The RNA helicases Has1, Mak5 and Spb4 mediate diverse functions at distinct regions of pre-ribosomal complexes

Assembly of eukaryotic ribosomal subunits is a highly dynamic and energy-consuming process involving numerous structural and compositional changes. Here, we identify the pre-ribosomal binding sites of three ATP-dependent RNA helicases Has1, Mak5 and Spb4 and by elucidating the precise targets of these enzymes, we uncover direct roles of Has1 in mediating release of the U14 snoRNA and dissociation of a cluster of early pre-60S biogenesis factors. We further discover pre-rRNA remodelling by Mak5 enables recruitment of the ribosomal protein Rpl10 in the cytoplasm and show that binding of Spb4 to a molecular hinge at the base of ES27 facilitates binding of the export adaptor Arx1 to pre-60S complexes. Our data highlight RNA helicases as key regulators of critical events during ribosome biogenesis and provide novel insights into the structural remodelling events that take place during assembly of the ribosomal subunits. Wild-type BY4741 yeast cells or cells expressing genomically His-Tev-ProtA-tagged Has1, Mak5 or Spb4 were grown in the presence of 4-thiouracil, crosslinked using light at 365 nm and analysed by CRAC.