Structural Heterogeneity in Pre-40S Ribosomes

Structure. 2017 Feb 7;25(2):329-340. doi: 10.1016/j.str.2016.12.011. Epub 2017 Jan 19.


Late-stage 40S ribosome assembly is a highly regulated dynamic process that occurs in the cytoplasm, alongside the full translation machinery. Seven assembly factors (AFs) regulate and facilitate maturation, but the mechanisms through which they work remain undetermined. Here, we present a series of structures of the immature small subunit (pre-40S) determined by three-dimensional (3D) cryoelectron microscopy with 3D sorting to assess the molecule's heterogeneity. These structures demonstrate an extensive structural heterogeneity of interface AFs that likely regulates subunit joining during 40S maturation. We also present structural models for the beak and the platform, two regions where the low resolution of previous studies did not allow for localization of AFs and the rRNA, respectively. These models are supported by biochemical analyses using point variants and suggest that maturation of the 18S 3' end is regulated by dissociation of the AF Dim1 from the subunit interface, consistent with previous biochemical analyses.

Keywords: cryo-EM; pre-40S ribosome; ribosome assembly; ribosome biogenesis; small subunit assembly.

MeSH terms

  • Amino Acid Motifs
  • Binding Sites
  • Cloning, Molecular
  • Cryoelectron Microscopy
  • Crystallography, X-Ray
  • Escherichia coli / genetics
  • Escherichia coli / metabolism
  • Gene Expression
  • Genetic Heterogeneity*
  • Humans
  • Models, Molecular
  • Nuclear Proteins / chemistry
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism
  • Protein Binding
  • Protein Interaction Domains and Motifs
  • Protein Structure, Secondary
  • RNA, Ribosomal / chemistry
  • RNA, Ribosomal / genetics
  • RNA, Ribosomal / metabolism
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Ribonucleoprotein, U5 Small Nuclear / chemistry
  • Ribonucleoprotein, U5 Small Nuclear / genetics
  • Ribonucleoprotein, U5 Small Nuclear / metabolism
  • Ribosomal Proteins / chemistry*
  • Ribosomal Proteins / genetics
  • Ribosomal Proteins / metabolism
  • Ribosome Subunits, Small, Eukaryotic / metabolism
  • Ribosome Subunits, Small, Eukaryotic / ultrastructure*
  • Saccharomyces cerevisiae / metabolism
  • Saccharomyces cerevisiae / ultrastructure*
  • Saccharomyces cerevisiae Proteins / chemistry
  • Saccharomyces cerevisiae Proteins / genetics
  • Saccharomyces cerevisiae Proteins / metabolism
  • Thermodynamics


  • NOB1 protein, S cerevisiae
  • Nuclear Proteins
  • PNO1 protein, S cerevisiae
  • RNA, Ribosomal
  • RPS3 protein, human
  • Recombinant Proteins
  • Ribonucleoprotein, U5 Small Nuclear
  • Ribosomal Proteins
  • Saccharomyces cerevisiae Proteins
  • TXNL4A protein, human
  • Tsr1 protein, human