16.6 Eukaryotic Translational and Post-translational Gene Regulation

Eukaryotic Translation and Initiation Complex

Eukaryotic translation converts the information in mRNA into a functional protein. While the basic steps mirror prokaryotic translation, eukaryotes have additional regulatory checkpoints, especially during initiation. The initiation complex acts as a gatekeeper, controlling which mRNAs get translated and how quickly.

Steps of Eukaryotic Translation

Initiation

1. The 40S ribosomal subunit binds to the 5' cap of the mRNA with the help of eukaryotic initiation factors (eIFs).
2. The initiator tRNA (Met-tRNAi), already loaded with methionine, recognizes and binds to the start codon (AUG) on the mRNA.
3. The 60S ribosomal subunit joins to form the complete 80S initiation complex, and translation can begin.

Elongation

1. Elongation factors (eEFs) help bring the correct aminoacyl-tRNA to the ribosome's A site.
2. Peptidyl transferase (a catalytic function of the ribosome itself) forms a peptide bond between the amino acid in the A site and the growing polypeptide chain in the P site.
3. The ribosome translocates along the mRNA in the 5' to 3' direction, moving to the next codon and repeating the cycle.

Note: Aminoacyl-tRNA synthetases charge each tRNA with its correct amino acid before the tRNA arrives at the ribosome. This happens in the cytoplasm, not on the ribosome itself.

Termination

1. The ribosome encounters a stop codon (UAA, UAG, or UGA), which does not code for any amino acid.
2. Release factors (eRFs) bind to the stop codon in the A site and trigger release of the completed polypeptide.
3. The ribosomal subunits dissociate from the mRNA and can be recycled for future rounds of translation.

The Initiation Complex as a Regulatory Checkpoint

Formation of the initiation complex is the most common point where eukaryotic translation is regulated. Cells control which mRNAs get translated and how fast by modifying the components of this complex.

Key players in initiation complex regulation:

• eIF4F complex (made up of eIF4E, eIF4A, and eIF4G) binds the 5' cap of the mRNA and unwinds secondary structures in the 5' untranslated region (UTR) so the ribosome can scan toward the start codon.
• eIF2 forms a ternary complex with Met-tRNAi and GTP, delivering the initiator tRNA to the 40S subunit at the AUG codon.

Two major ways cells shut down translation initiation:

• Phosphorylation of eIF2α: Stress-activated kinases (such as PKR, PERK, GCN2, and HRI) phosphorylate the alpha subunit of eIF2. This blocks formation of the ternary complex, causing a global decrease in protein synthesis during cellular stress.
• 4E-binding proteins (4E-BPs): These proteins bind and sequester eIF4E, preventing it from joining the eIF4F complex. Without eIF4F, the ribosome can't recognize the mRNA's 5' cap, and translation doesn't start.

Some mRNAs have special sequence features that fine-tune their translation independently of global signals:

• Upstream open reading frames (uORFs) in the 5' UTR can reduce translation of the main coding sequence by diverting ribosomes.
• Internal ribosome entry sites (IRES) allow ribosomes to bind directly to the mRNA without needing the 5' cap, enabling selective translation of certain mRNAs even when cap-dependent translation is shut down (for example, during stress).

Post-translational Gene Regulation in Eukaryotic Cells

Once a protein is synthesized, its job isn't necessarily done being regulated. Post-translational modifications (PTMs) chemically alter proteins after they leave the ribosome, changing their activity, stability, localization, or interactions. This gives cells a way to respond rapidly to signals without needing to make new mRNA.

Post-translational Modification Mechanisms

Phosphorylation

This is one of the most common and well-studied PTMs. Protein kinases add phosphate groups to specific amino acid residues (serine, threonine, or tyrosine) on target proteins. Phosphorylation can either activate or inactivate a protein depending on the target. Protein phosphatases reverse the modification by removing the phosphate group. This on/off switch makes phosphorylation a fast, reversible way to control protein function.

Ubiquitination

Ubiquitin is a small protein (76 amino acids) that gets covalently attached to lysine residues on target proteins through a cascade of three enzymes: E1 (activating), E2 (conjugating), and E3 (ligase).

• Polyubiquitination (a chain of ubiquitin molecules) typically marks a protein for destruction by the 26S proteasome.
• Monoubiquitination (a single ubiquitin) can alter protein function or localization without triggering degradation. For example, monoubiquitination of histones affects chromatin structure and gene expression.

Acetylation

Acetyltransferases add acetyl groups to lysine residues, most notably on histone proteins. Acetylation of histones loosens chromatin and tends to promote gene expression. Deacetylases reverse this modification. Beyond histones, acetylation also regulates non-histone proteins involved in metabolism and signaling.

Methylation

Methyltransferases add methyl groups to lysine or arginine residues, especially on histones. Unlike acetylation, methylation can either activate or repress gene expression depending on which residue is modified and how many methyl groups are added. Demethylases reverse the process, making this another dynamic regulatory mechanism.

Proteolytic Cleavage

Some proteins are synthesized as inactive precursors and only become functional after specific proteases cut them at defined sites. This is an irreversible modification. Examples include:

• Digestive enzymes like trypsinogen, which is cleaved to form active trypsin
• Insulin, which is processed from proinsulin by removal of the C-peptide
• Notch signaling, where cleavage of the Notch receptor releases an intracellular fragment that acts as a transcription factor

Subcellular Localization

Where a protein ends up in the cell matters for what it does. Proteins contain signal sequences that direct them to specific compartments (nucleus, mitochondria, ER, etc.). Post-translational modifications can also redirect a protein to a new location. For example, adding a nuclear localization signal allows a transcription factor to enter the nucleus only when the cell receives the right signal.

Protein Quality Control and Turnover

Cells don't just make proteins and hope for the best. There are systems in place to ensure proteins fold correctly, reach the right destination, and get removed when they're no longer needed.

• Protein folding: Chaperone proteins assist newly synthesized proteins in folding into their correct three-dimensional shape. If a protein misfolds, chaperones can attempt to refold it. Proteins that can't be rescued are targeted for degradation.
• Protein trafficking: Sorting signals and transport vesicles direct proteins to their proper cellular compartments. Correct localization is essential for function.
• Protein turnover: The steady-state level of any protein reflects the balance between its synthesis and degradation. The ubiquitin-proteasome system handles most targeted protein degradation, while lysosomes break down proteins taken in by autophagy or endocytosis. Together, these pathways remove damaged, misfolded, or unnecessary proteins.
• Additional modifications: Other PTMs like glycosylation (adding sugar groups) and lipidation (adding lipid groups) further influence protein stability, function, and membrane association, adding yet another layer of regulation.