The snoRNP chaperone snR190 and the Npa1 complex form a macromolecular 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 box C/D chaperone small nucleolar ribonucleoprotein (snoRNP) snR190 and the Npa1 complex, composed of the α-solenoid scaffold proteins Npa1 and Npa2, the DEAD-box helicase Dbp6, the RNA-binding protein Nop8, and Rsa3, are likely involved in early 25S ribosomal RNA (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 chaperone snoRNP snR190. Nop8 mediates the formation of this assembly and can associate on its own with free snR190 snoRNP. Moreover, Nop8 RNA Recognition Motif (RRM) helps tether the snR190 snoRNP to pre-ribosomal particles. The snR190 snoRNA features a specific central stem-loop structure, which is required for high-affinity binding between free snR190 snoRNP and the Npa1 complex. Deleting this extension does not prevent snR190 snoRNA 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.