26.3: Translational Regulation and Protein Degradation

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Learning Goals 

 

(Learning goals written by Claude, Sonnet 4.6, Anthropic)

Regulation of Translation: Ribosome Heterogeneity and mRNA Sequence Effects

• Explain the ribosome filter hypothesis: describe how ribosome composition varies between cell types and developmental states through differential expression of ribosomal protein (RP) paralogs, post-translational modifications of RPs (particularly phosphorylation), and tissue-specific patterns of rRNA modification (including 2′-OH methylation and pseudouridylation guided by snoRNAs)—and explain how these compositional differences may selectively modulate the translation of specific mRNA subsets rather than affecting global protein synthesis uniformly.
• Explain how miRNA-mediated RNA interference regulates translation: describe how pri-miRNAs are exported from the nucleus by Exportin-5, processed by Dicer to remove the hairpin loop, incorporated into RISC complexes with Argonaute proteins, and how the resulting miRNA:RISC complex base-pairs with complementary sequences in target mRNA 3′ UTRs to block translation and/or accelerate poly(A) tail deadenylation and mRNA degradation—and explain why this mechanism provides a post-transcriptional layer of gene expression control responsive to developmental and environmental signals.
• Describe how nascent peptide sequences influence ribosome elongation efficiency: explain how the ribosomal exit tunnel (~100 Å long, accommodating 30–60 amino acids) actively participates in translation by allowing specific peptide sequences to interact with tunnel walls and cause ribosome stalling, how proline-rich sequences stall elongation by destabilizing the P-site tRNA conformation in the PTC (rescued by EF-P in bacteria, which stabilizes the P-site tRNA), and how the macrolide class of antibiotics (e.g., erythromycin) binds within the exit tunnel at a constriction site formed by uL4 and uL22 to block protein synthesis.

The Ubiquitin-Proteasome System: Protein Tagging and Degradation

• Describe the three-enzyme cascade that targets proteins for proteasomal degradation: explain how E1 ubiquitin-activating enzyme first adenylates ubiquitin's C-terminal Gly76 carboxylate using ATP (forming the aminoacyl-AMP intermediate), then forms a high-energy E1-ubiquitin thioester with its active-site Cys; how the ubiquitin is transesterified to an E2 ubiquitin-conjugating enzyme Cys; and how E3 ubiquitin ligases (of which ~600–700 exist in humans, representing ~5% of the genome) direct substrate specificity by transferring ubiquitin from E2 to the ε-amino group of a substrate lysine via an isopeptide bond—and distinguish monoubiquitination, multi-monoubiquitination, and polyubiquitination (homogeneous vs. heterogeneous chains) and their different functional outcomes.
• Describe the architecture and catalytic mechanism of the 26S proteasome: explain how the 20S core particle consists of four stacked heptameric rings (α₁-α₇/β₁-β₇/β₁-β₇/α₁-α₇), how the α-rings regulate substrate access through hydrophobic loops that gate the barrel entrance (opened by 19S or PA28 activator caps), and how the three catalytic β-subunits (β1 with caspase-like/acidic residue specificity, β2 with trypsin-like/basic residue specificity, β5 with chymotrypsin-like/hydrophobic residue specificity) use their autocatalytically exposed N-terminal Thr as both nucleophile (hydroxyl) and catalytic base (amino group) to cleave peptide bonds—and describe how the 19S regulatory cap receives, deubiquitinates, unfolds, and translocates polyubiquitinated substrates into the 20S barrel using ATP hydrolysis.

Regulation of Translation

Heterogeneity of Ribosome Structure

Over the years, numerous studies in eukaryotes have provided evidence that ribosomes can vary in their protein and rRNA complements across different cell types and developmental states. These observations culminated in the postulation of the "ribosome filter hypothesis" by Mauro and Edelman in 2002. The authors propose that the ribosome composition functions as a determinant of translation. Depending on the RPs and rRNA sequences represented in the respective ribosome, the complex acts like a filter that selects for specific mRNAs and hence modulates translation, as shown in Figure \(\PageIndex{1}\). RP heterogeneity can arise from the differential expression of paralogs or homologs of RP proteins across different cell types, or from differential post-translational modifications of RPs, such as phosphorylation. The protein-to-rRNA ratio may also slightly vary within ribosomal composition, affecting translation efficiency and selectivity.

Figure \(\PageIndex{1}\): Components of the translation machinery that have the potential to contribute to functional heterogeneity. Structures were visualized with Polyview 3D software using the following maps: E. coli large ribosomal subunit (LSU; pdb accession code: 3d5a, rRNA in light gray, proteins in dark gray); small ribosomal subunit (SSU; pdb accession code: 3d5b); E. coli IF2 (pdb accession code: 1g7r); human eIF3 (EMD-2166); yeast tRNA^Phe (pdb accession code: 6tna).Sauert, M., Temmel, H., and Moll, I. (2015) Biochimie 114: 39-47.

