Eif4a
Encyclopedia
The eukaryotic initiation factor-4A (eIF4A) family consists of 3 closely related protein
s EIF4A1
, EIF4A2
, and EIF4A3
. These factors are required for the binding of mRNA
to 40S
ribosomal
subunits. In addition these proteins are helicase
s which function to unwind double stranded RNA
.
are immensely complex; it is unsurprising therefore that regulation occurs at a number of stages of protein synthesis – the regulation of translation has become a well studied field. Human translational control is of increasing research interest as it has connotations in a range of diseases. Orthologues of many of the factors involved in human translation are shared by a range of eukaryotic organisms; some of which are used as model systems
for the investigation of translation initiation, for example: sea urchin
eggs and rabbit reticulocytes. Monod and Jacob were among the first to propose that “the synthesis of individual proteins may be provoked or suppressed within a cell, under the influence of specific external agents, and the relative rates at which different proteins may be profoundly altered, depending upon external conditions” . Almost half a century after the flurry of postulations arising from the revelation of the central dogma of molecular biology
, of which the preceding supposition by Monod and Jacob is an example; contemporary researchers still have much to learn about the modulation of genetic expression. Synthesis of protein from mature messenger RNA
in eukaryotes is divided into translation initiation, elongation and termination, of these stages; the initiation of translation is the rate limiting step. Within the process of translation initiation; the bottleneck occurs shortly before the ribosome
binds to the 5’ m7GTP facilitated by a number of proteins; it is at this stage that constrictions born of stress, amino acid
starvation etc take effect.
with GTP
and the initiator Met
-tRNA – this process is regulated by guanine nucleotide exchange and phosphorylation
and serves as the main regulatory element
of the bottleneck of protein expression
. Before translation can progress to the elongation stage; a number of initiation factors must facilitate the synergy of the ribosome and the mRNA and ensure that the 5’ UTR of the mRNA is sufficiently devoid of secondary structure
. Binding in this way is facilitated by group 4 eukaryotic initiation factors; eIF4 has implications in the normal regulation of translation as well as the transformation and progression of cancerous cells; as such it represents an interesting field of research.
ribosomal subunit via eIF3. The mRNA cap is bound by eIF4E
(25 kDa), eIF4G (185 kDa) acts as a scaffold for the complex whilst the ATP-dependent RNA helicase eIF4A (46 kDa) processes the secondary structure of the mRNA 5’ UTR to render it more conducive to ribosomal binding and subsequent translation. Together these three proteins are referred to as eIF4F. For maximal activity; eIF4A also requires eIF4B
(80 kDa), which itself is enhanced by eIF4H
(25 kDa). A study conducted by Bi et al. into wheat germ
seemed to indicate that eIF4A had a higher binding affinity for ADP than ATP except in the presence of eIF4B which increased the ATP binding affinity tenfold without affecting ADP affinity. Once bound to the 5’ cap of mRNA, this 48S complex then searches for the (usually) AUG start codon
and translation begins.
has a transcript length of 1741bp, contains 11 exons and is located on chromosome 17. The genes for human isoforms II
and III
reside on chromosomes 3 and 17 respectively.
helicase family, so called due to their conserved four-residue D-E-A-D sequence. This family of helicases is found in a range of prokaryotic and eukaryotic organisms including humans, wherein they catalyse a variety of processes including embryogenesis and RNA splicing
as well as translation initiation. Crystallographic analysis of yeast eIF4A carried out by Carruthers et al. (2000) revealed that the molecule is approximately 80 Å in length and has a “dumbbell” shape where the proximal section represents an 11 residue (18 Å) linker postulated to confer a degree of flexibility and distension to the molecule in solution. eIF4A is an abundant cytoplasmic protein.
Three isoforms of eIF4A exist; I and II share 95% amino acid similarity and have been found simultaneously in rabbit reticulocyte eIF4F in a ratio of 4:1 respectively. The third isoform; eIF4A III, which shares only 65% similarity to the other isoforms is believed to be a core component of the exon junction complex involved in pre-mRNA splicing.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s EIF4A1
EIF4A1
Eukaryotic initiation factor 4A-I is a protein that in humans is encoded by the EIF4A1 gene.-Interactions:EIF4A1 has been shown to interact with EIF4E and Eukaryotic translation initiation factor 4 gamma.-Further reading:...
