Selective recruitment of endoplasmic reticulum-targeted and cytosolic mRNAs into membrane-associated stress granules

Abstract

Stress granules (SGs) are membraneless organelles composed of mRNAs and RNA binding proteins which undergo assembly in response to stress-induced inactivation of translation initiation. The biochemical criteria for mRNA recruitment into SGs are largely unknown. In general, SG recruitment is limited to a subpopulation of a given mRNA species and RNA-seq analyses of purified SGs revealed that signal sequence-encoding (i.e. endoplasmic reticulum (ER)-targeted) transcripts are significantly under-represented, consistent with prior reports that ER-localized mRNAs are excluded from SGs. Using translational profiling, cell fractionation, and single molecule mRNA imaging, we examined SG biogenesis during the unfolded protein response (UPR) and report that UPR-elicited SG formation is gene selective. Combined immunofluorescence-smFISH studies demonstrated that UPR-induced mRNA granules co-localized with SG protein markers and were in close physical proximity to or directly associated with the ER membrane. mRNA recruitment into ER-associated SGs required stress-induced translational inhibition, though translational inhibition was not solely predictive of mRNA accumulation in SGs. SG formation in response to UPR activation or arsenite addition was blocked by the transcriptional inhibitors actinomycin D or triptolide, suggesting a functional link between gene transcriptional state and SG biogenesis. These data demonstrate that ER-targeted mRNAs can be recruited into SGs and identify the ER as a subcellular site of SG assembly. On the basis of the transcriptional inhibitor studies, we propose that newly transcribed mRNAs undergoing nuclear export during conditions of suppressed translation initiation are key substrates for SG biogenesis.