- To probe translation initiation, investigators compared two capped mRNAs encoding the same reporter protein: one contains a strong Kozak consensus surrounding the AUG start codon, and the other contains multiple mismatches in the Kozak sequence. Equal mRNA amounts were transfected into cells, and mRNA stability was similar. Which outcome is most consistent with the role of the Kozak sequence in eukaryotic translation initiation?
Answer: The mismatched Kozak mRNA produces less protein due to reduced start-codon recognition during scanning
This question tests understanding of translation and post-translational modification in biological systems. The Kozak consensus sequence surrounds the AUG start codon and facilitates its recognition by the scanning ribosome during translation initiation. A strong Kozak sequence enables efficient start codon recognition, while mismatches reduce the efficiency of AUG selection by the ribosomal scanning complex. Choice A correctly explains that poor Kozak sequences lead to reduced protein output due to inefficient start codon recognition during the scanning process. Choice B incorrectly suggests that weak recognition increases translation, contradicting established mechanisms. Choice C confuses transcription (RNA polymerase II function) with translation initiation. Choice D is incorrect because Kozak recognition occurs during initiation, not after termination. Understanding Kozak sequence function helps predict translation efficiency from mRNA sequence features.
- Investigators examined translation initiation of an mRNA engineered to lack a 5' cap but retain a poly(A) tail. Compared with the capped version, the uncapped mRNA showed markedly reduced protein output in a cell-free eukaryotic translation system. Which process is most likely involved in the reduced translation of the uncapped mRNA?
Answer: Impaired recruitment of initiation factors and the small ribosomal subunit to the 5' end, reducing initiation frequency
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with the 5' cap facilitating initiation factor recruitment for efficient translation. In this passage, the focus is on how lacking a 5' cap reduces mRNA translation. Choice C is correct because uncapped mRNAs impair initiation complex recruitment, decreasing initiation. Choice B is incorrect because uncapped mRNAs are exported but translated poorly. Ensure understanding of cap-dependent initiation. Remember that cell-free systems isolate translational effects.
- To investigate protein targeting, researchers expressed a secreted cytokine precursor containing an N-terminal signal peptide. When cells were treated with a compound that prevents recognition of signal peptides, the cytokine accumulated in the cytosol as an unprocessed precursor and was not detected in the culture medium. Which process is most likely involved in the normal localization of this cytokine?
Answer: Co-translational targeting of the ribosome–nascent chain complex to the ER membrane, enabling translocation and signal peptide cleavage
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with co-translational targeting directing nascent chains to specific compartments like the ER for secreted proteins. In this passage, the focus is on how signal peptide recognition is essential for cytokine localization and processing. Choice D is correct because the signal peptide enables co-translational ER targeting and translocation, followed by cleavage for secretion. Choice B is incorrect because it describes mRNA capping, which aids export but not protein targeting. Ensure understanding of the signal recognition particle's role in ER targeting. Remember that disrupting targeting leads to cytosolic accumulation of precursors.
- In a signaling study, Protein R is phosphorylated at a single threonine residue following receptor activation. The phosphorylated form shows increased binding to a downstream effector in co-immunoprecipitation assays. A phospho-dead Thr→Ala mutant fails to bind the effector. How does phosphorylation most likely alter Protein R function?
Answer: It creates a specific recognition site that promotes Protein R–effector binding, enabling downstream signaling
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, while post-translational modifications like phosphorylation can create binding sites for effectors in signaling. In this passage, the focus is on how threonine phosphorylation enhances Protein R–effector interaction. Choice D is correct because phosphorylation likely forms a recognition site for binding. Choice B is incorrect because it misattributes the effect to translation inhibition. Ensure understanding of phosphorylation in protein-protein interactions. Remember that phospho-mutants can abolish specific functions.
- A signaling protein (Protein S) is phosphorylated after exposure to cytokine. Mass spectrometry shows phosphorylation at two serines within a regulatory region; following stimulation, Protein S is rapidly degraded, and proteasome inhibition restores Protein S levels. A Ser→Ala mutant is stable after cytokine exposure. Based on the passage, what effect would phosphorylation have on Protein S?