, EIF4A2
EIF4A2
Eukaryotic initiation factor 4A-II is a protein that in humans is encoded by the EIF4A2 gene.-Further reading:...
, and EIF4A3
EIF4A3
Eukaryotic initiation factor 4A-III is a protein that in humans is encoded by the EIF4A3 gene.-Further reading:...
. These factors are required for the binding of mRNA
Messenger RNA
Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...
to 40S
40S
40S is the small subunit of eukaryotic ribosomes.It interacts with the internal ribosome entry site of the hepatitis C virus.The following is a list of proteins contained in the 40S ribosome:...
ribosomal
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
subunits. In addition these proteins are helicase
Helicase
Helicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands using energy derived from ATP hydrolysis.-Function:Many cellular processes Helicases are a...
s which function to unwind double stranded RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
.
Background
The mechanisms governing the basic subsistence of eukaryotic cellsEukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
are immensely complex; it is unsurprising therefore that regulation occurs at a number of stages of protein synthesis – the regulation of translation has become a well studied field. Human translational control is of increasing research interest as it has connotations in a range of diseases. Orthologues of many of the factors involved in human translation are shared by a range of eukaryotic organisms; some of which are used as model systems
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
for the investigation of translation initiation, for example: sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
eggs and rabbit reticulocytes. Monod and Jacob were among the first to propose that “the synthesis of individual proteins may be provoked or suppressed within a cell, under the influence of specific external agents, and the relative rates at which different proteins may be profoundly altered, depending upon external conditions” . Almost half a century after the flurry of postulations arising from the revelation of the central dogma of molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
, of which the preceding supposition by Monod and Jacob is an example; contemporary researchers still have much to learn about the modulation of genetic expression. Synthesis of protein from mature messenger RNA
Mature messenger RNA
Mature messenger RNA, often abbreviated as mature mRNA is a eukaryotic RNA transcript that has been spliced and processed and is ready for translation in the course of protein synthesis...
in eukaryotes is divided into translation initiation, elongation and termination, of these stages; the initiation of translation is the rate limiting step. Within the process of translation initiation; the bottleneck occurs shortly before the ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
binds to the 5’ m7GTP facilitated by a number of proteins; it is at this stage that constrictions born of stress, amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
starvation etc take effect.
Function
Eukaryotic initiation factor (eIF) complex 2 forms a ternary complexTernary complex
A Ternary complex refers to a protein complex containing three different molecules which are bound together. In structural biology ternary complex can be used to describe a crystal containing a protein with two small molecules bound, for example cofactor and substrate; or a complex formed between...
with GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
and the initiator Met
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
-tRNA – this process is regulated by guanine nucleotide exchange and phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
and serves as the main regulatory element
Regulatory sequence
A regulatory sequence is a segment of DNA where regulatory proteins such as transcription factors bind preferentially. These regulatory proteins bind to short stretches of DNA called regulatory regions, which are appropriately positioned in the genome, usually a short distance 'upstream' of the...
of the bottleneck of protein expression
Protein expression
Protein expression is a subcomponent of gene expression. It consists of the stages after DNA has been translated into polypeptide chains, which are ultimately folded into proteins...
. Before translation can progress to the elongation stage; a number of initiation factors must facilitate the synergy of the ribosome and the mRNA and ensure that the 5’ UTR of the mRNA is sufficiently devoid of secondary structure
Secondary structure
In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...
. Binding in this way is facilitated by group 4 eukaryotic initiation factors; eIF4 has implications in the normal regulation of translation as well as the transformation and progression of cancerous cells; as such it represents an interesting field of research.
