MRNA escape from stress granule sequestration is dictated by localization to the endoplasmic reticulum

Abstract

In mammalian cells, cytotoxic stress triggers several signaling cascades that converge in the phosphorylation of translation initiation factor 2alpha, shuttling of nuclear RNA-binding proteins such as TIA-1 to the cytoplasm, and aggregation of most cellular mRNAs into TIA-1-containing stress granules (SGs). As a result, protein synthesis is greatly impaired. Here we describe different dynamics of endogenous transcripts according to their cellular location, in response to stress. While cytosolic mRNAs aggregate into SGs, endoplasmic reticulum (ER) -bound transcripts escape sequestration. This has been specifically demonstrated using the multidrug resistance transporter gene (MDR1) as a model and showing that chimeric RNA constructs can be directed to the cytosol or tethered to the ER depending on the nature of the chimera, in response to stress. In addition, polysome profile analyses indicate that, on stress, ribosomes do not disengage from ER-associated transcripts (puromycin insensitive) and recover their translation status faster than SG-targeted cytosolic mRNAs once the stress is lifted. These findings have important implications for cell survival given that many membrane proteins, which are translated at the ER, have important roles in detoxification.