Translation and Post-Translational Modifications

Core Explanation

The Genetic Code and tRNA Charging

The genetic code is nearly universal, read in triplets (codons), and degenerate — most amino acids are specified by multiple codons. The wobble hypothesis (Crick, 1966) explains how a single tRNA can recognize multiple codons: the third codon position can form non-Watson-Crick base pairs with the first anticodon position. Inosine (I), a modified adenosine, is common at the wobble position and can pair with U, C, or A.

Aminoacyl-tRNA synthetases (aaRSs) are the true translators of the genetic code — they couple each amino acid to its cognate tRNA(s). This is a two-step reaction requiring ATP:

1. Amino acid + ATP → aminoacyl-AMP + PPᵢ (activation)
2. Aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP (transfer)

aaRSs achieve extraordinary specificity (~1 error per 10⁴ charging events) through a double-sieve mechanism: the synthetic active site excludes amino acids larger than the cognate substrate, while a separate editing site (present in Class I synthetases) hydrolyzes incorrectly charged amino acids that are smaller than the cognate. This double discrimination is essential — valine and isoleucine differ by a single methylene group, yet IleRS mischarges Val only once per ~40,000 events.

Ribosome Structure

The eukaryotic 80S ribosome comprises a 60S large subunit and 40S small subunit:

The peptidyl transferase center (PTC) is composed entirely of 28S rRNA — it is a ribozyme. No ribosomal protein contributes chemically to peptide bond formation. The PTC positions the aminoacyl-tRNA in the A site and the peptidyl-tRNA in the P site such that the α-amino group of the A-site amino acid attacks the ester carbonyl of the P-site peptidyl-tRNA. The reaction is entropically driven and does not require ATP or GTP at this step.

Three tRNA-binding sites span both subunits:

• A (aminoacyl) site: binds incoming aminoacyl-tRNA.
• P (peptidyl) site: holds the peptidyl-tRNA (the growing chain).
• E (exit) site: holds the deacylated tRNA before release.

The Translation Cycle

Initiation

Eukaryotic translation initiation is the most complex and highly regulated step. It requires at least 12 eukaryotic initiation factors (eIFs).

1. 43S preinitiation complex formation: eIF1, eIF1A, eIF3, eIF5, and eIF2–GTP–Met-tRNAᵢ (ternary complex) bind the 40S subunit.
2. mRNA activation: eIF4F complex (eIF4E cap-binding protein + eIF4A helicase + eIF4G scaffold) binds the m⁷G cap. PABPC1 at the poly(A) tail binds eIF4G, circularizing the mRNA.
3. Recruitment: eIF3 on the 40S subunit binds eIF4G, recruiting the 43S complex to the 5′ end of the mRNA.
4. Scanning: The 43S complex scans 5′→3′ for the start codon (AUG) within a Kozak consensus context (GCCRCCAUGG). Scanning requires ATP (eIF4A helicase unwinds 5′ UTR secondary structure).
5. Start codon recognition: Base pairing between the AUG and Met-tRNAᵢ anticodon triggers eIF1 release and eIF5-mediated GTP hydrolysis by eIF2. eIF2–GDP dissociates.
6. 60S joining: eIF5B–GTP promotes 60S subunit joining, forming the 80S initiation complex. eIF5B hydrolyzes GTP, and all remaining initiation factors dissociate. Translation-competent 80S ribosome is now assembled with Met-tRNAᵢ in the P site.

Regulation: eIF2α phosphorylation (by PERK, PKR, GCN2, or HRI in response to ER stress, viral infection, amino acid starvation, or heme deficiency) inhibits eIF2B (GEF), reducing ternary complex formation and globally suppressing translation — part of the integrated stress response (ISR). mTORC1 phosphorylates 4E-BPs, causing their dissociation from eIF4E and allowing cap-dependent translation; rapamycin inhibits mTORC1 → 4E-BP hypophosphorylation → eIF4E sequestration → reduced translation of growth-promoting mRNAs.

