Eukaryotic small ribosomal subunit (40S)

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The eukaryotic small ribosomal subunit (40S) is the smaller subunit of the eukaryotic 80S ribosomes, with the other major component being the large ribosomal subunit (60S). The "40S" and "60S" names originate from the convention that ribosomal particles are denoted according to their sedimentation coefficients in Svedberg units. It is structurally and functionally related to the 30S subunit of 70S prokaryotic ribosomes.[1][2][3][4][5] However, the 40S subunit is much larger than the prokaryotic 30S subunit and contains many additional protein segments, as well as rRNA expansion segments.


The 40S subunit contains the decoding center which monitors the complementarity of tRNA and mRNA in protein translation. It is the largest component of several translation initiation complexes, including the 43S and 48S preinitiation complexes (PICs), being bound by several eukaryotic initiation factors, including eIF1, eIF1A, and eIF3.[6] The 40S ribosomal subunit is also tightly bound by the HCV IRES to form a binary complex mediate by protein-mRNA and rRNA-mRNA interactions.[7] More information can be found in the articles on the ribosome, the eukaryotic ribosome (80S), and the article on protein translation.

Overall structure[edit]

The shape of the small subunit can be subdivided into two large segments, the head and the body. Characteristic features of the body include the left and right feet, the shoulder and the platform. The head features a pointed protrusion reminiscent of a bird's beak. The mRNA binds in the cleft between the head and the body, and there are three binding sites for tRNA, the A-site, P-site and E-site (see article on protein translation for details). The core of the 40S subunit is formed by the 18S ribosomal RNA (abbreviated 18S rRNA), which is homologous to the prokaryotic 16S rRNA. This rRNA core is decorated with dozens of proteins. In the figure "Crystal Structure of the Eukaryotic 40S Ribosomal Subunit from T. thermophila", the ribosomal RNA core is represented as a grey tube and expansion segments are shown in red. Proteins which have homologs in eukaryotes, archaea and bacteria are shown as blue ribbons. Proteins shared only between eukaryotes and archaea are shown as orange ribbons and proteins specific to eukaryotes are shown as red ribbons.

40S ribosomal proteins[edit]

The table "40S ribosomal proteins" shows the individual protein folds of the 40S subunit colored by conservation. Proteins which have homologs in eukaryotes, archaea and bacteria (EAB) are shown as blue ribbons. Proteins shared only between eukaryotes and archaea (EA) are shown as orange ribbons and proteins specific to eukaryotes (E) are shown as red ribbons. Eukaryote-specific extensions of conserved proteins, ranging from a few residues or loops to very long alpha helices and additional domains, are highlighted in red.[2] For a details, refer to the article on the eukaryotic ribosome. Historically, different nomenclatures have been used for ribosomal proteins. For instance, proteins have been numbered according to their migration properties in gel electrophoresis experiments. Therefore, different names may refer to homologous proteins from different organism, while identical names not necessarily denote homologous proteins. The table "40S ribosomal proteins" crossreferences the human ribosomal protein names with yeast, bacterial and archaeal homologs.[8] Further information can be found in the ribosomal protein gene database (RPG).[8]

40S ribosomal proteins
Structure (Eukaryotic)[9] H. sapiens[8][10] Conservation[11] S. cerevisiae[12] Bacterial homolog (E. coli) Archaeal homolog
RPS2 EAB S2 S5p S5p
RPS3 EAB S3 S3p S3p
RPS3A EA S1 n/a S3Ae
RPS4 (RPS4X, RPS4Y1, RPS4Y2) EA S4 n/a S4e
RPS5 EAB S5 S7p S5p
RPS6 EA S6 n/a S6e
RPS7 E S7 n/a n/a
RPS8 EA S8 n/a S8e
RPS9 EAB S9 S4p S4p
RPS10 E S10 n/a n/a
RPS11 EAB S11 S17p S17p
RPS12 E S12 n/a n/a
RPS13 EAB S13 S15p S15p
RPS14 EAB S14 S11p S11p
RPS15 EAB S15 S19p S19p
RPS15A EAB S22 S8p S8p
RPS16 EAB S16 S9p S9p
RPS17 EA S17 n/a S17e
RPS18 EAB S18 S13p S13p
RPS19 EA S19 n/a S19e
RPS20 EAB S20 S10p S10p
RPS21 E S21 n/a n/a
RPS23 EAB S23 S12p S12p
RPS24 EA S24 n/a S24e
RPS25 EA S25 n/a S25e
RPS26 EA S26 n/a S26e
RPS27 EA S27 n/a S27e
RPS27A EA S31 n/a S27ae
RPS28 EA S28 n/a S28e
RPS29 EAB S29 S14p S14p
RPS30 EA S30 n/a S30e
RACK1 E Asc1 n/a n/a

See also[edit]


  1. ^ 40S+Ribosomal+Subunits at the US National Library of Medicine Medical Subject Headings (MeSH)
  2. ^ a b Rabl, J; Leibundgut, M; Ataide, SF; Haag, A; Ban, N (Feb 2011). "Crystal structure of the eukaryotic 40S ribosomal subunit in complex with initiation factor 1". Science. 331 (6018): 730–736. doi:10.1126/science.1198308. hdl:20.500.11850/153130. PMID 21205638.
  3. ^ Ben-Shem, A; Garreau; de Loubresse, N; Melnikov, S; Jenner, L; Yusupova, G; Yusupov, M (Dec 2011). "The structure of the eukaryotic ribosome at 3.0 Å resolution". Science. 334 (6062): 1524–1529. doi:10.1126/science.1212642. PMID 22096102.
  4. ^ Wimberly, BT; Brodersen, DE; Clemons, WM Jr; Morgan-Warren, RJ; Carter, AP; Vonrhein, C; Hartsch, T; Ramakrishnan, V (Sep 2000). "Structure of the 30S ribosomal subunit". Nature. 407 (6802): 327–339. doi:10.1038/35030006. PMID 11014182.
  5. ^ Schmeing, TM; Ramakrishnan, V (Oct 2009). "What recent ribosome structures have revealed about the mechanism of translation". Nature. 461 (7268): 1234–1242. doi:10.1038/nature08403. PMID 19838167.
  6. ^ Aitken, Colin E.; Lorsch, Jon R. (2012). "A mechanistic overview of translation initiation in eukaryotes". Nat. Struct. Mol. Biol. 19 (6): 568–576. doi:10.1038/nsmb.2303. PMID 22664984.
  7. ^ Lytle JR, Wu L, Robertson HD (August 2002). "Domains on the hepatitis C virus internal ribosome entry site for 40s subunit binding". RNA. 8 (8): 1045–1055. doi:10.1017/S1355838202029965. PMC 1370315. PMID 12212848.
  8. ^ a b c Nakao, A; Yoshihama, M; Kenmochi, N (2004). "RPG: the Ribosomal Protein Gene database". Nucleic Acids Res. 32 (Database issue): D168–70. doi:10.1093/nar/gkh004. PMC 308739. PMID 14681386.
  9. ^ Structure of the T. thermophila,' proteins from the structures of the large subunit PDBS 417, 4A19 and small subunit PDB 2XZM
  10. ^ Nomenclature according to the ribosomal protein gene database, applies to H. sapiens and T. thermophila
  11. ^ EAB means conserved in eukaryotes, archaea and bacteria, EA means conserved in eukaryotes and archaea and E means eukaryote-specific protein
  12. ^ Traditionally, ribosomal proteins were named according to their apparent molecular weight in gel electrophoresis, leading to different names for homologous proteins from different organisms. The RPG offers a unified nomenclature for ribosomal protein genes based on homology.

External links[edit]