RNA genes are also present in multiple copies throughout the genomes of organisms from all domains of life. For example, the bacteria Streptomyces coelicolor harbor six copies of divergent large subunit (LSU) rRNA genes that constitute at least five different LSU rRNA species in a cell. These genes were shown to be differentially expressed during the organism's morphological development. Similarly, B. subtilis harbors ten rRNA operons, and their reduction to one copy increased the doubling time as well as the sporulation frequency and the motility of the resulting mutant.

Modification of the rRNA also provides another avenue of ribosomal heterogeneity. Similar to tRNA, rRNA residues can be chemically modified and commonly have 2-OH methylation. The conversion of uridine to pseudouridine is also common. In eukaryotes, the modifications are facilitated by snoRNAs, and their tissue-specific expression might be a source for ribosome specialization. In light of growing evidence, ribosome heterogeneity, although still far from fully understood, is an integral mechanism for modulating and fine-tuning protein synthesis in response to environmental signals across all organisms.

Effects of Sequence and Secondary Structure in mRNA

The amount of protein produced from any given mRNA (i.e., the translational output) is influenced by multiple factors specified by the primary nucleotide sequence. These factors include GC content, codon usage, codon pairs, and secondary structure. For example, 5' UTR sequences in the mRNA may interact with small miRNAs, leading to RNA interference. miRNA interactions may also target mRNA for degradation(Figure \(\PageIndex{2}\)). This process is facilitated by protein chaperones known as argonautes. This antisense-based process involves steps that first process the miRNA so that it can base-pair with a region of its target mRNA. Once the base pairing occurs, other proteins direct the mRNA to be destroyed by nucleases. Fire and Mello were awarded the 2006 Nobel Prize in Physiology or Medicine for this discovery.

Figure \(\PageIndex{2}\): Role of Micro RNA (miRNA) in the Inhibition of Eukaryotic mRNA Translation. Wikimedia Commons

• Step (1) shows how Exportin-5 transports a primary microRNA (pri-miRNA) hairpin out of the nucleus and into the cytoplasm.
• Step (2) shows how Dicer (not shown) trims the pri-miRNA, removing the hairpin loop. A group of proteins known as Argonautes forms a miRNA/protein complex.
• Steps (3,4) illustrate how the miRNA/protein complex forms hydrogen bonds with mRNA via complementary base pairing, thereby blocking translation.
• Step (5) shows that the miRNA/protein complex accelerates the breakdown of the mRNA polyA tail, causing the mRNA to be degraded sooner.

Effects of the Nascent Peptide on Ribosome Efficiency

Since the Peptidyl Transferase Center (PTC) is buried within the large subunit, during translation, the nascent peptide chain (NC) exits through a 100 Å-long tunnel (Figure \(\PageIndex{3}\)). The exit tunnel plays an active role in protein synthesis. Certain peptide sequences specifically interact with tunnel walls and induce ribosome stalling. Furthermore, the exit tunnel is a binding site for a clinically important class of antibiotics known as the macrolides.

Figure \(\PageIndex{3}\): The Ribosomal Exit Tunnel (a) Scheme of the ribosome exit tunnel with several proteins highlighted. NC, nascent peptide chain; ERY, erythromycin; PTC, peptidyl transferase center. (b) Context of the erythromycin (ERY, in green) binding; figure based on PDB: 5JTE. Several large subunit nucleotides are highlighted in bold red. Two proteins uL4 and uL22 form a constriction site. The nascent peptide is shown as a transparent surface structure. Bock, L.V., Kolár, M.H., Grubmüller, H. (2018) Cur. Op. Struc. Bio. 49:27-35

When synthesizing proteins containing proline stretches (i.e., several prolines in a row), ribosomes become stalled. Stalling is alleviated by the specialized elongation factor EF-P in bacteria. Recently, cryo-EM structures of a ribosome stalled by a proline stretch with and without EF-P were resolved. In simulations of the PTC region, elongation factor P (EF-P) was observed to stabilize the P-site tRNA in a conformation compatible with peptide bond formation. In contrast, in the absence of EF-P, the P-site tRNA moved away from the A-site tRNA.

The exit tunnel can accommodate 30–60 AAs, depending on the level of NC compaction. The rate of translation of about 4–22 AA per second in bacteria provides the NC with sufficient time to explore its conformational space and to start folding when still bound to the ribosome-tRNA complex.