Answer: It likely promotes Protein S degradation by enabling recognition by degradation machinery, reducing steady-state protein levels
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, while post-translational modifications like phosphorylation can target proteins for degradation. In this passage, the focus is on how serine phosphorylation leads to Protein S degradation. Choice A is correct because it enables recognition by degradation machinery. Choice D is incorrect because phosphorylation can promote, not always block, degradation. Ensure understanding of phospho-dependent ubiquitination. Remember that proteasome inhibition confirms degradation pathways.
- Investigators compared translation initiation on two otherwise identical mRNAs differing only in their 5' untranslated region (UTR). The mRNA with a stable 5' UTR secondary structure showed reduced protein output and fewer ribosomes per mRNA in polysome profiling. Which process is most likely involved in the reduced translation of the structured 5' UTR mRNA?
Answer: Impaired scanning by the small ribosomal subunit from the 5' end to the start codon, decreasing initiation frequency
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with 5' UTR structures affecting initiation efficiency. In this passage, the focus is on how stable 5' UTR structure reduces translation. Choice A is correct because it impairs ribosomal scanning to the start codon. Choice C is incorrect because structures do not create stop codons. Ensure understanding of 5' UTR regulation in initiation. Remember that polysome analysis detects initiation barriers.
- To test cap-independent translation, researchers inserted an internal ribosome entry site (IRES) upstream of a reporter ORF and translated the mRNA in a lysate where eIF4E was inhibited. The IRES-containing reporter was still translated, whereas a cap-dependent control reporter was not. Which process is most likely involved in the IRES reporter's continued translation under eIF4E inhibition?
Answer: Direct recruitment of ribosomal subunits to an internal mRNA structure, bypassing cap recognition
This question tests understanding of translation and post-translational modification in biological systems. Internal ribosome entry sites (IRES) enable cap-independent translation by directly recruiting ribosomes to internal mRNA sequences, bypassing the need for cap recognition by eIF4E. This alternative translation mechanism is particularly important during cellular stress when cap-dependent translation is inhibited. Choice C is correct because IRES elements recruit ribosomal subunits directly to the mRNA interior, allowing translation without eIF4E-mediated cap recognition. Choice B is incorrect because DNA replication doesn't occur in cell-free translation lysates and wouldn't explain selective translation of IRES-containing mRNA. Remember that IRES-mediated translation represents an important alternative to canonical cap-dependent initiation.
- A cytosolic transcription factor (TF) is retained in the cytoplasm under basal conditions. Upon stress, a kinase phosphorylates TF at two residues, and TF accumulates in the nucleus within 10 min without a change in TF protein abundance. A nonphosphorylatable TF mutant fails to enter the nucleus. Based on the passage, what effect would phosphorylation have on TF?
Answer: It promotes nuclear import by enabling interaction with nuclear transport machinery (e.g., importins) or unmasking an NLS
This question tests understanding of translation and post-translational modification in biological systems. Post-translational phosphorylation can regulate protein localization by creating or exposing nuclear localization signals (NLS) that are recognized by nuclear import machinery like importins. In this case, stress-induced phosphorylation enables the transcription factor to translocate from cytoplasm to nucleus without changing protein levels. Choice A correctly identifies that phosphorylation promotes nuclear import through interaction with transport machinery or NLS exposure. Choice B incorrectly relates phosphorylation to translation inhibition, which wouldn't explain nuclear accumulation. Choice C contradicts the observation - phosphorylation increases rather than decreases nuclear localization. Choice D impossibly suggests phosphorylating mRNA before transcription. This exemplifies how post-translational modifications dynamically control protein localization in response to cellular signals.
- Researchers express a secreted cytokine in cultured mammalian cells. The wild-type cytokine is efficiently detected in the culture medium. A mutant cytokine lacking its N-terminal hydrophobic signal peptide is synthesized at similar levels (cell lysate immunoblot) but is not detected in the medium. Which process is most likely involved in the normal secretion of the wild-type cytokine?
(Assume the mutant protein remains soluble and is not rapidly degraded.)
