Structure of a human pre-40S particle points to a role for RACK1 in the final steps of 18S rRNA processing

Synthesis of ribosomal subunits in eukaryotes is a complex and tightly regulated process that has been mostly characterized in yeast. The discovery of a growing number of diseases linked to defects in ribosome biogenesis calls for a deeper understanding of these mechanisms and of the specificities of human ribosome maturation. We present the 19 Å resolution cryo-EM reconstruction of a cytoplasmic precursor to the human small ribosomal subunit, purified by using the tagged ribosome biogenesis factor LTV1 as bait. Compared to yeast pre-40S particles, this first three-dimensional structure of a human 40S subunit precursor shows noticeable differences with respect to the position of ribosome biogenesis factors and uncovers the early deposition of the ribosomal protein RACK1 during subunit maturation. Consistently, RACK1 is required for efficient processing of the 18S rRNA 3'-end, which might be related to its role in translation initiation. This first structural analysis of a human pre-ribosomal particle sets the grounds for high-resolution studies of conformational transitions accompanying ribosomal subunit maturation.