The eukaryotic initiation factor-4A (eIF4A) family consists of 3 closely related

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

EIF4A1 Eukaryotic initiation factor 4A-I (also known as eIF4A1 or DDX2A) is a 46 kDa cytosolic protein that, in humans, is encoded by the ''EIF4A1'' gene, which is located on chromosome 17. It is the most prevalent member of the eIF4A family of ATP-depe ...

EIF4A2 Eukaryotic initiation factor 4A-II is a protein that in humans is encoded by the ''EIF4A2'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''gene ...

, and

EIF4A3 Eukaryotic initiation factor 4A-III is a protein that in humans is encoded by the ''EIF4A3'' gene. Function This gene encodes a member of the DEAD box protein family. DEAD box proteins, characterized by the conserved motif Asp-Glu-Ala-Asp (DEA ...

. These factors are required for the binding of

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...

40S 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 pa ...

ribosomal 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 for ...

subunits. In addition these proteins are

helicase Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separatin ...

s that function to unwind double-stranded

RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

Background

The mechanisms governing the basic subsistence of

eukaryotic cells Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...

are immensely complex; it is therefore unsurprising that regulation occurs at a number of stages of

protein synthesis Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside Cell (biology), cells, homeostasis, balancing the loss of cellular proteins (via Proteolysis, degradation or Protein targeting, export) through the product ...

– 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. Orthologs 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 and elongation, for example:

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

eggs upon fertilization, rodent brain 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 is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...

, 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 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 ...

binds to the 5’ m7GTP facilitated by a number of proteins; it is at this stage that constrictions born of stress,

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

starvation etc. take effect.

Function

Eukaryotic initiation factor complex 2 (eIF2) forms a

ternary complex A ternary complex is a protein complex containing three different molecules that are bound together. In structural biology, ''ternary complex'' can also be used to describe a crystal containing a protein with two small molecules bound, for example ...

with GTP and the initiator Met-

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 ...

– this process is regulated by guanine nucleotide exchange and

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

and serves as the main

regulatory element A regulatory sequence is a segment of a nucleic acid molecule which is capable of increasing or decreasing the expression of specific genes within an organism. Regulation of gene expression is an essential feature of all living organisms and v ...

of the bottleneck of

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

. 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 Protein secondary structure is the three dimensional conformational isomerism, form of ''local segments'' of proteins. The two most common Protein structure#Secondary structure, secondary structural elements are alpha helix, alpha helices and beta ...

. Binding in this way is facilitated by group 4 eukaryotic initiation factors;

eIF4F Eukaryotic initiation factor 4F (eIF4F) is a heterotrimeric protein complex that binds the 5' cap of messenger RNAs (mRNAs) to promote eukaryotic translation initiation. The eIF4F complex is composed of three non-identical subunits: the DEAD- ...

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;

is responsible for the binding of capped mRNA to the

ribosomal subunit via

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 mRNA cap is bound by

eIF4E Eukaryotic translation initiation factor 4E, also known as eIF4E, is a protein that in humans is encoded by the ''EIF4E'' gene. Structure and function Most eukaryotic cellular mRNAs are blocked at their 5'-ends with the 7-methyl-guanosine fi ...

(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

. For maximal activity; eIF4A also requires

eIF4B Eukaryotic translation initiation factor 4B is a protein that in humans is encoded by the ''EIF4B'' gene. Interactions eIF4B has been shown to interact with and stimulate the activity of eIF4A and bind to the eIF3 complex through the eIF3A subu ...

(80 kDa), which itself is enhanced by

eIF4H Eukaryotic translation initiation factor 4H is a protein that in humans is encoded by the ''EIF4H'' gene. This gene encodes one of the translation initiation factors, which function to stimulate the initiation of protein synthesis at the level of ...

(25 kDa). A study conducted by Bi ''et al.'' in

wheat germ Cereal germ or Wheat 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 ...

seemed to indicate that eIF4A has 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 The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and Archaea and a N-formylmethionine (fMet) in bacteria, mitochondria and plastids. The ...

and translation begins.

Genes

In humans, the gene encoding eIF4A isoform I 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.

Proteins

The 407 residue, 46 kDa, protein eIF4A is the prototypical member of the

DEAD box Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

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 RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). It works by removing all the introns (non-coding regions of RNA) and ''splicing'' b ...

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

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.

References