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Kornprobst, Markus; Turk, Martin; Kellner, Nikola; Cheng, Jingdong; Flemming, Dirk; Kos-Braun, Isabelle; Kos, Martin; Thoms, Matthias; Berninghausen, Otto; Beckmann, Roland; Hurt, Ed (2016): Architecture of the 90S Pre-ribosome: A Structural View on the Birth of the Eukaryotic Ribosome. In: Cell, Vol. 166, No. 2: pp. 380-393
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Abstract

The 90S pre-ribosome is an early biogenesis intermediate formed during co-transcriptional ribosome formation, composed of similar to 70 assembly factors and several small nucleolar RNAs ( snoRNAs) that associate with nascent pre-rRNA. We report the cryo-EM structure of the Chaetomium thermophilum 90S pre-ribosome, revealing how a network of biogenesis factors including 19 beta-propellers and large a-solenoid proteins engulfs the pre-rRNA. Within the 90S pre-ribosome, we identify the UTP-A, UTP-B, Mpp10-Imp3-Imp4, Bms1-Rcl1, and U3 snoRNP modules, which are organized around 5'-ETS and partially folded 18S rRNA. The U3 snoRNP is strategically positioned at the center of the 90S particle to perform its multiple tasks during pre-rRNA folding and processing. The architecture of the elusive 90S pre-ribosome gives unprecedented structural insight into the early steps of pre-rRNA maturation. Nascent rRNA that is co-transcriptionally folded and given a particular shape by encapsulation within a dedicated mold-like structure is reminiscent of how polypeptides use chaperone chambers for their protein folding.