12. During heat shock, mammalian cells shut down global protein synthesis while inducing heat shock proteins (Hsps). The possible molecular regulation(s) that could explain the phenomenon are:
A. mRNA of all proteins, except those of Hsps, undergoes degradation during heat shock.
B. Cap-dependent translation of most mRNAs is affected during heat shock due to denaturation of cap binding protein, elF-4E.
C. Translation initiation of Hsp mRNAs takes place through their internal ribosome entry sites (IRES)
D. Hsp mRNAs are abundant during heat shock and thus they compete out other mRNAs for ribosome binding and translation.
Which of the following sets is correct?
(1) A and D (2) B and C
(3) C and D (4) A and D
Introduction
Heat shock triggers a rapid cellular response that halts general protein synthesis to conserve resources and prevent accumulation of misfolded proteins. Simultaneously, cells selectively translate heat shock proteins (Hsps), which help protect and refold damaged proteins. Understanding the molecular mechanisms behind this selective translation is crucial for insights into stress responses and diseases involving protein misfolding.
Global Translation Shutdown During Heat Shock
During heat shock, mammalian cells suppress cap-dependent translation initiation, the primary mechanism for most mRNAs. This shutdown prevents the synthesis of most proteins, conserving energy and reducing the load on the protein folding machinery.
Contrary to some misconceptions, mRNAs for most proteins are not degraded during heat shock; rather, their translation is inhibited. The cap-binding protein eIF4E remains functional but is part of a complex regulatory network that reduces cap-dependent initiation.
Selective Translation of Heat Shock Proteins
Heat shock protein mRNAs bypass this global inhibition through the presence of internal ribosome entry sites (IRES) within their 5′ untranslated regions. IRES elements allow ribosomes to initiate translation independently of the 5′ cap structure, enabling Hsps to be synthesized even when cap-dependent translation is compromised.
Additionally, the abundance of Hsp mRNAs increases dramatically during heat shock due to transcriptional upregulation. This increased mRNA pool further enhances their competitive advantage for the translational machinery.
Conclusion
The selective translation of heat shock proteins during cellular stress involves:
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Inhibition of cap-dependent translation for most mRNAs.
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IRES-mediated translation initiation for Hsp mRNAs.
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Increased abundance of Hsp mRNAs, allowing them to outcompete other transcripts.
Together, these mechanisms ensure that protective heat shock proteins are produced efficiently to help cells survive stressful conditions.
Correct answer: (3) C and D
This article summarizes the key molecular events that allow cells to prioritize heat shock protein synthesis during stress, highlighting the importance of IRES elements and mRNA abundance.
