The eukaryotic small ribosomal subunit (40S) is the smaller subunit of the eukaryotic ribosome (80S), 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. However, the 40S subunit is much larger than the prokaryotic 30S subunit and contains many additional protein segments, as well as rRNA expansion segments.
Function 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. 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. More information can be found in the articles on the ribosome, the eukaryotic ribosome (80S), and the article on protein translation.
Overall structure 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 Tetrahymena 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 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. 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. Further information can be found in the ribosomal protein gene database (RPG).
+ 40S ribosomal proteins Structure (Eukaryotic)Structure of the 'T. thermophila,' proteins from the structures of the large subunit PDBS 417, 4A19 and small subunit PDB 2XZM H. sapiensNomenclature according to the ribosomal protein gene database, applies to H. sapiens and T. thermophila Universal name ConservationEAB means conserved in eukaryotes, archaea and bacteria, EA means conserved in eukaryotes and archaea and E means eukaryote-specific protein S. cerevisiaeTraditionally, 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. Bacterial homolog (E. coli) Archaeal homolog RPSA uS2 EABS0S2pS2 RPS2 uS5 EABS2S5pS5p RPS3 uS3 EABS3S3pS3p RPS3A eS1 EAS1 n/aS3Ae RPS4 (RPS4X, RPS4Y1, RPS4Y2) eS4 EAS4 n/aS4e RPS5 uS7 EABS5S7p S5p RPS6 eS6 EAS6n/aS6e RPS7 eS7 ES7 n/a n/a RPS8 eS8 EAS8 n/a S8e RPS9 uS4 EABS9S4p S4p RPS10 eS10 ES10 n/a n/a RPS11 uS17 EABS11S17p S17p RPS12 eS12 ES12 n/a n/a RPS13 uS15 EABS13S15p S15p RPS14 uS11 EABS14S11p S11p RPS15 uS19 EABS15S19pS19p RPS15A uS8 EABS22S8p S8p RPS16 uS9 EABS16S9p S9p RPS17 eS17 EAS17 n/aS17e RPS18 uS13 EABS18S13pS13p RPS19 eS19 EAS19n/aS19e RPS20 uS10 EABS20S10pS10p RPS21 eS21 ES21n/an/a RPS23 uS12 EABS23S12pS12p RPS24 eS24 EAS24n/aS24e RPS25 eS25 EAS25n/aS25e RPS26 eS26 EAS26n/aS26e RPS27 eS27 EAS27n/aS27e RPS27A eS31 EAS31n/aS27ae RPS28 eS28 EAS28n/aS28e RPS29 uS14 EABS29S14pS14p RPS30 eS30 EAS30n/aS30e RACK1 RACK1 EAsc1n/an/a
See also Eukaryotic large ribosomal subunit (60S)
References
External links Ribosomal Protein Gene Database (RPG)