Abstract
Mechanistic understanding of eukaryotic ribosome formation requires a detailed structural knowledge of the numerous assembly intermediates, generated along a complex pathway. Here, we present the structure of a late pre-40S particle at 3.6 Å resolution, revealing in molecular detail how assembly factors regulate the timely folding of pre-18S rRNA. The structure shows that, rather than sterically blocking 40S translational active sites, the associated assembly factors Tsr1, Enp1, Rio2 and Pno1 collectively preclude their final maturation, thereby preventing untimely tRNA and mRNA binding and error prone translation. Moreover, the structure explains how Pno1 coordinates the 3'end cleavage of the 18S rRNA by Nob1 and how the late factor's removal in the cytoplasm ensures the structural integrity of the maturing 40S subunit.
Highlights
Ribosomes are the cellular machines that translate mRNAs into proteins
The synthesis of eukaryotic ribosomal subunits requires the concerted activity of ~200 assembly factors that drive ribosome biogenesis through a series of pre-rRNA cleavage, folding and modification reactions, which are coupled to the incorporation of ribosomal proteins (Henras et al, 2015; Woolford and Baserga, 2013; Zemp and Kutay, 2007)
Initial steps of 40S biogenesis occur in the nucleolus, which leads to the formation of the first stable assembly intermediate, called the 90S pre-ribosome (Dragon et al, 2002; Grandi et al, 2002; Kornprobst et al, 2016), within which many of the early assembly steps for the 40S take place. This process requires between 50–70 different ribosome biogenesis factors (RBFs) (Woolford and Baserga, 2013; Grandi et al, 2002), which were shown by recent cryo-electron microscopy analysis to engulf the nascent pre-18S rRNA (Kornprobst et al, 2016; Sun et al, 2017; Chaker-Margot et al, 2017)
Summary
Ribosomes are the cellular machines that translate mRNAs into proteins. In eukaryotes, they consist of a small 40S and large 60S subunit, which carry the decoding and peptidyl transferase activity, respectively, and together comprise four ribosomal (r)RNAs (18S, 5.8S, 25S and 5S rRNA) and 78 ribosomal proteins in yeast. Initial steps of 40S biogenesis occur in the nucleolus, which leads to the formation of the first stable assembly intermediate, called the 90S pre-ribosome (Dragon et al, 2002; Grandi et al, 2002; Kornprobst et al, 2016), within which many of the early assembly steps for the 40S take place This process requires between 50–70 different ribosome biogenesis factors (RBFs) (Woolford and Baserga, 2013; Grandi et al, 2002), which were shown by recent cryo-electron microscopy (cryo-EM) analysis to engulf the nascent pre-18S rRNA (Kornprobst et al, 2016; Sun et al, 2017; Chaker-Margot et al, 2017). Detailed insight into the molecular interactions of the RBFs and the conformation of the pre-rRNA in late 40S pre-ribosomes was lacking
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