Hafner, J.; Thoms, M.; Hamze, H.; Forstner, A.; Alidou-D'Anjou, I.; Hebbachi, H.; Kalinina, M.; Hausharter, A.; Favre, S.; Lebaron, S.; Ismail, S.; Denk, T.; Schlick, K.; Fröhlich, T.; Bhutada, P.; Farquar, E.; Schindlmaier, K.; Sormaz, S.; Hurt, E.; Kressler, D.; Henry, Y.; Woodson, S.; Dragon, F.; Beckmann, R.; Henras, A. K.; Pertschy, B.

The early steps of eukaryotic large ribosomal subunit assembly remain poorly understood due to the structural flexibility of pre-60S intermediates, whose rRNA is extensively modified by small nucleolar RNPs (snoRNPs). Some snoRNPs, however, lack any modification function and instead scaffold ribosome assembly through largely unknown mechanisms. Here, we show that the H/ACA snoRNP snR37 integrates both modifying and scaffolding roles. Biochemical and structural analyses reveal a canonical H/ACA core that pseudouridylates a conserved uridine in the A site of the peptidyl transferase center, the catalytic heart of the 60S subunit. Additional RNA helices recruit non-core proteins, the Upa1-Upa2 heterodimer and Rbp95, which mediate stable snR37 association with pre-60S complexes. These proteins cooperate with the Npa1 rRNA chaperone complex to link four rRNA domains, thereby structurally organizing early pre-60S intermediates and promoting proper formation of the PTC. This dual organization establishes a paradigm for snoRNPs combining rRNA modification and scaffolding functions.