Stress granules are dispensable for mRNA stabilization during cellular stress

Abstract

During cellular stress, protein synthesis is severely reduced and bulk mRNA is recruited to stress granules (SGs). Previously, we showed that the SG-recruited IGF2 mRNA-binding protein 1 (IGF2BP1) interferes with target mRNA degradation during cellular stress. Whether this requires the formation of SGs remained elusive. Here, we demonstrate that the sustained inhibition of visible SGs requires the concomitant knockdown of TIA1, TIAR and G3BP1. FRAP and photo-conversion studies, however, indicate that these proteins only transiently associate with SGs. This suggests that instead of forming a rigid scaffold for mRNP recruitment, TIA proteins and G3BP1 promote SG-formation by constantly replenishing mRNPs. In contrast, RNA-binding proteins like IGF2BP1 or HUR, which are dispensable for SG-assembly, are stably associated with SGs and the IGF2BP1/HUR-G3BP1 association is increased during stress. The depletion of IGF2BP1 enhances the degradation of target mRNAs irrespective of inhibiting SG-formation, whereas the turnover of bulk mRNA remains unaffected when SG-formation is impaired. Together these findings indicate that the stabilization of mRNAs during cellular stress is facilitated by the formation of stable mRNPs, which are recruited to SGs by TIA proteins and/or G3BP1. Importantly, however, the aggregation of mRNPs to visible SGs is dispensable for preventing mRNA degradation.

Stress and the nonsense-mediated RNA decay pathway

“…For example, translationally repressed mRNAs may have intrinsic properties that cause them to aggregate to form SGs, but the SGs do not have a function in RNA decay per se . In support, one study found that inhibition of SG formation had no effect on mRNA degradation rates [107]. One possibility is that eIF2α phosphorylation—the upstream driver of SG formation—is responsible for inhibiting RNA decay, not SGs themselves.…”

Casein Kinase 2 Is Linked to Stress Granule Dynamics through Phosphorylation of the Stress Granule Nucleating Protein G3BP1

“…Many components of SGs have very short residence times within stress granules (seconds), despite the long-term persistence of the SG itself (minutes to hours), creating challenges to explain these properties (25,26). Posttranslational modifications are commonly found on key SG nucleating proteins; thus, attention is turning to how such modifications may influence SG dynamics.…”

“…Since they are very dynamic structures where many incorporated proteins quickly exchange with cytoplasmic pools of those proteins (25)(26)(27), posttranslational modifications are implicit in regulating these protein-protein associations. A recent report indicated that the SG nucleating proteins are exchanged more rapidly than those that are incapable of inducing SG aggregation (i.e., ZBP1 and YB1) (25). What regulatory mechanisms promote stress granule disassembly and resumed protein synthesis during the poststress recovery phase is not well understood.…”

AMP Kinase Activation Alters Oxidant-Induced Stress Granule Assembly by Modulating Cell Signaling and Microtubule Organization

“…crucial for SG assembly (Bley et al, 2015). Moreover, an increase in granule-nucleating proteins stimulates SG production (Kedersha et al, 1999;Tourrière et al, 2003).…”