Eukaryotic Small Ribosomal Subunit (40S)

Ribosomal particles are denoted according to their sedimentation coefficients in Svedberg units. The 40S subunit is the small subunit of eukaryotic 80S ribosomes. 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.

Function

The 40S subunit contains the decoding center which monitors the complementarity of tRNA and mRNA in protein translation. It is the target of most eukaryotic initiation factors and also interacts with the internal ribosome entry site of the hepatitis C virus.^[6] More information can be found in the articles on the 80S eukaryotic ribosome, the ribosome in general 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 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.

Crystal Structure of the Eukaryotic 40S Ribosomal Subunit from T. thermophila
40S subunit viewed from the subunit interface side, PDB identifier 2XZM  40S subunit viewed from the solvent-exposed side, PDB identifier 2XZM

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.^[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.^[7] Further information can be found in the ribosomal protein gene database (RPG).^[7]

40S ribosomal proteins

Structure (Eukaryotic)[8]	H. sapiens[7][9]	Conservation[10]	S. cerevisiae[11]	Bacterial homolog (E. coli)	Archaeal homolog
	RPSA	EAB	S0	S2p	S2
	RPS2	EAB	S2	S5p	S5p
	RPS3	EAB	S3	S3p	S3p
	RPS3A	EA	S1	n/a	S3Ae
	RPS4	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

External links

• Ribosomal Protein Gene Database (RPG)

References

1. ↑ 40S Ribosomal Subunits at the US National Library of Medicine Medical Subject Headings (MeSH)
2. ↑ 2.0 2.1 Rabl J, Leibundgut M, Ataide SF, Haag A, Ban N. Crystal structure of the eukaryotic 40S ribosomal subunit in complex with initiation factor 1. Science. 2011 Feb 11;331(6018):730-6. doi: 10.1126/science.1198308. Epub 2010 Dec 23. PubMed PMID 21205638.
3. ↑ Ben-Shem A, Garreau de Loubresse N, Melnikov S, Jenner L, Yusupova G, Yusupov M. The structure of the eukaryotic ribosome at 3.0 Å resolution. Science. 2011 Dec 16;334(6062):1524-9. doi: 10.1126/science.1212642. Epub 2011 Nov 17. PubMed PMID 22096102.
4. ↑ Wimberly BT, Brodersen DE, Clemons WM Jr, Morgan-Warren RJ, Carter AP, Vonrhein C, Hartsch T, Ramakrishnan V. Structure of the 30S ribosomal subunit. Nature. 2000 Sep 21;407(6802):327-39. PubMed PMID 11014182.
5. ↑ Schmeing TM, Ramakrishnan V. What recent ribosome structures have revealed about the mechanism of translation. Nature. 2009 Oct 29;461(7268):1234-42. doi:10.1038/nature08403. Epub 2009 Oct 18. Review. PubMed PMID 19838167.
6. ↑ 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): 1045–55. doi:10.1017/S1355838202029965. PMC 1370315. PMID 12212848. 
7. ↑ 7.0 7.1 7.2 Nakao A, Yoshihama M, Kenmochi N. RPG: the Ribosomal Protein Gene database. Nucleic Acids Res. 2004 Jan 1;32(Database issue):D168-70. PubMed PMID 14681386; PubMed Central PMCID: PMC308739.
8. ↑ Structure of the T. thermophila,' proteins from the structures of the large subunit PDBS 417, 4A19 and small subunit PDB 2XZM
9. ↑ Nomenclature according to the ribosomal protein gene database, applies to H. sapiens and T. thermophila
10. ↑ EAB means conserved in eukaryotes, archaea and bacteria, EA means conserved in eukaryotes and archaea and E means eukaryote-specific protein
11. ↑ 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.