Answer: Co-translational targeting of the nascent polypeptide to the rough ER via signal recognition particle and translocation into the ER lumen
This question tests understanding of translation and post-translational modification in biological systems. Secreted proteins contain N-terminal signal peptides that direct them to the endoplasmic reticulum (ER) for co-translational translocation, which is essential for their eventual secretion. In this passage, the wild-type cytokine with its signal peptide is secreted normally, while the mutant lacking this sequence remains intracellular despite similar synthesis levels. Choice D is correct because the signal peptide enables recognition by the signal recognition particle (SRP), which targets the ribosome-nascent chain complex to the ER for co-translational import into the secretory pathway. Choice B is incorrect because phosphorylation in the nucleus would not explain the requirement for an N-terminal signal peptide, which functions during translation at the ER. When analyzing protein secretion, remember that signal peptides act co-translationally to direct proteins into the ER-Golgi secretory pathway.
- A newly synthesized plasma membrane receptor contains an N-terminal signal peptide and multiple hydrophobic segments. When cells were treated with an agent that disrupts ER translocation, the receptor accumulated as a cytosolic protein and was not detected at the cell surface. Which process is most likely involved in normal localization of this receptor?
Answer: Co-translational insertion into the ER membrane followed by trafficking through the secretory pathway to the plasma membrane
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with co-translational ER insertion directing membrane proteins through the secretory pathway. In this passage, the focus is on how disrupting ER translocation prevents receptor surface localization. Choice A is correct because the signal peptide enables ER membrane insertion and subsequent trafficking. Choice B is incorrect because receptors do not diffuse through nuclear pores. Ensure understanding of secretory pathway for membrane proteins. Remember that translocation defects cause cytosolic accumulation.
- A secreted growth factor is produced as an inactive precursor requiring cleavage by a specific convertase in the Golgi. In cells lacking the convertase, the precursor is secreted but fails to activate its receptor. Which statement best explains this observation?
Answer: The convertase cleaves the precursor to remove an inhibitory pro-domain, producing an active ligand
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, while post-translational proteolytic processing activates precursors in the secretory pathway. In this passage, the focus is on how convertase cleavage is required for growth factor activation. Choice B is correct because cleavage removes an inhibitory pro-domain, enabling receptor binding. Choice A is incorrect because convertases act on proteins, not mRNA translation. Ensure understanding of proprotein processing in the Golgi. Remember that unprocessed precursors are often inactive.
- In a cell-free translation assay using capped eukaryotic mRNA encoding a cytosolic kinase, investigators titrated an inhibitor that selectively prevents assembly of the eIF4F complex at the 5' cap. Protein output was quantified by immunoblot after 20 min; mRNA abundance was unchanged by RT-qPCR. Which process is most likely involved in the observed reduction in protein synthesis under these conditions?
Answer: Recruitment of the 40S ribosomal subunit to the mRNA 5' end followed by scanning to the start codon
This question tests understanding of translation and post-translational modification in biological systems. Translation initiation in eukaryotes requires the eIF4F complex to bind the 5' cap of mRNA, which then recruits the 40S ribosomal subunit for scanning to the start codon. The eIF4F complex inhibitor prevents this cap-dependent initiation process, blocking the recruitment and scanning mechanism described in choice A. Choice B is incorrect because splicing occurs during mRNA processing in the nucleus, not during translation in the cytoplasm. Choice C describes a post-translational modification that would occur after translation, not a mechanism that explains reduced protein synthesis. Choice D is incorrect because peptidyl transferase activity occurs during elongation after proper initiation, not before initiator tRNA binding. Understanding cap-dependent translation initiation is crucial for comprehending how protein synthesis is regulated in eukaryotic cells.
- A research group monitored translation initiation of an mRNA with a strong Kozak consensus sequence. When eIF2 function was inhibited, global protein synthesis decreased and the mRNA shifted from heavy to light polysome fractions without a change in mRNA level. Which step is most likely directly affected by eIF2 inhibition?
