Translation

Translation or proteosynthesis is the translation of a nucleotide sequence of mRNA into a sequence of amino acid proteins. The process takes place on ribosomes and individual amino acids are arranged according to the rules of the genetic code.

The following are required for translation:


 * mRNA;
 * tRNA with bound amino acids from the cytoplasm;
 * ribosome components and proteins conditioning individual reactions (eIF, GTP, ATP, etc.).

Prokaryotes vs. Eukaryotes

 * In prokaryotes
 * translation takes place simultaneously with transcription, i.e. translation is already taking place at one end of the nascent mRNA molecule, while transcription is still continuing at the other.


 * In eukaryotes
 * is produced by transcription of hnRNA (pre-mRNA), which is then post-transcriptionally modified. The definitive mRNA molecule is transported from the nucleus to the cytoplasm using transport proteins. Only then do n and parts of the ribosome bind and translation begins.

Proteins that are to remain in the cell are created on free ribosomes, while proteins are synthesized on the ribosomes of the endoplasmic reticulum, which the cell then transports into the extracellular space.

Translation Progress




Several Ribosomes are usually attached to one mRNA molecule in a row, so a polysome is formed. Under optimal conditions, translation takes place at a rate of up to 40 inserted amino acids per second. Less than 1% of amino acids are misclassified.

Pre-initiation process

 * Before translation can begin, amino acids must be activated, for which energy from ATP is used;
 * activated AMKs are then attached to the 3'OH end of their tRNA by aminoacyl-tRNA-synthetase enzymes.

Initiation

 * In eukaryotes, a number of proteins called eukaryotic initiation factors (eIFs, numerically distinguished) are used during translation;
 * proteosynthesis (we are only talking about eukaryotes) is initiated by joining:
 * initiating tRNA (special tRNA carrying AMK Methionine: Met-tRNAiMet);
 * GTP (required energy source);
 * eIF2 (see above) to  the complex;
 * the complex is bound to the small subunit of the (40S) ribosome;
 * then, with the participation of other eIFs, the molecule mRNA is attached to this small ribosome subunit, where its "cap" (7-methyl-guanosine) and the attached "eIF4E and eIF4G" play an important role '';
 * with the help of the energy obtained by splitting ATP, the mRNA molecule moves from the 5' end along the small unit of the ribosome until it encounters the first triplet AUG (triplet for Met) → there is an opening of the reading frame (a mechanism ensuring the reading of information after triplet mRNA bases) and the start of translation;
 * the resulting complex is subsequently connected to the larger subunit of the ribosome with the help of the energy released by GTP cleavage, and at the same time eIF is released;


 * → this is how a complete ribosome is formed, where:
 * Met-tRNAiMet is located at the peptide site (P site).

Elongation

 * The tRNA corresponding to the second mRNA triplet is inserted into the amino acid site (A site) with the help of elongation factor (EFα) and energy from GTP;
 * on the ribosome, two AMKs are connected to their tRNA at the same time;


 * on the ribosome we describe the P (protein) site and the A (amino acid site) site:
 * P site is the binding region for the tRNA carrying the peptide;
 * A site is the region where the new tRNA binds with the new AMK;
 * in the beginning, the tRNA carrying the AMK methionine gets to the P site → with the help of a series of ribosomal peptides, a nucleophilic attack of the amino acid from the A site to the amino acid in the P site of the peptide bond occurs between the carboxyl group of methionine and the amino group of the second AMK (the tRNA of this AMK has bound to the A site) → then the Met is released from its tRNA and at the same time there is a transfer of the second AMK (this AMK is already linked to methionine by a peptide bond) with its tRNA from A to P instead → the whole complex shifts three bases to the 3' end of the mRNA → to A instead, according to the rules of the genetic code, another tRNA is included with its AMK.

Termination

 * the whole process (codon for mRNA - anticodon for tRNA system) is repeated until a stop-codon - termination codon is found on the mRNA molecule ''' (UAA, UAG, UGA);
 * then another protein factor (RF) comes in, which releases the finished polypeptide from the ribosomal complex.

Post-translational modifications
In order for the newly synthesized polypeptide to become functional, it undergoes a series of modifications:


 * a common post-translational modification is the removal of the first methionine from the N terminus of the polypeptide
 * further e.g. covalent attachment of chemical groups and splitting of the polypeptide

Chemical modifications of a protein include

 * methylation
 * phosphorylation
 * acetylation
 * attachment of larger molecular pods
 * lipids
 * oligosaccharides (glycosylation)

Post-translational modifications are related to the function the protein is supposed to perform

 * glycosylation
 * typical of proteins that are secreted from the cell or transported to lysosomes, the Golgi apparatus, or the plasma membrane


 * lipids
 * lipid groups are mainly added to membrane proteins
 * serves to anchor the protein


 * split
 * during cleavage of the polypeptide there may be removal of internal peptides or signal peptides at the N end (methionine)

Proteins that are to be secreted (e.g. hormones) or transported to a certain area of ​​the cell (histones to the nucleus, DNA polymerases as well) must be provided with a certain signal sequence (signal peptide).


 * this signal sequence is called the leader sequence, it consists of 15-30 AMK arranged in a spiral hairpin
 * after transporting the protein to the right place, it is cleaved by a special peptidase

Proteins intended for secretion are first transported to the endoplasmic reticulum (ER) by the signal recognition particle (SRP) - a complex of small cytoplasmic RNAs and proteins


 * this complex binds to the growing polypeptide and the ribosome and via the SRP receptor on the surface of the rough ER (docking protein) enters the ER lumen and then out of cell
 * similarly, other proteins are directed to different target sites through other signal sequences (e.g. nuclear localization signals - transport to the nucleus, lysosomal proteins - transport to the Golgi apparatus and to the lysosome, etc.)

Protein transport

 * many of the polypeptides created by the process of proteosynthesis have their application in a different place than the place of their creation;
 * space endoplasmic reticulum is used for transport;
 * co-translational regression occurs, when at the beginning of translation the signal peptide (containing 15-30 AMK) is conformed into the shape of a spiral hairpin, which is caught in the double layer of the membrane of the endoplasmic reticulum (ER) → then the transport is started;
 * during further translation, this signal peptide is then separated;
 * once it reaches the ER lumen, it is further modified;
 * translation is controlled by SRP (particle recognition signal):
 * it is a complex of 7SL RNA and 6 different proteins;
 * has the ability to bind to the ribosome and stop further translation until it can come into contact with the so-called docking protein that forms part of the membrane ER → thereby frees it from binding to the ribosome and translation can continue.

Related Articles

 * Translation in eukaryotes
 * Translation in prokaryotes
 * DNA
 * Structure of DNA
 * DNA Replication
 * Transcription factors
 * Transcription
 * Post-transcriptional modifications
 * Post-translational modifications
 * RNA
 * mRNA