Elongation

Elongation proceeds through repeated cycles, each consuming two GTP molecules:

1. Decoding: eEF1A–GTP delivers aminoacyl-tRNA to the A site. The decoding center monitors the codon–anticodon helix, specifically checking the first two base pairs for Watson-Crick geometry (induced fit). Correct pairing triggers GTP hydrolysis by eEF1A, and eEF1A–GDP dissociates. The aminoacyl-tRNA is accommodated into the A site.
2. Peptide bond formation: The PTC catalyzes nucleophilic attack by the A-site aminoacyl-tRNA's α-amino group on the P-site peptidyl-tRNA's ester carbonyl. The peptide chain is transferred to the A-site tRNA, elongating by one amino acid. The P-site tRNA is now deacylated.
3. Translocation: eEF2–GTP (eEF2 is the eukaryotic homolog of bacterial EF-G) binds and hydrolyzes GTP, driving the ribosome to move exactly three nucleotides along the mRNA. The peptidyl-tRNA moves from A→P site, deacylated tRNA from P→E site, and the A site is vacated for the next round.

Elongation rate: ~4–6 amino acids per second in eukaryotes. The ribosome is processive — it typically translates the entire ORF without dissociating.

Termination

When a stop codon (UAA, UAG, or UGA) enters the A site:

1. eRF1 (eukaryotic release factor 1) recognizes all three stop codons. eRF1 is a structural mimic of tRNA — its N-terminal domain reads the stop codon while a conserved GGQ motif reaches into the PTC.
2. eRF3–GTP binds eRF1. GTP hydrolysis by eRF3 promotes eRF1 repositioning.
3. The GGQ motif coordinates a water molecule in the PTC, hydrolyzing the ester bond between the polypeptide and the P-site tRNA. The completed polypeptide is released.
4. ABCE1 (ATP-binding cassette protein) uses ATP hydrolysis to split the ribosomal subunits, recycling them for new rounds of initiation.

Protein Folding

The nascent polypeptide begins folding co-translationally within the ribosomal exit tunnel, which is ~100 Å long and ~10–20 Å wide. The tunnel can accommodate limited α-helix formation but not tertiary structure. Upon emergence, the polypeptide is met by molecular chaperones.

Chaperone Systems

Hsp70 system (Hsp70 + Hsp40 co-chaperone + nucleotide exchange factor):

• Hsp70 (ATP-bound) has low substrate affinity; Hsp40 delivers the substrate and stimulates ATP hydrolysis → Hsp70 (ADP-bound) has high affinity for exposed hydrophobic patches.
• Nucleotide exchange factors (NEFs; e.g., BAG-1, Hsp110) catalyze ADP→ATP exchange, releasing the substrate.
• Repeated cycles of binding and release prevent aggregation and provide opportunities for folding.

Chaperonin system (TRiC/CCT in eukaryotes; GroEL/GroES in bacteria):

• TRiC/CCT is a large double-ring complex (~1 MDa) that encapsulates the substrate (~60 kDa capacity) in a central chamber, providing an isolated environment where folding can proceed without aggregation.
• ATP binding and hydrolysis drive conformational changes that alter the chamber's chemical environment, promoting folding. Substrates include actin, tubulin, and other obligate chaperonin clients.

Hsp90: acts downstream of Hsp70, binding partially folded clients (kinases, steroid hormone receptors, telomerase) and stabilizing them in a near-native, activatable state. Hsp90 inhibitors (geldanamycin, 17-AAG) are anti-cancer agents because many oncogenic kinases depend on Hsp90.

Quality Control and Degradation

Terminally misfolded proteins are targeted for degradation:

• Ubiquitin–proteasome system (UPS): E3 ligases ubiquitinate misfolded proteins, targeting them to the 26S proteasome.
• ER-associated degradation (ERAD): Misfolded ER proteins are retrotranslocated to the cytosol and degraded.
• Autophagy: large aggregates and organelles are degraded via the lysosome.
• Unfolded protein response (UPR) sensors (IRE1, PERK, ATF6) detect ER stress and signal to expand ER folding capacity or trigger apoptosis if stress is irremediable.

Post-Translational Modifications (PTMs)

PTMs covalently modify amino acid side chains or the polypeptide backbone, regulating protein activity, localization, stability, and interactions. Over 400 known PTM types exist.

