Translation and Post-Translational Modification (1B)
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MCAT Biological and Biochemical Foundations of Living Systems › Translation and Post-Translational Modification (1B)
A research group expressed a receptor-associated adaptor protein (Adap1) in mammalian cells. After stimulation with a growth factor, Adap1 shifted to a slower-migrating band on SDS-PAGE, and the shift was eliminated by phosphatase treatment. A mutant Adap1 in which a single serine in a consensus kinase motif was replaced with alanine failed to show the shift and showed prolonged receptor signaling. Based on the findings, what effect would the modification have on Adap1 function?
Phosphorylation directly increases Adap1 transcription by recruiting RNA polymerase to the Adap1 gene
Phosphorylation must increase receptor signaling because adding negative charge always activates adaptor proteins
Phosphorylation likely creates or disrupts a binding interface that promotes signal termination, reducing pathway activity
Phosphorylation prevents Adap1 translation by blocking ribosomal scanning of the Adap1 mRNA 5′ UTR
Explanation
This question tests understanding of translation and post-translational modification in biological systems. Post-translational phosphorylation adds negatively charged phosphate groups to proteins, often altering their conformation and function. The gel shift indicates phosphorylation occurred, and the serine-to-alanine mutation preventing both the shift and prolonged signaling suggests phosphorylation normally terminates signaling. Choice A is correct because phosphorylation commonly creates or disrupts protein-protein interactions that regulate signaling cascades, and the prolonged signaling in the mutant indicates phosphorylation normally promotes signal termination. Choice D is incorrect because phosphorylation effects are context-dependent and don't always activate proteins - here it appears to have an inhibitory effect. Understanding that post-translational modifications can both activate and inhibit protein function depending on the specific context is crucial for MCAT success.
Researchers compared two mRNAs encoding the same cytosolic enzyme (EnzQ) but with different 5′ UTRs. mRNA-1 has a short, unstructured 5′ UTR; mRNA-2 has a long, GC-rich 5′ UTR predicted to form stable secondary structure. In a translation assay with equal mRNA input, mRNA-2 produced less EnzQ protein. Which process is most likely involved in the reduced translation of mRNA-2?
Decreased EnzQ catalytic activity because GC-rich sequences reduce active-site flexibility
Enhanced translation because increased secondary structure always increases ribosome binding affinity
Impaired 40S scanning and start-codon recognition due to stable 5′ UTR secondary structure
Reduced EnzQ gene transcription because GC-rich 5′ UTRs inhibit promoter clearance
Explanation
This question tests understanding of translation and post-translational modification in biological systems. Translation efficiency is strongly influenced by 5' UTR structure, as stable secondary structures impede ribosomal scanning from the cap to the start codon. GC-rich sequences form particularly stable structures that can block ribosome progression. Choice A is correct because stable secondary structures in the 5' UTR inhibit 40S ribosomal subunit scanning and AUG recognition, reducing translation efficiency. Choice D is incorrect because increased secondary structure typically decreases, not increases, ribosome accessibility and translation efficiency. Remember that 5' UTR structure is a major determinant of translational control, with more structured UTRs generally correlating with reduced translation.
A signaling enzyme (EnzR) is inactive when purified from unstimulated cells but becomes active after cells are exposed to a ligand. Mass spectrometry identifies ligand-dependent phosphorylation at a single threonine. A phosphomimetic mutant (Thr→Glu) shows high activity without ligand, while a nonphosphorylatable mutant (Thr→Ala) remains low activity even with ligand. Based on the passage, what effect would phosphorylation have on EnzR most consistently?
Phosphorylation increases EnzR mRNA synthesis by acting as a transcription factor in the nucleus
Phosphorylation shifts EnzR toward an active conformation, increasing catalytic activity in the absence of other changes
Phosphorylation decreases EnzR activity because phosphorylation universally inhibits enzymes
Phosphorylation must occur before EnzR translation begins, otherwise the ribosome cannot initiate at the start codon
Explanation
This question tests understanding of translation and post-translational modification in biological systems. Post-translational phosphorylation is a common mechanism for regulating enzyme activity by inducing conformational changes. The phosphomimetic mutation (Glu) constitutively activates while the nonphosphorylatable mutation (Ala) prevents activation, demonstrating phosphorylation's activating role. Choice D is correct because the data clearly show phosphorylation shifts EnzR to an active conformation, with the phosphomimetic maintaining high activity without ligand stimulation. Choice B is incorrect because phosphorylation effects are enzyme-specific - many enzymes are activated rather than inhibited by phosphorylation, as shown here. Understanding that post-translational modifications like phosphorylation can bidirectionally regulate protein function is crucial for the MCAT.
A secreted glycoprotein is produced in cultured cells. Treatment with an inhibitor that blocks N-linked glycosylation in the ER yields a protein of lower apparent molecular weight on SDS-PAGE and markedly reduced secretion, while total mRNA levels remain constant. Which process is most likely involved in the secretion defect?
Glycan addition occurring before translation so that the ribosome can recognize the signal peptide
Impaired folding and quality-control passage through the ER due to loss of N-linked glycan addition
Reduced transcription initiation because glycosylation is required for RNA polymerase II activation
Enhanced secretion because removal of glycans always increases vesicular transport efficiency
Explanation
This question tests understanding of translation and post-translational modification in biological systems. N-linked glycosylation in the ER is a co-translational modification essential for proper protein folding and quality control of many secreted proteins. Glycans assist in protein folding and are recognized by ER chaperones and quality control machinery. Choice A correctly explains that blocking glycosylation impairs protein folding and ER quality control, reducing secretion efficiency. Choice B incorrectly suggests glycan removal always enhances secretion, contradicting the observed decrease. Choice C confuses post-translational modification with transcriptional regulation. Choice D impossibly places glycosylation before translation, when it actually occurs co-translationally in the ER. This illustrates how post-translational modifications like glycosylation are critical for protein maturation and secretion.