Answer: Loading of the initiator Met-tRNA onto the small ribosomal subunit to form an initiation complex competent to scan for AUG
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with initiation requiring factors like eIF2 for loading initiator tRNA onto the ribosome. In this passage, the focus is on how eIF2 inhibition reduces global protein synthesis and polysome association. Choice A is correct because eIF2 is essential for forming the initiation complex that scans for the start codon. Choice B is incorrect because it describes elongation, which occurs after initiation. Ensure understanding of eIF2's role in ternary complex formation. Remember that polysome shifts indicate initiation defects.
- A receptor-associated cytosolic protein (Protein X) is rapidly phosphorylated on a single serine residue within 2 min of ligand addition. In cells expressing a Ser→Ala mutant at this site, Protein X still accumulates to the same level but fails to bind a downstream SH2-domain adaptor and signaling is attenuated. Based on the passage, what effect would phosphorylation at this serine most likely have on Protein X?
Answer: It creates a negatively charged recognition element that promotes adaptor binding and complex assembly
This question tests understanding of translation and post-translational modification in biological systems. Post-translational phosphorylation adds negatively charged phosphate groups to proteins, creating recognition sites for specific protein-protein interactions. In this case, serine phosphorylation creates a negatively charged element that allows SH2-domain adaptor binding, as SH2 domains specifically recognize phosphorylated residues. Choice A correctly identifies this mechanism where phosphorylation enables adaptor recruitment and signaling complex assembly. Choice B is incorrect because phosphorylation of a protein doesn't affect mRNA cap structure or translation of that same protein. Choice C contradicts the basic chemistry - phosphorylation adds negative charge rather than eliminating it. Choice D confuses post-translational modification with co-translational events during protein synthesis. Remember that phosphorylation is a key post-translational modification that regulates protein-protein interactions through charge-based recognition mechanisms.
- Investigators study a mitochondrial matrix enzyme encoded in the nucleus. The enzyme is synthesized in the cytosol as a precursor with an N-terminal amphipathic targeting sequence. When this sequence is deleted, the enzyme remains cytosolic and mitochondrial activity of the enzyme is lost. Which process is most likely involved in normal localization of the wild-type enzyme?
Answer: Post-translational import through mitochondrial translocases guided by an N-terminal targeting sequence
This question tests understanding of translation and post-translational modification in biological systems. Nuclear-encoded mitochondrial proteins are synthesized in the cytosol and imported post-translationally using N-terminal targeting sequences recognized by the mitochondrial import machinery. In this passage, deletion of the N-terminal amphipathic sequence causes the enzyme to remain cytosolic, demonstrating this sequence is required for mitochondrial targeting. Choice D is correct because the N-terminal targeting sequence directs post-translational import through the TOM/TIM translocase complexes in the outer and inner mitochondrial membranes. Choice B is incorrect because mitochondrial matrix proteins do not use the ER secretory pathway - they are imported directly from the cytosol through specialized mitochondrial translocases. When analyzing mitochondrial protein import, remember that most matrix proteins use cleavable N-terminal presequences for post-translational targeting.
- A coagulation-related protease is synthesized as an inactive proenzyme in hepatocytes and secreted into plasma. In a patient sample, the proenzyme level is normal but enzymatic activity is low; sequencing reveals a mutation at the cleavage site required for activation. Which outcome is most consistent with this mutation?
Answer: The proenzyme is secreted but fails to be converted into the active protease because the activating cleavage cannot occur
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, while post-translational cleavage activates proenzymes like coagulation proteases. In this passage, the focus is on how a cleavage site mutation impairs activation. Choice D is correct because the mutation prevents conversion to the active form. Choice B is incorrect because cleavage is required for activation, not constitutive activity. Ensure understanding of zymogen activation in plasma. Remember that mutations at cleavage sites block processing.
- A lab investigates a digestive protease produced as an inactive zymogen (ProZ). In vitro, purified ProZ has minimal catalytic activity until incubated with a small amount of active protease, after which activity increases sharply. A ProZ mutant in which a specific N-terminal cleavage site is altered fails to activate under the same conditions. Which process is most likely involved in generating the active enzyme from ProZ?