Phosphorylation in Detail

Phosphorylation is the most prevalent and best-studied PTM. The human kinome comprises ~518 protein kinases. Phosphorylation adds a bulky, doubly negatively charged group, which can:

• Allosterically activate/inactivate enzymes (e.g., glycogen phosphorylase activation by phosphorylase kinase).
• Create docking sites for phospho-binding domains: SH2 domains (bind pTyr), 14-3-3 proteins (bind pSer/pThr in specific contexts), WW, FHA, and BRCT domains.
• Induce conformational change (e.g., phosphorylation of the activation loop of many kinases reorients the catalytic residues).
• Target proteins for degradation (e.g., phosphorylation of IκBα → ubiquitination → degradation → NF-κB release).

Ubiquitination in Detail

Ubiquitin is a 76-residue protein ligated via its C-terminal Gly to a substrate Lys ε-amino group. The cascade:

• E1 (ubiquitin-activating enzyme): uses ATP to adenylate ubiquitin, forming a thioester bond with ubiquitin.
• E2 (ubiquitin-conjugating enzyme): accepts ubiquitin from E1 via transthioesterification.
• E3 (ubiquitin ligase): transfers ubiquitin from E2 to substrate Lys. E3s (~600+ in humans) provide substrate specificity. RING E3s catalyze direct transfer; HECT and RBR E3s form a ubiquitin–E3 thioester intermediate.

Ubiquitin itself has seven Lys residues, enabling chain diversity. K48-linked chains (≥4 ubiquitins) target substrates to the 26S proteasome. K63-linked chains function in signaling (NF-κB activation, DNA repair). Linear (M1-linked) chains are generated by the LUBAC complex and regulate NF-κB.

Experimental Evidence

• Nirenberg and Matthaei (1961): Poly-U RNA directed synthesis of poly-phenylalanine in a cell-free translation system, cracking the first codon (UUU = Phe). Nobel Prize, 1968.
• In vitro reconstitution of translation: Purified ribosomes, tRNAs, aminoacyl-tRNA synthetases, initiation/elongation/termination factors, and mRNA are sufficient to synthesize protein, demonstrating that all necessary components have been identified.
• Puromycin: An aminoacyl-tRNA analog that enters the A site and forms a peptide bond with the P-site peptidyl-tRNA, causing premature termination. Puromycin's mechanism proved that peptide bond formation occurs at the PTC.
• Ribosome high-resolution X-ray crystallography, particularly of the 50S subunit (Steitz, Moore) and 30S subunit (Ramakrishnan, Yonath), revealed that the PTC is an RNA-only active site — a ribozyme. Nobel Prize in Chemistry, 2009.
• Folding reporter assays: GFP fused to a protein of interest — only correctly folded GFP fluoresces, providing a direct readout of folding efficiency.
• ΔF508 CFTR: The most common cystic fibrosis mutation deletes Phe508, causing a folding defect. At permissive temperature (27°C), ΔF508 CFTR folds and reaches the surface, demonstrating that the mutation impairs the folding pathway, not the activity of folded CFTR. This insight led to corrector drug development (lumacaftor, tezacaftor, elexacaftor).

Disease and Clinical Connections

High-Yield Summary

• Ribosome: 80S = 40S (decoding) + 60S (PTC). PTC is a ribozyme (28S rRNA). A, P, E sites.
• aaRSs charge tRNAs: double-sieve editing. ATP-dependent.
• Initiation: 43S PIC → cap binding (eIF4F) → scanning (Kozak) → AUG recognition → 60S joining. GTP-dependent (eIF2, eIF5B). eIF2α phosphorylation = global translational control (ISR).
• Elongation: eEF1A–GTP delivers aa-tRNA → decoding → peptide bond (PTC) → eEF2–GTP translocation. 2 GTP/cycle.
• Termination: eRF1 recognizes stop → eRF3–GTP → peptide hydrolysis → ABCE1 splits ribosome.
• Folding: co-translational. Hsp70/Hsp40 (prevent aggregation), chaperonins (TRiC; encapsulated folding), Hsp90 (late-stage maturation).
• PTMs: phosphorylation (kinase/phosphatase switch), ubiquitination (E1–E2–E3 cascade; K48 = degradation, K63 = signaling), glycosylation (ER N-linked, Golgi O-linked), lipidation (membrane targeting), proteolytic cleavage (zymogen activation).
• PTMs regulate activity, localization, stability, interactions, and degradation.

