Abstract
Eukaryotic ribosome biogenesis begins with the co-transcriptional assembly of the 90S pre-ribosome. The ‘U three protein’ (UTP) complexes and snoRNP particles arrange around the nascent pre-ribosomal RNA chaperoning its folding and further maturation. The earliest event in this hierarchical process is the binding of the UTP-A complex to the 5'-end of the pre-ribosomal RNA (5'-ETS). This oligomeric complex predominantly consists of β-propeller and α-solenoidal proteins. Here we present the structure of the Utp4 subunit from the thermophilic fungus Chaetomium thermophilum at 2.15 Å resolution and analyze its function by UV RNA-crosslinking (CRAC) and in context of a recent cryo-EM structure of the 90S pre-ribosome. Utp4 consists of two orthogonal and highly basic β-propellers that perfectly fit the EM-data. The Utp4 structure highlights an unusual Velcro-closure of its C-terminal β-propeller as relevant for protein integrity and potentially Utp8 recognition in the context of the pre-ribosome. We provide a first model of the 5'-ETS RNA from the internally hidden 5'-end up to the region that hybridizes to the 3'-hinge sequence of U3 snoRNA and validate a specific Utp4/5'-ETS interaction by CRAC analysis.
Highlights
The ribosome is the cellular machinery responsible protein synthesis and is vital to cellular function
Following up on our reconstitution of entire ctUTP-A and ctUTP-B complexes from heterologously expressed components in yeast [12] we set out to determine the structure of Utp4 and to derive the molecular basis of its interactions within the 90S pre-ribosome
The crystal structure of Utp4 was solved at a resolution of 2.15 Å by the single wavelength anomalous dispersion (SAD) method using seleno-methionine labelled protein (Table 1)
Summary
The ribosome is the cellular machinery responsible protein synthesis and is vital to cellular function. In prokaryotes is less complex and to a certain degree spontaneous, while ribosome biogenesis in eukaryotes is a hierarchical and elaborate process It involves more than 200 non-ribosomal protein factors, which act at different steps of maturation [1, 2]. The earliest intermediate is the huge 90S pre-ribosome or small-subunit processome (SSU) [3, 4] that is co-transcriptionally assembled in the nucleolus by incorporation of several modular subcomplexes onto the nascent pre-rRNA (Fig 1)
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