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 coefficient
The sedimentation coefficient () of a particle characterizes its sedimentation during centrifugation. It is defined as the ratio of a particle's sedimentation velocity to the applied acceleration causing the sedimentation.
: s = \frac
The sedime ...
s 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 Eukaryotic initiation factors (eIFs) are proteins or protein complexes involved in the initiation phase of eukaryotic translation. These proteins help stabilize the formation of ribosomal preinitiation complexes around the start codon and are an i ...
, including
eIF1,
eIF1A
Eukaryotic translation initiation factor 1A, X-chromosomal (eIF1A) is a protein that in humans is encoded by the ''EIF1AX'' gene. This gene encodes an essential eukaryotic translation initiation factor. The protein is a component of the 43S pre-in ...
, and
eIF3
Eukaryotic initiation factor 3 (eIF3) is a multiprotein complex that functions during the initiation phase of eukaryotic translation. It is essential for most forms of Eukaryotic translation#Cap-dependent initiation, cap-dependent and Eukaryotic t ...
.
The 40S ribosomal subunit is also tightly bound by the
HCV IRES
The Hepatitis C virus internal ribosome entry site, or HCV IRES, is an RNA structure within the 5'UTR of the HCV genome that mediates cap-independent translation initiation.
Protein translation of most eukaryotic mRNAs occurs by a cap-dependen ...
to form a binary complex mediate by protein-mRNA and rRNA-mRNA interactions.
More information can be found in the articles on the
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
, the
eukaryotic ribosome (80S)
Ribosomes are a large and complex molecular machine that catalyzes the synthesis of proteins, referred to as translation. The ribosome selects aminoacylated transfer RNAs (tRNAs) based on the sequence of a protein-encoding messenger RNA (mRNA) an ...
, and the article on
protein translation
In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire process is ...
.
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
Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
, the A-site, P-site and E-site (see article on
protein translation
In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire process is ...
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
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
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
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).
See also
*
Eukaryotic large ribosomal subunit (60S) Ribosomal particles are denoted according to their sedimentation coefficients in Svedberg units. The 60S subunit is the large subunit of eukaryotic 80S ribosomes. It is structurally and functionally related to the 50S subunit of 70S prokaryotic ...
References
External links
Ribosomal Protein Gene Database (RPG)
{{Ribosome subunits
Ribosomal RNA