Proteasome

The 26S proteasome is the central element of proteostasis regulation in eukaryotic cells. It is required for the degradation of protein factors in multiple cellular pathways and plays a fundamental role in cellular stability. The 26S proteasome has a structural configuration that confines the proteolytic active sites to a location inaccessible to native and functional proteins, thus preventing uncontrolled degradation. The proteolytically active sites are found in the interior of a barrel-shaped core particle (CP or 20S). The tunnel entrances, located at the distal ends of the barrel, are commonly occupied by the regulatory particle (RP or 19S), a complex protein assembly that functions as a substrate-processing machine. The regulatory particle plays a crucial role in receiving, deubiquitinating, unfolding, and translocating substrates to the CP, adopting different configurations depending on its activity state. This process typically requires ATP hydrolysis. Moreover, conformationally distinct proteasomes may exhibit different subcellular distributions, depending on the functional requirements of each cell type and environmental situation. Proteasomes are distributed throughout the cell, detected in both the cytoplasm and the nucleus, and can localize to specific sites with high protein metabolism or with distinct protein degradation requirements.

The core of the proteasome consists of a symmetrical cylinder-shaped structure composed of four stacked rings, each containing seven different subunits. It is called the 20S proteasome, as shown in Figure \(\PageIndex{4}\). The two outer rings are each composed of seven α-subunits (α1-α7 or PSMA1-7). During proteasome assembly, the α-rings serve as the backbone for the incorporation of β-subunits, followed by the dimerization of two half-proteasomes. In mature proteasomes, the α-rings regulate substrate entry, as the α-subunits have hydrophobic loops that close the 20S barrel, preventing random substrate entry.

Figure \(\PageIndex{4}\): Sabine et al., Frontiers in Molecular Biosciences, 6 (2019), https://www.frontiersin.org/articles...olb.2019.00056. DOI=10.3389/fmolb.2019.00056. Creative Commons Attribution License (CC BY).

The 20S core of the proteasome consists of 4 stacked rings. The outer rings contain seven α-subunits (white) while the inner rings contain seven β-subunits (purple). The catalytic subunits, β1, β2, and β5, are depicted in shades of blue. Gate opening of the 20S core occurs via capping by proteasome activators such as the 19S cap or PA28. The 19S cap is the most abundant activator and forms the 26S proteasome with the 20S core. Different cells have different caps. For example, interferoIFN-γ stimulation induces de novo formation of immunoproteasomes, which contain the immune subunits β1i (LMP2), β5i (LMP7), and β2i (MECL-1) (shades of red), as well as proteasome activation by PA28αβ (shades of green). Proteasomes in neural tissue are discussed below.

In general, protein entry can be established only after gate opening by proteasome activators (PAs), such as the 19S cap, allowing substrates to enter the interior of the 20S core for degradation. The inner two rings of the 20S barrel consist of the subunits β1-β7 (PSMB1-7). Each β-ring contains three catalytic subunits, termed β1, β2, and β5. In mature 20S complexes, the pro-peptides of these catalytic β-subunits are auto-catalytically removed. Upon autocatalytic processing, the N-terminal threonine residues become exposed as the catalytically reactive residues, harboring both the nucleophile (the hydroxyl group) and the catalytic base (the N-terminal amine) involved in peptide bond cleavage. Each catalytic subunit has selectivity toward specific residues. β1 has caspase- or peptidyl-glutamyl peptidase-like activity, preferring cleavage at the C-terminus of acidic residues. β2 has trypsin-like activity and cleaves after basic residues, while β5 has chymotrypsin-like activity and prefers cleavage after hydrophobic residues. Figure \(\PageIndex{5}\) shows an interactive iCn3D model of the Human 20S Proteasome (6RGQ).

Figure \(\PageIndex{5}\): Human 20S Proteasome (6RGQ). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...9gDp6nj7Co2Ud9

The alpha subunits are shown in light gray, and the beta subunits are in light cyan. Active site residues, as defined by databases, are shown for one of the alpha chains (L37, Q53, K55, H68, V170) and one of the beta chains (T1, D17, R19, K33, S130, D167, S170, G171).

A possible generic mechanism for the cleavage of peptide bonds by the beta chain is shown in Figure \(\PageIndex{6}\). The mechanism illustrates how the newly exposed N-terminal threonine functions as both the nucleophile (via its hydroxyl group) and the catalytic base.

