Maturation of pre‐40S particles in yeast and humans

Abstract

checkpoints/quality control steps in the pre-40S particle maturation pathway. A very recent review provides a complementary view to ours on small subunit biogenesis in yeast (Chaker-Margot, 2018). 2 | PRE-40S PARTICLE MATURATION IN SACCHAROMYCES CEREVISIAE 2.1 | Maturation pathway of pre-40S particles in S. cerevisiaeThe first precursor to the small ribosomal subunit is termed the small subunit (SSU) processome, also referred to as the 90S pre-ribosomal particle. It is formed by the co-transcriptional assembly of most of the small subunit ribosomal proteins (RPSs), AFs, and small nucleolar ribonucleoprotein particles (snoRNPs) on the nascent pre-rRNA transcript. The assembly and structure of this fascinating entity has been reviewed recently (Barandun, Hunziker, & Klinge, 2018) and will not be discussed further here. We will mostly concentrate on the maturation steps that follow the disassembly of the SSU processome, which leads to the release of the first pre-40S particle that contains the 20S pre-rRNA. This RNA species is generated by endonucleolytic cleavages of the RNA polymerase I transcript at sites A1, the 5 0 end of 18S rRNA and A2, within internal transcribed spacer 1 (ITS1). 20S pre-rRNA thus encompasses the 18S rRNA flanked at its 3 0 extremity by a 211-nucleotide-long fragment of ITS1. To yield the mature 40S ribosomal subunit, the evolving pre-40S particle undergoes maturation steps in the nucleus and the cytoplasm that include RNA conformational rearrangements, stabilization of RP integration, elimination of the ITS1 fragment, and removal of all AFs.The first pre-40S particle generated following disassembly of the SSU processome contains, in addition to the 20S pre-rRNA, the following AFs that have been recruited during formation of the SSU processome: Enp1p, the endonuclease Nob1p and its partner Pno1p/Dim2p, the methyltransferase Dim1p, and probably the helicase Prp43p. These AFs are then joined in the nucleus by Ltv1p, the kinase Hrr25p, the ATPase/kinase Rio2p, and the GTPase-like protein Tsr1p. The potential roles of these AFs in the maturation process and their dissociation stage will be discussed below. Of particular note is the fact that Nob1p is the endonuclease responsible for the cleavage at the 3 0 end of the 18S rRNA sequence (termed site D) that eliminates the ITS1 fragment and thus releases 18S rRNA. Nob1p is required for D-site cleavage in vivo and is able on its own to specifically perform this cleavage on a naked in vitro-transcribed RNA substrate (Fatica,

Ribosome Biogenesis in Plants: From Functional 45S Ribosomal DNA Organization to Ribosome Assembly Factors

“…Assembly might initiate on full-length rRNA precursor or after cleavage at A 2 /A 3 sites that release processome factor precursors and pre-40S particles. The figure was adapted and drawn from models of the assembly steps of the 90S/ SSU-processome and pre-40S in yeast and mammalian cells (Soltanieh et al, 2014;Cerezo et al, 2019). Accession numbers for all the Arabidopsis factors are provided in Supplemental Data Set 2. specific forms of NUCLEOLIN-U3 snoRNP bind rDNA while others associate with the SSU-processome.…”

“…The cytoplasmic export of the pre-40S particle requires the module Noc4/Utp19-Nop14/Utp2 and Crm1, which recognizes nuclear export signals (Schäfer et al, 2003;Kühn et al, 2009). In the cytoplasm, Enp1p/Bystin, Nob1, Ltv1, and other proteins dissociate from pre-40S particles while RACK1 and, later, RPS are incorporated to form mature 40S particles (Figure 4; reviewed in Cerezo et al, 2019).…”

Good Vibrations: Structural Remodeling of Maturing Yeast Pre-40S Ribosomal Particles Followed by Cryo-Electron Microscopy

“…These RBFs are proteins that drive pre-rRNA cleavage, modification and folding, as well as RPs chaperoning and integration into nascent ribosomal subunit precursors. Although they were quasi-exhaustively identified two decades ago by genetic screens or proteomics studies [1][2][3], and have been predicted to harbor various activities such as NTPase, nuclease, helicase or kinase based on their secondary structures (for reviews, [4][5][6][7]), the precise role of most RBFs in ribosome biogenesis remains elusive.…”

Erosion of human X chromosome inactivation causes major remodelling of the iPSC proteome

“…4e & f). Interestingly, these proteins are involved in cytoplasmic quality control of ribosomes (Cerezo et al, 2019;Karbstein, 2013;Pena et al, 2017).…”