Mechanism
The repertoire of compounds involved in eukaryotic translation consists of initiation factor classes 1 – 6; eIF4 is responsible for the binding of capped mRNA to the 40S40S
40S is the small subunit of eukaryotic ribosomes.It interacts with the internal ribosome entry site of the hepatitis C virus.The following is a list of proteins contained in the 40S ribosome:...
ribosomal subunit via eIF3. The mRNA cap is bound by eIF4E
EIF4E
Eukaryotic translation initiation factor 4E, also known as eIF4E, is a protein which in humans is encoded by the eIF4E gene.- Function :...
(25 kDa), eIF4G (185 kDa) acts as a scaffold for the complex whilst the ATP-dependent RNA helicase eIF4A (46 kDa) processes the secondary structure of the mRNA 5’ UTR to render it more conducive to ribosomal binding and subsequent translation. Together these three proteins are referred to as eIF4F. For maximal activity; eIF4A also requires eIF4B
EIF4B
Eukaryotic translation initiation factor 4B is a protein that in humans is encoded by the EIF4B gene.-External links:* from Nature Reviews Microbiology. A good image and overview of the function of initiation factors-Further reading:...
(80 kDa), which itself is enhanced by eIF4H
EIF4H
Eukaryotic translation initiation factor 4H is a protein that in humans is encoded by the EIF4H gene.EIF4H appears analogous to drr-2 in C. elegans which regulates the mTOR pathway and affects longevity.-Further reading:...
(25 kDa). A study conducted by Bi et al. into wheat germ
Cereal germ
The germ of a cereal is the reproductive part that germinates to grow into a plant; it is the embryo of the seed. Along with bran, germ is often a by-product of the milling that produces refined grain products. Cereal grains and their components, such as wheat germ, rice bran, and maize may be used...
seemed to indicate that eIF4A had a higher binding affinity for ADP than ATP except in the presence of eIF4B which increased the ATP binding affinity tenfold without affecting ADP affinity. Once bound to the 5’ cap of mRNA, this 48S complex then searches for the (usually) AUG start codon
Start codon
The start codon is generally defined as the point, sequence, at which a ribosome begins to translate a sequence of RNA into amino acids.When an RNA transcript is "read" from the 5' carbon to the 3' carbon by the ribosome the start codon is the first codon on which the tRNA bound to Met,...
and translation begins.
Genes
In humans the gene encoding eIF4A isoform IEIF4A1
Eukaryotic initiation factor 4A-I is a protein that in humans is encoded by the EIF4A1 gene.-Interactions:EIF4A1 has been shown to interact with EIF4E and Eukaryotic translation initiation factor 4 gamma.-Further reading:...
has a transcript length of 1741bp, contains 11 exons and is located on chromosome 17. The genes for human isoforms II
EIF4A2
Eukaryotic initiation factor 4A-II is a protein that in humans is encoded by the EIF4A2 gene.-Further reading:...
and III
EIF4A3
Eukaryotic initiation factor 4A-III is a protein that in humans is encoded by the EIF4A3 gene.-Further reading:...
reside on chromosomes 3 and 17 respectively.
Proteins
The 407 residue, 46 kDa, protein eIF4A is the prototypical member of the DEAD boxDEAD box
DEAD box proteins are involved in an assortment of metabolic processes that involve RNA. They are highly conserved in nine domains and can be found in both prokaryotes and eukaryotes, but not all...
helicase family, so called due to their conserved four-residue D-E-A-D sequence. This family of helicases is found in a range of prokaryotic and eukaryotic organisms including humans, wherein they catalyse a variety of processes including embryogenesis and RNA splicing
Splicing (genetics)
In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation...
as well as translation initiation. Crystallographic analysis of yeast eIF4A carried out by Carruthers et al. (2000) revealed that the molecule is approximately 80 Å in length and has a “dumbbell” shape where the proximal section represents an 11 residue (18 Å) linker postulated to confer a degree of flexibility and distension to the molecule in solution. eIF4A is an abundant cytoplasmic protein.
Three isoforms of eIF4A exist; I and II share 95% amino acid similarity and have been found simultaneously in rabbit reticulocyte eIF4F in a ratio of 4:1 respectively. The third isoform; eIF4A III, which shares only 65% similarity to the other isoforms is believed to be a core component of the exon junction complex involved in pre-mRNA splicing.