Transient arrest in proteasomal degradation during inhibition of translation in the unfolded protein response

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The unfolded protein response (UPR) is a cellular stress response that typically upregulates protein degradation pathways.
• However, the precise impact of early UPR on proteasomal degradation remains incompletely understood.

Purpose of the Study:

• To investigate the early effects of the unfolded protein response (UPR) on the proteasomal degradation of specific cytosolic and ER-associated substrates.
• To elucidate the molecular mechanisms underlying any observed inhibition of proteasomal degradation during early UPR.

Main Methods:

• Utilized cell-based assays to monitor the degradation of p53, p27kip1, and an ER-associated substrate.
• Employed protein synthesis inhibitors and manipulated E3 ligase (Mdm2) expression and function.
• Assessed ubiquitination levels of target proteins.

Main Results:

• Early UPR stages transiently inhibit proteasomal degradation of cytosolic (p53, p27kip1) and ER-associated substrates.
• This inhibition correlates with a block in protein synthesis and is reversible upon recovery from translation arrest.
• Ubiquitination is impaired during the translation block, implicating short-lived E3 ligases like Mdm2.
• Overexpression of functional Mdm2 restored degradation, while a mutant Mdm2 variant partially restored ubiquitination but not full degradation.

Conclusions:

• Early UPR induces a temporary shutdown of proteasomal degradation, likely to conserve essential short-lived proteins during translation arrest.
• The stabilization of p27kip1 may contribute to cell cycle arrest observed under these conditions.
• This finding reveals a novel regulatory mechanism of proteasomal degradation linked to the UPR and protein synthesis.