Hamze, H. | bioRxiv Science

bioRxiv2025

Sni445 recruits box C/D snoRNPs snR4 and snR45 to guide ribosomal RNA acetylation by Kre33

Eukaryotic ribosome synthesis is a highly complex, multistep process that is best characterized in the yeast Saccharomyces cerevisiae. It is orchestrated by over 200 ribosome assembly factors and 75 small nucleolar ribonucleoproteins (snoRNPs), which guide site-specific chemical modifications of precursor ribosomal RNA (pre-rRNA). While canonical box C/D snoRNPs direct 2-O-methylation, the atypical box C/D snoRNPs snR4 and snR45 mediate acetylation of 18S rRNA residues C1280 and C1773, respectively, catalyzed by the acetyltransferase Kre33. Here, we identify and characterize Ynl050c/Sni445 as a novel ribosome assembly factor and previously unrecognized auxiliary component of the snR4 and snR45 box C/D snoRNPs. Sni445 associates with snR4 and snR45 in their free form and is required for their stable incorporation into 90S pre-ribosomes. Genetic interactions link Sni445 and the snR4 and snR45 snoRNAs to ribosomal proteins Rps20 (uS10) and Rps14 (uS11), which are positioned near the respective acetylation sites in the 40S subunit. Moreover, Sni445 physically interacts with Kre33 within the 90S pre-ribosome, and its absence abolishes acetylation of C1280 and C1773. Our findings suggest that Sni445 facilitates the recruitment of snR4 and snR45 snoRNPs to 90S particles and might promote their interaction with Kre33, thereby enabling the site-specific acetylation of 18S rRNA by Kre33.

molecular biology↗

bioRxiv2024

The snoRNP chaperone snR190 and the Npa1 complex form a macromomecular assembly required for 60S ribosomal subunit maturation

The early steps of large-ribosomal-subunit assembly feature among the least understood steps of ribosome synthesis in eukaryotes. In Saccharomyces cerevisiae, the snR190 box C/D snoRNP chaperone and the Npa1 complex, composed of the -solenoid scaffold proteins Npa1 and Npa2, the DEAD-box helicase Dbp6, the RNA-binding protein Nop8 and Rsa3, are likely involved in early 25S rRNA folding events. Here, we report for the first time the existence outside pre-ribosomal particles of an independent macromolecular assembly constituted by the Npa1 complex and the snR190 snoRNP chaperone. Nop8 mediates the formation of this assembly and can associate on its own with free snR190. Moreover, Nop8 RRM domain helps tether the snR190 snoRNP to pre-ribosomal particles. snR190 features a specific central stem-loop structure, which is required for high-affinity binding between free snR190 and the Npa1 complex. Deleting this extension does not prevent snR190 association with pre-ribosomal particles but impairs snR190 activity in early pre-rRNA processing events. This work establishes the importance of association with auxiliary protein complexes for optimum snoRNP chaperone activity during rRNA folding events.

molecular biology↗