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?
It provides a site for recruitment of downstream signaling proteins, enabling propagation of a membrane-localized signaling complex
It increases Protein Y levels by enhancing DNA replication of the Protein Y gene at the plasma membrane
It activates phosphorylation of unrelated cytosolic enzymes regardless of localization, because kinases are nonspecific
It decreases downstream signaling by increasing scaffold binding affinity through removal of negative charge
Explanation
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 lab engineered a cytosolic enzyme (Enzyme Y) with a single Tyr phosphorylation site required for catalytic activity. After stimulating cells with a growth factor, Enzyme Y activity increased within minutes without a change in Enzyme Y protein abundance. Treatment with a broad tyrosine kinase inhibitor blocked the activity increase. How does phosphorylation most likely alter Enzyme Y function in this context?
It increases Enzyme Y gene transcription by enhancing RNA polymerase II binding to the promoter
It prevents Enzyme Y translation by blocking tRNA charging for tyrosine
It induces a conformational change that increases catalytic efficiency or substrate access at the active site
It decreases Enzyme Y activity by destabilizing the folded state and promoting proteasomal degradation
Explanation
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 induce conformational changes affecting enzyme activity. In this passage, the focus is on how tyrosine phosphorylation rapidly activates Enzyme Y without altering protein levels. Choice C is correct because phosphorylation likely causes a conformational shift that enhances catalytic efficiency or substrate access. Choice D is incorrect because it confuses phosphorylation with tRNA charging, which is unrelated to post-translational effects. Ensure understanding of how phosphorylation can allosterically regulate enzyme function. Remember that rapid activity changes without abundance shifts often indicate post-translational mechanisms.
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?
Impaired scanning by the small ribosomal subunit from the 5' end to the start codon, decreasing initiation frequency
Increased transcription initiation because structured 5' UTRs recruit RNA polymerase more efficiently
Enhanced DNA replication of the gene encoding the mRNA, diluting ribosomes across more templates
Increased peptide chain termination because stable 5' structures create additional stop codons
Explanation
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 probe ER targeting, scientists fused an N-terminal signal peptide to a normally cytosolic fluorescent protein. The fusion protein localized to the ER and was detected in the secretory pathway. Which process is most likely involved in this change in localization?
Activation of the fluorescent protein by proteolytic cleavage in the nucleus, enabling ER entry
Increased transcription of ER genes leading to passive diffusion of the protein into the ER lumen
Recognition of the signal peptide on the nascent chain and docking of the translating ribosome to the ER translocon
Addition of the signal peptide by a Golgi-resident enzyme after translation is complete
Explanation
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with signal peptides directing co-translational ER targeting for secretory proteins. In this passage, the focus is on how fusing a signal peptide redirects a cytosolic protein to the ER. Choice A is correct because the signal peptide enables ribosome docking to the ER translocon during translation. Choice B is incorrect because signal peptides are part of the nascent chain, not added post-translationally in the Golgi. Ensure understanding of co-translational translocation. Remember that engineered signals can alter protein localization.
A newly synthesized lysosomal hydrolase is translated on ER-bound ribosomes and trafficked through the Golgi. In cells with defective Golgi sorting, the hydrolase is secreted instead of being delivered to lysosomes, despite normal translation. Which process is most likely involved in normal delivery of this hydrolase to lysosomes?
Post-translational phosphorylation of cytosolic ribosomal proteins to increase hydrolase translation rate
Targeting information acquired during secretory pathway processing that directs packaging into vesicles destined for lysosomes
Activation of the hydrolase by nuclear proteases prior to ER entry
Direct import of the hydrolase into lysosomes through a mitochondrial-like translocon
Explanation
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with post-translational modifications and sorting directing lysosomal enzymes via the secretory pathway. In this passage, the focus is on how Golgi sorting ensures lysosomal delivery of the hydrolase. Choice B is correct because targeting information acquired in the secretory pathway directs vesicular packaging to lysosomes. Choice A is incorrect because it describes ribosomal phosphorylation, unrelated to trafficking. Ensure understanding of mannose-6-phosphate sorting for lysosomes. Remember that sorting defects lead to secretion.
Investigators studied a mitochondrial matrix enzyme (Protein M) encoded in the nucleus. A mutant Protein M lacking its N-terminal targeting sequence accumulated in the cytosol and remained enzymatically inactive in mitochondrial assays, despite normal translation. Which process is most likely involved in the normal localization of Protein M?
Insertion into the ER membrane through a signal peptide and subsequent trafficking via Golgi vesicles to mitochondria
Recognition of an N-terminal targeting sequence by mitochondrial import machinery, followed by translocation into the matrix
Phosphorylation of mitochondrial rRNA to increase ribosome binding to Protein M mRNA
Alternative splicing that adds a mitochondrial targeting sequence after translation is complete
Explanation
This question tests understanding of translation and post-translational modification in biological systems. Translation involves the synthesis of proteins from mRNA, with post-translational targeting signals directing proteins to organelles like mitochondria. In this passage, the focus is on how the N-terminal sequence is crucial for Protein M's mitochondrial import and activity. Choice D is correct because the targeting sequence facilitates recognition and translocation into the mitochondrial matrix. Choice B is incorrect because it describes ER-Golgi trafficking, which is not typical for mitochondrial proteins. Ensure understanding of mitochondrial import pathways. Remember that loss of targeting sequences causes cytosolic mislocalization.