IT'S 2 for the price of 1: Multifaceted ITS2 processing machines in RNA and DNA maintenance

Cells utilize sophisticated RNA processing machines to ensure the quality of RNA. Many RNA processing machines have been further implicated in regulating the DNA damage response signifying a strong link between RNA processing and genome maintenance. One of the most intricate and highly regulated RNA processing pathways is the processing of the precursor ribosomal RNA (pre-rRNA), which is paramount for the production of ribosomes. Removal of the Internal Transcribed Spacer 2 (ITS2), located between the 5.8S and 25S rRNA, is one of the most complex steps of ribosome assembly. Processing of the ITS2 is initiated by the newly discovered endoribonuclease Las1, which cleaves at the C2 site within the ITS2, generating products that are further processed by the polynucleotide kinase Grc3, the 5'→3' exonuclease Rat1, and the 3'→5' RNA exosome complex. In addition to their defined roles in ITS2 processing, these critical cellular machines participate in other stages of ribosome assembly, turnover of numerous cellular RNAs, and genome maintenance. Here we summarize recent work defining the molecular mechanisms of ITS2 processing by these essential RNA processing machines and highlight their emerging roles in transcription termination, heterochromatin function, telomere maintenance, and DNA repair.

Characterization of the molecular crosstalk within the essential Grc3/Las1 pre-rRNA processing complex

Grc3 is an essential well-conserved eukaryotic polynucleotide kinase (PNK) that cooperates with the endoribonuclease Las1 to process the preribosomal RNA (rRNA). Aside from being dependent upon Las1 for coordinated kinase and nuclease function, little is known about Grc3 substrate specificity and the molecular mechanisms governing kinase activity. Here we characterize the kinase activity of Grc3 and identify key similarities and differences between Grc3 and other polynucleotide kinase family members. In contrast to other PNK family members, Grc3 has distinct substrate preference for RNA substrates in vitro. By disrupting conserved residues found at the Grc3 kinase active site, we identified specific residues required to support Grc3-directed Las1-mediated pre-rRNA cleavage in vitro and in vivo. The crosstalk between Grc3 and Las1 ensures the direct coupling of cleavage and phosphorylation during pre-rRNA processing. Taken together, our studies provide key insight into the polynucleotide kinase activity of the essential enzyme Grc3 and its molecular crosstalk with the endoribonuclease Las1.

Nuclease integrated kinase super assemblies (NiKs) and their role in RNA processing

Here we highlight the Grc3/Las1 complex, an essential RNA processing machine that is well conserved across eukaryotes and required for processing the pre-ribosomal RNA (pre-rRNA). Las1 is an endoribonuclease that cleaves the pre-rRNA while Grc3 is a polynucleotide kinase that phosphorylates the Las1-cleaved RNA product. Recently we showed that Grc3 and Las1 assemble into a higher-order complex composed of a dimer of Grc3/Las1 heterodimers that is required for nuclease and kinase activity. Unexpectedly, we found that the Grc3/Las1 complex draws numerous parallels with two other eukaryotic nucleases, Ire1 and RNase L. In this perspective we explore the similarities and differences between this family of nuclease integrated kinase super assemblies (NiKs) and their distinct roles in RNA cleavage.