Figure \(\PageIndex{6}\): Possible mechanism for the beta chain proteasome cleavage of proteins. after Arjun Saha, Gabriel Oanca, Dibyendu Mondal, and Arieh Warshel. The Journal of Physical Chemistry B 2020124 (27), 5626-5635. DOI: 10.1021/acs.jpcb.0c04435

Proteins are normally targeted to the proteasome via ubiquitin labels attached covalently to a lysine residue, usually in a linear chain that produces a polyubiquitinated protein. The process of protein polyubiquitination is carried out by a highly specialized and diverse enzymatic system, which
includes families of ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), and ubiquitin ligases (E3).

The covalent attachment of ubiquitin to specific target proteins is primarily achieved through a stepwise enzymatic cascade, in which ubiquitin is attached to substrates via the concerted action of the ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligase (E3). The attachment of ubiquitin or ubiquitin chains to the substrate is a successive process, as shown in Figure \(\PageIndex{7}\). First, the C-terminal carboxylic acid is adenylated with ATP, forming an adenylate (AMP-) intermediate. The adenylate acts as a good leaving group during the next reaction, where an E1-ubiquitin thioester bond is formed between the C-terminal Gly carboxyl group of ubiquitin and the active site Cys of the E1 enzyme. AMP leaves the active site at this point. Note that ATP is used in many reactions to activate carboxylic acid functional groups through the formation of an adenylate intermediate, and that this will be seen as a theme in many different types of reactions throughout this textbook. Once ubiquitin is docked as a thioester on E1, it can be transferred to a Cys residue of E2 to form an E2-ubiquitin thioester-linked intermediate. This enzymatic reaction is known as a transesterification.

Figure \(\PageIndex{7}\): Enzymatic Activity of the E1 Ubiquitin Activating Enzyme. Conversion of Gly 76 on ubiquitin to E1 and E2 thioesters. After Hann, Z.S., et. al. (2019) PNAS 116(31)15475-15484.

Eventually, the E2 transfers the ubiquitin to the substrate protein by the E3. Ubiquitin is conjugated to the target protein through an isopeptide bond between its C-terminal glycine (Gly76) and the ε-amino group of a lysine residue. There are three typical ways of linking ubiquitin with the substrate, as shown in Figure \(\PageIndex{8}\). The first is called mono-ubiquitination, which refers to the modification of a single site on a substrate by a single ubiquitin molecule. The second is multi-mono-ubiquitination, which involves repeatedly adding multiple ubiquitin molecules to distinct sites. The third is called polyubiquitination, which includes both linear and branched polyubiquitination.  Ubiquitin molecules are added to the same site on a substrate. In the second and third ways of linking, the previously attached ubiquitin serves as the “acceptor” for the subsequent addition of ubiquitin. Of course, polyubiquitin chains linked by the same Lys are homogeneous, while those linked at different Lys are heterogeneous or mixed ones.

Figure \(\PageIndex{8}\): The Ubiquitin-Proteasome System. Liu, W., et.al. (2020) Int J Mol Sci 21(8):2894

Three major enzymes catalyze ubiquitination, from activation to attachment to the substrate. Ubiquitin-labeled substrates are degraded by the 26S proteasome or function in non-degradative roles in other processes. Abbreviations: APC, Anaphase-promoting complex; DUBs, Deubiquitinating enzymes; E1, Ubiquitin-activating enzyme; E2, Ubiquitin-conjugating enzyme; E3, Ubiquitin-ligase enzyme; Cul-based, Cullin-RING box1-Ligase; HECT, Homology to E6-AP C Terminus; Ub, Ubiqitin; SUMO, Small ubiquitin-related modifier; RBX1, RING-Box 1; RING, Really interesting new gene; RBR, RING1-IBR(cysteine/histidine-rich region)-RING2.

Subsequently, the ubiquitin-tagged substrate complex is either degraded by the 26S proteasome or performs non-proteolytic functions, such as regulating gene expression, cellular trafficking, or other biological processes. In most cases, polyubiquitinated proteins are recognized and degraded by the 26S proteasome, and the ubiquitin or ubiquitin chain is hydrolyzed and freed by deubiquitinating enzymes (DUBs) for reuse in further conjugation cycles after being removed from the substrate protein (Figure 27.3.6).

The family of E3 enzymes is large and diverse. It is estimated that there are 600-700 E3 enzymes in humans, representing approximately 5% of the human genome. Thus, E3 enzymes can be highly substrate-specific, leading to the specialized degradation of a small subset of proteins within the cell. E2 enzymes are the link between E1 and E3 enzymes, helping determine the ubiquitination of specific target proteins by interacting with various E3 enzymes.

