No19 Is a Spatial Regulator for the Human ITS2 Pre-rRNA Endonuclease-Kinase Complex

The ribosome plays a universal role in translating the cellular proteome. Defects in the ribosome assembly factor Las1L are associated with congenital lethal motor neuron disease and X-linked intellectual disability disorders, yet its role in processing precursor ribosomal RNA (pre-rRNA) is largely unclear. The Las1L endoribonuclease associates with the No19 polynucleotide kinase to form the internal transcribed spacer 2 (ITS2) pre-rRNA endonuclease kinase machinery. Together, Las1L No19 catalyzes RNA cleavage and phosphorylation to mark the ITS2 for degradation. While ITS2 processing is critical for the production of functional ribosomes, the regulation of mammalian Las1L No19 remains obscure. Here we characterize the human Las1L No19 complex and identify critical molecular features that regulate its assembly and spatial organization. We establish that Las1L and No19 form a higher-order complex and identify the regions responsible for orchestrating this intricate architecture. Structural analysis by high-resolution imaging defines the intricate spatial pattern of Las1L No19 within the nucleolar sub-structure linked with late pre-rRNA processing events. Furthermore, we uncover a No19-encoded nucleolar localization sequence that is responsible for nucleolar transport of the assembled Las1L No19 complex. Together, these data provide a mechanism for the assembly and nucleolar localization of the human ITS2 pre-rRNA endonuclease kinase complex. Published by Elsevier Ltd.