7.5 Translation
5 min read•august 22, 2024
Translation is the final step in gene expression, where mRNA is decoded to produce proteins. This process involves ribosomes, tRNAs, and various factors working together to build polypeptide chains based on the genetic code.
The translation machinery assembles at the start codon, elongates the protein by adding amino acids, and terminates at a stop codon. Post-translational modifications further diversify protein function, highlighting the complexity of gene expression and regulation.
Translation Process
Initiation
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- Translation is the process by which the genetic information encoded in mRNA is used to synthesize a polypeptide chain
- Initiation of translation involves the assembly of the translation machinery at the start codon (AUG) of the mRNA, which is recognized by the initiator tRNA carrying the amino acid methionine
- Eukaryotic initiation factors (eIFs) and prokaryotic initiation factors (IFs) help recruit the small ribosomal subunit to the mRNA and facilitate the binding of the initiator tRNA
- The large ribosomal subunit joins the complex, forming the complete ribosome ready for elongation (example: 80S ribosome in eukaryotes, 70S ribosome in prokaryotes)
Elongation and Termination
- Elongation is the process of adding amino acids to the growing polypeptide chain based on the codons in the mRNA
- The ribosome has three sites: A (aminoacyl), P (peptidyl), and E (exit) sites, which accommodate tRNAs during the elongation process
- Elongation factors (EFs) in prokaryotes and eukaryotic elongation factors (eEFs) in eukaryotes assist in the binding of aminoacyl-tRNAs and the translocation of the ribosome along the mRNA (example: EF-Tu and EF-G in prokaryotes, eEF1A and eEF2 in eukaryotes)
- Termination occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) in the mRNA, signaling the end of the polypeptide chain
- Release factors (RFs in prokaryotes and eRFs in eukaryotes) recognize the stop codons and facilitate the release of the completed polypeptide chain from the ribosome
- The ribosomal subunits dissociate from the mRNA and are recycled for future rounds of translation
Components of Translation Machinery
Ribosomes and tRNAs
- Ribosomes are the primary organelles responsible for protein synthesis, consisting of a small and a large subunit
- Prokaryotic ribosomes (70S) are composed of 30S and 50S subunits, while eukaryotic ribosomes (80S) are composed of 40S and 60S subunits
- Ribosomes contain ribosomal RNA (rRNA) and proteins, which contribute to their structure and function
- Transfer RNAs (tRNAs) are adapter molecules that bridge the genetic code in mRNA to the amino acid sequence of proteins
- tRNAs have a cloverleaf secondary structure and an L-shaped tertiary structure, with an anticodon loop that base-pairs with the mRNA codon and an acceptor stem that carries the corresponding amino acid
- Each tRNA is specific for a particular amino acid, and the anticodon is complementary to the mRNA codon that specifies that amino acid (example: tRNA-Phe with anticodon GAA for phenylalanine, tRNA-Leu with anticodon CAA for leucine)
Aminoacyl-tRNA Synthetases
- Aminoacyl-tRNA synthetases (aaRSs) are enzymes that catalyze the attachment of amino acids to their cognate tRNAs, ensuring the fidelity of protein synthesis
- There are 20 different aaRSs, each specific for one amino acid and its corresponding tRNAs
- The aminoacylation reaction occurs in two steps: the activation of the amino acid with ATP and the transfer of the amino acid to the 3' end of the tRNA
- Examples of aaRSs include leucyl-tRNA synthetase (LeuRS), tyrosyl-tRNA synthetase (TyrRS), and glutamyl-tRNA synthetase (GluRS)
Genetic Code and Properties
Codon-Amino Acid Correspondence
- The genetic code is the set of rules that defines the correspondence between the nucleotide sequence of mRNA and the amino acid sequence of proteins
- The genetic code is a triplet code, meaning that each amino acid is specified by a sequence of three nucleotides (a codon) in the mRNA
- The genetic code is degenerate or redundant, meaning that multiple codons can specify the same amino acid (synonymous codons)
- 61 codons specify 20 amino acids, while 3 codons (UAA, UAG, and UGA) serve as stop codons, signaling the end of the polypeptide chain
Universality and Reading Frame
- The genetic code is nearly universal, meaning that it is used by almost all living organisms, with only minor variations in some species (mitochondrial genetic code)
- The genetic code is non-overlapping, meaning that each nucleotide in the mRNA is part of only one codon (except in some cases of programmed ribosomal frameshifting)
- The genetic code is comma-less, meaning that there are no punctuation marks between codons, and the reading frame is determined by the start codon (AUG for methionine)
Post-Translational Modifications
Types and Functions of PTMs
- Post-translational modifications (PTMs) are chemical alterations of proteins that occur after their synthesis on the ribosome, expanding the diversity of the proteome and regulating protein function
- PTMs can affect protein folding, stability, localization, activity, and interactions with other molecules, thereby modulating their roles in various cellular processes
- Common types of PTMs include:
- Phosphorylation: the addition of a phosphate group to serine, threonine, or tyrosine residues, often regulating enzyme activity and protein-protein interactions (example: phosphorylation of kinases in signaling cascades)
- Glycosylation: the attachment of carbohydrate moieties to proteins, affecting their stability, solubility, and interactions with other molecules (example: glycosylation of cell surface receptors)
- Ubiquitination: the covalent attachment of ubiquitin to lysine residues, targeting proteins for degradation by the proteasome or altering their function (example: ubiquitination of cell cycle regulators)
- Acetylation: the addition of an acetyl group to lysine residues, often regulating gene expression by modifying histones and transcription factors (example: acetylation of histones in chromatin remodeling)
- Methylation: the addition of methyl groups to lysine or arginine residues, modulating protein-protein interactions and gene expression (example: methylation of histones in epigenetic regulation)
Regulation and Dysregulation of PTMs
- PTMs are reversible and dynamically regulated by specific enzymes (kinases, phosphatases, transferases) in response to various cellular signals and stimuli
- Dysregulation of PTMs can lead to various diseases, including cancer, neurodegeneration, and metabolic disorders, making them important targets for therapeutic intervention
- Examples of PTM dysregulation in diseases:
- Hyperphosphorylation of tau protein in Alzheimer's disease
- Aberrant glycosylation patterns in cancer cells
- Impaired ubiquitination of misfolded proteins in neurodegenerative disorders