Figure \(\PageIndex{9}\) shows an interactive iCn3D model of the Yeast proteasome in resting state (C1-a) (6J2X). (long load)

Figure \(\PageIndex{9}\): Yeast proteasome in resting state (C1-a) (6J2X). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/icn3d/share.html?1AmGmkghbZD2zute7

It's too complex to discuss in much detail. The chains are presented in different colors. You should be able to find the alpha and beta chains of the core 20S particle. The regulatory cap proteins, comprising the lid, are positioned atop the alpha ring.

Summary

(Summary written by Claude, Sonnet 4.6, Anthropic)

The quantity and identity of proteins in a cell are regulated not only at the level of transcription and mRNA processing but also through control of translation efficiency and targeted protein degradation—two mechanisms explored in this chapter.

Translation is not a uniform process: ribosome composition varies across cell types and developmental states, selectively modulating which mRNAs are translated. The ribosome filter hypothesis proposes that compositional heterogeneity—arising from differential expression of RP paralogs, post-translational modifications of RPs such as phosphorylation, and tissue-specific patterns of rRNA chemical modification (2′-OH methylation and pseudouridylation, guided in eukaryotes by snoRNAs)—allows specialized ribosomes to preferentially translate subsets of mRNAs. This provides a layer of control over gene expression beyond transcriptional regulation.

At the level of individual mRNA molecules, translational output is shaped by primary sequence features including GC content, codon usage, codon pair bias, and secondary structure in the 5′ UTR. miRNA-mediated RNA interference provides a powerful post-transcriptional regulatory mechanism: Exportin-5 exports stem-loop pri-miRNA precursors from the nucleus; Dicer removes the hairpin loop to produce the mature ~22 nt miRNA duplex; one strand is incorporated into an Argonaute-containing RISC complex, which then base-pairs with partially complementary sequences in the 3′ UTR of target mRNAs to block translation initiation or elongation and accelerate deadenylation and mRNA degradation. Andrew Fire and Craig Mello received the 2006 Nobel Prize in Physiology or Medicine for the discovery of this mechanism.

The ribosomal exit tunnel adds another layer of translational regulation at the level of the elongating peptide chain itself. The ~100 Å tunnel accommodates 30–60 amino acids and actively influences translation: specific nascent peptide sequences interact with the tunnel walls, causing ribosome stalling, most notably proline-rich sequences that destabilize the P-site tRNA conformation required for peptide bond formation. The specialized bacterial elongation factor EF-P rescues stalled ribosomes by stabilizing the P-site tRNA in a conformation compatible with the peptidyl transferase reaction. The clinically important macrolide antibiotics exploit the exit tunnel by binding at a constriction site formed by uL4 and uL22 proteins, physically blocking the elongating peptide and halting bacterial protein synthesis.

Proteins that are misfolded, damaged, or no longer needed are removed by the ubiquitin-proteasome system (UPS). Substrate proteins are marked for destruction through covalent attachment of ubiquitin chains by a three-enzyme cascade: E1 ubiquitin-activating enzyme adenylates ubiquitin's C-terminal Gly76 using ATP, forming an E1-ubiquitin thioester via its active-site Cys; ubiquitin is then transesterified to an E2 ubiquitin-conjugating enzyme; and finally E3 ubiquitin ligases—of which ~600–700 exist in humans (~5% of the genome), conferring exquisite substrate specificity—transfer ubiquitin from E2 to the ε-amino group of a substrate lysine via an isopeptide bond. Progressive addition of ubiquitin monomers produces polyubiquitin chains (linked through different Lys residues of ubiquitin) that are recognized by the 26S proteasome.

The 26S proteasome consists of a barrel-shaped 20S core particle capped by one or two 19S regulatory particles. The 20S core contains four stacked heptameric rings (α₁-α₇:β₁-β₇:β₁-β₇:α₁-α₇): the outer α-rings gate substrate entry through hydrophobic loop closures that are opened upon binding of regulatory activators, and the inner β-rings contain three catalytic subunits (β1, β2, β5) that cleave after acidic, basic, and hydrophobic residues, respectively. These subunits use their autocatalytically exposed N-terminal threonine residues as dual-function catalysts—the hydroxyl group as a nucleophile and the α-amino group as a catalytic base—for peptide bond hydrolysis. The 19S regulatory cap receives polyubiquitinated substrates, deubiquitinates them (releasing ubiquitin for reuse), uses ATPase activity to unfold the substrate, and translocates it into the 20S barrel for processive degradation. Deubiquitinating enzymes (DUBs) complete the cycle by recycling free ubiquitin. Specialized immunoproteasomes—induced by interferon-γ and containing alternative catalytic β-subunits (β1i, β2i, β5i) capped by PA28αβ—generate peptides optimized for MHC class I antigen presentation, illustrating how proteasome composition itself can be modulated in response to cellular signals.

 

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