Practice Questions

1. A patient harbors a mutation in eIF2B (the GEF for eIF2) that reduces its activity by 50%. What cellular condition does this mimic, and what would be the predicted effect on global translation?

Answer: Reduced eIF2B activity mimics eIF2α phosphorylation, which inhibits eIF2B. Normally, eIF2α phosphorylation occurs during the integrated stress response (ISR): PERK activation during ER stress, PKR during viral infection, GCN2 during amino acid starvation, and HRI during heme deficiency. In all cases, phosphorylated eIF2α binds eIF2B tightly, sequestering it and reducing eIF2–GTP–Met-tRNAᵢ ternary complex levels. With reduced eIF2B activity, even basal eIF2α phosphorylation would excessively suppress ternary complex formation, causing global translational attenuation and activation of the ISR transcriptional program (ATF4 target genes). This is the molecular basis of vanishing white matter disease (childhood ataxia with CNS hypomyelination, CACH/VWM) — eIF2B mutations cause constitutive ISR activation in oligodendrocytes and astrocytes.

2. Bortezomib, a proteasome inhibitor, is effective in multiple myeloma. Why are plasma cells (antibody-secreting B cells) selectively sensitive to proteasome inhibition?

Answer: Plasma cells are professional secretory cells — they synthesize enormous quantities of immunoglobulins. A significant fraction of nascent immunoglobulin heavy chains fail to fold properly, especially before light chain pairing. These misfolded proteins are constitutively retrotranslocated from the ER and degraded by the proteasome via ERAD. Blocking the proteasome causes accumulation of misfolded immunoglobulins in the ER, triggering the terminal unfolded protein response (UPR) — sustained PERK/eIF2α signaling and IRE1 activation leading to CHOP-mediated apoptosis. Non-secretory cells have lower ER protein flux and are less dependent on constitutive ERAD, making them relatively resistant. This differential sensitivity is the therapeutic window.

3. The H3K27M mutation in histone H3.3 dominantly inhibits Polycomb Repressive Complex 2 (PRC2). What type of post-translational modification is PRC2's normal catalytic function, on what residue does it act, and why does a single M-for-K substitution have dominant-negative effects?

Answer: PRC2 (specifically its EZH2 catalytic subunit) is a histone methyltransferase — it catalyzes the addition of methyl groups (a PTM) to lysine 27 of histone H3 (H3K27), generating H3K27me1/me2/me3. The H3K27M mutation substitutes methionine for lysine at position 27. The methionine side chain is hydrophobic and is not a methylation substrate. Critically, the H3K27M mutant histone is incorporated into nucleosomes and binds the PRC2 active site with higher affinity than wild-type H3, but cannot be methylated. This sequesters PRC2 in an unproductive complex, globally reducing H3K27me3 levels. Only ~3–17% of total H3 carries the mutation, yet H3K27me3 is nearly abolished — this is why a single-copy heterozygous mutation can have such a profound dominant effect. The resulting epigenetic dysregulation drives pediatric diffuse intrinsic pontine glioma (DIPG).

Common Misconceptions

"The ribosome is a passive platform; proteins do the chemistry." No. The peptidyl transferase center is composed entirely of rRNA — it is a ribozyme. Ribosomal proteins provide structural scaffolding and assist in assembly but do not catalyze peptide bond formation. This is a foundational argument for the RNA world hypothesis.

"Chaperones fold proteins." Chaperones do not impart folding information — the amino acid sequence determines the folded structure (Anfinsen's dogma). Chaperones prevent off-pathway aggregation (kinetic partitioning), providing the polypeptide repeated opportunities to reach its native state. Chaperonins (TRiC/GroEL) provide an isolated environment but the folding is still spontaneous.

"Ubiquitination = degradation." This is true only for K48-linked polyubiquitin chains. K63-linked chains function in signaling (NF-κB, DNA repair, endocytosis). Monoubiquitination regulates histones and membrane protein trafficking. SUMOylation, a related modification, often antagonizes ubiquitination and has distinct functions. The ubiquitin code is complex and context-dependent.