 7% of snoRNA-rRNA interactions recovered were predicted to guide methylation. We propose that Mtr4 functions in dissociating inappropriate snoRNA-target interactions. Numerous snoRNA-snoRNA interactions were recovered, indicating potential cross regulation. The snoRNAs snR4 and snR45 were recently implicated in site-directed rRNA acetylation, and hybrids were identified adjacent to the acetylation sites. We also identified 1,368 reproducible snoRNA-mRNA interactions, representing 448 sites of interaction involving 39 snoRNAs and 382 mRNAs. Depletion of the snoRNAs U3, U14 or snR4 each altered the levels of numerous mRNAs. Targets identified by CLASH were over-represented among these species, but causality has yet to be established. Conclusions: Systematic mapping of snoRNA-target binding provides a catalogue of high-confidence binding sites and indicates numerous potential regulatory interactions

RNA substrate length as an indicator of exosome interactions <i>in vivo</i>

Background: The exosome complex plays key roles in RNA processing and degradation in Eukaryotes and Archaea. Outstanding structural studies identified multiple pathways for RNA substrates into the exosome in vitro, but identifying the pathway followed by individual RNA species in vivo remains challenging. Methods: We attempted to address this question using RNase protection. In vivo RNA-protein crosslinking (CRAC) was applied to the exosome component Rrp44/Dis3, which has both endonuclease and exonuclease activity. During CRAC, the exosome was purified under native conditions and subjected to RNase digestion, prior to protein denaturation and cDNA cloning. The resulting high-throughput sequence reads were stratified by length of the cDNA sequence. This should reflect RNA fragment lengths, and therefore the RNA region that was protected by exosome binding. We anticipated major read lengths of ~30nt and ~10nt, reflecting the “central channel” and “direct access” routes to the Rrp44 exonuclease active site observed in vitro. Results: Unexpectedly, no clear peak was observed at 30nt, whereas a broad peak was seen around 20nt. The expected ~10nt peak was seen, and showed strong elevation in strains lacking exonuclease activity. Unexpectedly, this peak was suppressed by point mutations in the Rrp44 endonuclease active site. This indicates that the short fragments are degraded by the exonuclease activity of Rrp44, but also suggests that at least some may be generated by endonuclease activity. Conclusions: The absence of 30nt protected fragments may reflect obligatory binding of cofactors at the entrance to the exosome central channel in vivo. The presence of ~20nt fragments apparently indicates an access route not yet reported from in vitro studies. Confident mapping of 10nt reads is challenging, but they are clearly derived from a subset of exosome targets. In particular, pre-rRNA species, which are major exosome targets, are strongly disfavored for the generation of short reads

DIS3 isoforms vary in their endoribonuclease activity and are differentially expressed within haematological cancers

DIS3 is the catalytic subunit of the exosome, a protein complex involved in the 3’ to 5’ degradation of RNAs. DIS3 is a highly conserved exoribonuclease, also known as Rrp44. Global sequencing studies have identified DIS3 as being mutated in a range of cancers, with a considerable incidence in multiple myeloma. In this work, we have identified two proteincoding isoforms of DIS3. Both isoforms are functionally relevant and result from alternative splicing. They differ from each other in the size of their N-terminal PIN domain, which has been shown to have endoribonuclease activity and tether DIS3 to the exosome. Isoform 1 encodes a full-length PIN domain, whereas the PIN domain of isoform 2 is shorter and is missing a segment with conserved amino-acids. We have carried out biochemical activity assays on both isoforms of full-length DIS3 as well as the isolated PIN domains. We find that isoform 2, despite missing part of the PIN domain, has greater endonuclease activity compared to isoform 1. Examination of the available structural information allows us to provide a hypothesis to explain this altered behaviour. Our results also show that multiple myeloma patient cells and all cancer cell lines tested have higher levels of isoform 1 compared to isoform 2 whereas Acute Myeloid Leukemia (AML) and chronic myelomonocytic leukaemia (CMML) patient cells and samples from healthy donors have similar levels of isoforms 1 and 2. Together, our data indicate that significant changes in the ratios of the two isoforms could be symptomatic of haematological cancers

Substrate specificity of the TRAMP nuclear surveillance complexes

During nuclear surveillance in yeast and human cells, the RNA exosome functions together with the TRAMP complexes. Here, the authors defined the protein composition of the TRAMP complexes and identified specific RNA binding sites for the different TRAMP components

Transcriptome-wide analysis of alternative routes for RNA substrates into the exosome complex

<div><p>The RNA exosome complex functions in both the accurate processing and rapid degradation of many classes of RNA. Functional and structural analyses indicate that RNA can either be threaded through the central channel of the exosome or more directly access the active sites of the ribonucleases Rrp44 and Rrp6, but it was unclear how many substrates follow each pathway <i>in vivo</i>. We used CRAC (UV crosslinking and analysis of cDNA) in growing cells to identify transcriptome-wide interactions of RNAs with the major nuclear exosome-cofactor Mtr4 and with individual exosome subunits (Rrp6, Csl4, Rrp41 and Rrp44) along the threaded RNA path. We compared exosome complexes lacking Rrp44 exonuclease activity, carrying a mutation in the Rrp44 S1 RNA-binding domain predicted to disfavor direct access, or with multiple mutations in Rrp41 reported to impede RNA access to the central channel <i>in vitro</i>. Preferential use of channel-threading was seen for mRNAs, 5S rRNA, scR1 (SRP) and aborted tRNAs transcripts. Conversely, pre-tRNAs preferentially accessed Rrp44 directly. Both routes participated in degradation and maturation of RNAPI transcripts, with hand-over during processing. Rrp41 mutations blocked substrate passage through the channel to Rrp44 only for cytoplasmic mRNAs, supporting the predicted widening of the lumen in the Rrp6-associated, nuclear complex. Many exosome substrates exhibited clear preferences for a specific path to Rrp44. Other targets showed redundancy, possibly allowing the efficient handling of highly diverse RNA-protein complexes and RNA structures. Both threading and direct access routes involve the RNA helicase Mtr4. mRNAs that are predominately nuclear or cytoplasmic exosome substrates can be distinguished <i>in vivo</i>.</p></div

Purification of cross-linked RNA-protein complexes by phenol-toluol extraction

Recent methodological advances allowed the identification of an increasing number of RNA-binding proteins (RBPs) and their RNA-binding sites. Most of those methods rely, however, on capturing proteins associated to polyadenylated RNAs which neglects RBPs bound to non-adenylate RNA classes (tRNA, rRNA, pre-mRNA) as well as the vast majority of species that lack poly-A tails in their mRNAs (including all archea and bacteria). We have developed the Phenol Toluol extraction (PTex) protocol that does not rely on a specific RNA sequence or motif for isolation of cross-linked ribonucleoproteins (RNPs), but rather purifies them based entirely on their physicochemical properties. PTex captures RBPs that bind to RNA as short as 30 nt, RNPs directly from animal tissue and can be used to simplify complex workflows such as PAR-CLIP. Finally, we provide a global RNA-bound proteome of human HEK293 cells and the bacterium Salmonella Typhimurium