Substrate discrimination and quality control require each catalytic activity of TRAMP and the nuclear RNA exosome

The making and breaking of tRNAs by ribonucleases

Ribonucleases (RNases) play important roles in supporting canonical and non-canonical roles of tRNAs by catalyzing the cleavage of the tRNA phosphodiester backbone. Here, we highlight how recent advances in cryo-electron microscopy (cryo-EM), protein structure prediction, reconstitution experiments, tRNA sequencing, and other studies have revealed new insight into the nucleases that process tRNA. This represents a very diverse group of nucleases that utilize distinct mechanisms to recognize and cleave tRNA during different stages of a tRNA's life cycle including biogenesis, fragmentation, surveillance, and decay. In this review, we provide a synthesis of the structure, mechanism, regulation, and modes of tRNA recognition by tRNA nucleases, along with open questions for future investigation.

Conserved Residues at the Mtr4 C-Terminus Coordinate Helicase Activity and Exosome Interactions

Mtr4 is an essential RNA helicase involved in nuclear RNA processing and degradation and is a member of the Ski2-like helicase family. Ski2-like helicases share a common core architecture that includes two RecA-like domains, a winged helix, and a helical bundle (HB) domain. In Mtr4, a short C-terminal tail immediately follows the HB domain and is positioned at the interface of the RecA-like domains. The tail ends with a SLYΦ sequence motif that is highly conserved in a subset of Ski2-like helicases. Here, we show that this sequence is critical for Mtr4 function. Mutations in the C-terminus result in decreased RNA unwinding activity. Mtr4 is a key activator of the RNA exosome complex, and mutations in the SLYΦ motif produce a slow growth phenotype when combined with a partial exosome defect in S. cerevisiae, suggesting an important role of the C-terminus of Mtr4 and the RNA exosome. We further demonstrate that C-terminal mutations impair RNA degradation activity by the major RNA exosome nuclease Rrp44 in vitro. These data demonstrate a role for the Mtr4 C-terminus in regulating helicase activity and coordinating Mtr4-exosome interactions.

SKI complex: A multifaceted cytoplasmic RNA exosome cofactor in mRNA metabolism with links to disease, developmental processes, and antiviral responses

RNA stability and quality control are integral parts of gene expression regulation. A key factor shaping eukaryotic transcriptomes, mainly via 3'-5' exoribonucleolytic trimming or degradation of diverse transcripts in nuclear and cytoplasmic compartments, is the RNA exosome. Precise exosome targeting to various RNA molecules requires strict collaboration with specialized auxiliary factors, which facilitate interactions with its substrates. The predominant class of cytoplasmic RNA targeted by the exosome are protein-coding transcripts, which are carefully scrutinized for errors during translation. Normal, functional mRNAs are turned over following protein synthesis by the exosome or by Xrn1 5'-3'-exonuclease, acting in concert with Dcp1/2 decapping complex. In turn, aberrant transcripts are eliminated by dedicated surveillance pathways, triggered whenever ribosome translocation is impaired. Cytoplasmic 3'-5' mRNA decay and surveillance are dependent on the tight cooperation between the exosome and its evolutionary conserved co-factor-the SKI (superkiller) complex (SKIc). Here, we summarize recent findings from structural, biochemical, and functional studies of SKIc roles in controlling cytoplasmic RNA metabolism, including links to various cellular processes. Mechanism of SKIc action is illuminated by presentation of its spatial structure and details of its interactions with exosome and ribosome. Furthermore, contribution of SKIc and exosome to various mRNA decay pathways, usually converging on recycling of ribosomal subunits, is delineated. A crucial physiological role of SKIc is emphasized by describing association between its dysfunction and devastating human disease-a trichohepatoenteric syndrome (THES). Eventually, we discuss SKIc functions in the regulation of antiviral defense systems, cell signaling and developmental transitions, emerging from interdisciplinary investigations. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Turnover and Surveillance > Regulation of RNA Stability RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Caught in the act-Visualizing ribonucleases during eukaryotic ribosome assembly

Ribosomes are essential macromolecular machines responsible for translating the genetic information encoded in mRNAs into proteins. Ribosomes are composed of ribosomal RNAs and proteins (rRNAs and RPs) and the rRNAs fulfill both catalytic and architectural functions. Excision of the mature eukaryotic rRNAs from their precursor transcript is achieved through a complex series of endoribonucleolytic cleavages and exoribonucleolytic processing steps that are precisely coordinated with other aspects of ribosome assembly. Many ribonucleases involved in pre-rRNA processing have been identified and pre-rRNA processing pathways are relatively well defined. However, momentous advances in cryo-electron microscopy have recently enabled structural snapshots of various pre-ribosomal particles from budding yeast (Saccharomyces cerevisiae) and human cells to be captured and, excitingly, these structures not only allow pre-rRNAs to be observed before and after cleavage events, but also enable ribonucleases to be visualized on their target RNAs. These structural views of pre-rRNA processing in action allow a new layer of understanding of rRNA maturation and how it is coordinated with other aspects of ribosome assembly. They illuminate mechanisms of target recognition by the diverse ribonucleases involved and reveal how the cleavage/processing activities of these enzymes are regulated. In this review, we discuss the new insights into pre-rRNA processing gained by structural analyses and the growing understanding of the mechanisms of ribonuclease regulation. This article is categorized under: Translation > Ribosome Biogenesis RNA Processing > rRNA Processing.

Structural basis for RNA surveillance by the human nuclear exosome targeting (NEXT) complex

RNA quality control relies on co-factors and adaptors to identify and prepare substrates for degradation by ribonucleases such as the 3' to 5' ribonucleolytic RNA exosome. Here, we determined cryogenic electron microscopy structures of human nuclear exosome targeting (NEXT) complexes bound to RNA that reveal mechanistic insights to substrate recognition and early steps that precede RNA handover to the exosome. The structures illuminate ZCCHC8 as a scaffold, mediating homodimerization while embracing the MTR4 helicase and flexibly anchoring RBM7 to the helicase core. All three subunits collaborate to bind the RNA, with RBM7 and ZCCHC8 surveying sequences upstream of the 3' end to facilitate RNA capture by MTR4. ZCCHC8 obscures MTR4 surfaces important for RNA binding and extrusion as well as MPP6-dependent recruitment and docking onto the RNA exosome core, interactions that contribute to RNA surveillance by coordinating RNA capture, translocation, and extrusion from the helicase to the exosome for decay.