Answer: Autocatalytic or protease-mediated cleavage that removes an inhibitory propeptide, exposing the active site
This question tests understanding of translation and post-translational modification in biological systems. Zymogens are inactive enzyme precursors that require proteolytic cleavage to become active, a common regulatory mechanism for digestive proteases to prevent premature activation. In this passage, ProZ becomes active only after incubation with active protease, and mutation of the cleavage site prevents activation, indicating proteolytic processing is required. Choice A is correct because proteolytic cleavage removes an inhibitory propeptide that blocks the active site, allowing the enzyme to adopt its catalytically competent conformation. Choice D is incorrect because cleavage cannot occur before translation - post-translational modifications by definition occur after the protein is synthesized. Understanding zymogen activation requires recognizing that proteolytic removal of inhibitory sequences is a key post-translational mechanism for regulating enzyme activity, particularly for potentially harmful proteases.
- A kinase phosphorylates a specific tyrosine on Protein Y only when Protein Y is bound to a scaffold at the plasma membrane. A mutation that disrupts scaffold binding abolishes phosphorylation but does not alter Protein Y expression. Based on the passage, what effect would phosphorylation most likely have on Protein Y function in this context?
Answer: It provides a site for recruitment of downstream signaling proteins, enabling propagation of a membrane-localized signaling complex
This question tests understanding of translation and post-translational modification in biological systems. Post-translational phosphorylation of tyrosine residues creates docking sites for proteins containing phosphotyrosine-binding domains, enabling assembly of signaling complexes at specific cellular locations. The scaffold-dependent phosphorylation ensures spatial organization of signaling, with the phosphorylated tyrosine recruiting downstream effectors. Choice A correctly identifies that phosphorylation creates a recruitment site for membrane-localized signaling complex formation. Choice B incorrectly suggests kinases are nonspecific, when this example shows precise substrate recognition requiring scaffold binding. Choice C contradicts the signaling role of phosphorylation by suggesting it decreases activity. Choice D impossibly relates protein phosphorylation to DNA replication at the plasma membrane. This demonstrates how post-translational modifications coordinate spatially restricted signaling cascades.
- A team studied cap-dependent translation of an mRNA encoding an inflammatory mediator. After pharmacologic activation of a pathway that phosphorylates an initiation factor-binding protein, translation of the mediator increased without changes in mRNA abundance. Which mechanism is most consistent with increased translation initiation in this setting?
Answer: Enhanced recruitment of the cap-binding initiation complex to the mRNA, increasing ribosome loading
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with cap-dependent initiation regulated by factors and phosphorylation events. In this passage, the focus is on how pathway activation enhances translation of the inflammatory mediator. Choice A is correct because phosphorylation likely improves cap-binding complex recruitment, increasing ribosome loading. Choice B is incorrect because it suggests reduced protein via ubiquitination, contradicting the increase observed. Ensure understanding of translational control via initiation factors. Remember that mRNA abundance changes are distinct from translational efficiency.
- A cytosolic enzyme is engineered with a short C-terminal peroxisomal targeting signal (PTS1). After expression in mammalian cells, fluorescence microscopy shows punctate localization consistent with peroxisomes. A second construct lacking the C-terminal tripeptide remains diffuse in the cytosol. Which process is most likely involved in targeting the engineered enzyme to peroxisomes?
Answer: Post-translational recognition of the C-terminal targeting signal by a cytosolic receptor that mediates import into peroxisomes
This question tests understanding of translation and post-translational modification in biological systems. Peroxisomal proteins are synthesized on free ribosomes in the cytosol and imported post-translationally using specific targeting signals like the C-terminal PTS1 sequence. In this passage, the enzyme with PTS1 localizes to peroxisomes while the construct lacking this signal remains cytosolic, demonstrating the requirement for this targeting sequence. Choice D is correct because PTS1 signals are recognized by cytosolic receptors (like Pex5) that mediate post-translational import into peroxisomes through the peroxisomal membrane import machinery. Choice B is incorrect because peroxisomal proteins do not use the ER/SRP pathway - they are imported directly from the cytosol after translation is complete. When analyzing organellar protein targeting, distinguish between co-translational ER/mitochondrial import and post-translational peroxisomal import mechanisms.
