Spinetti, E. | bioRxiv Science

bioRxiv2023

IGF2BP1 phosphorylation regulates ribonucleoprotein condensate formation by impairing low-affinity protein and RNA interactions

The insulin-like growth factor 2 mRNA binding protein (IGF2BP1) is a conserved RNA-binding protein that regulates RNA stability, localization, and translation. IGF2BP1 is part of various ribonucleoprotein (RNP) condensates regulating RNA outputs. However, the mechanism that regulates its assembly into condensates remains unknown. Here we found, using proteomics, that IGF2BP1 phosphorylation at S181 in a disordered linker is regulated in a stress-dependent manner. Phosphomimetic mutations in two disordered linkers, S181E and Y396E, modulated RNP condensate formation by IGF2BP1 without impacting its binding affinity for RNA. Intriguingly, the S181E mutant, which lies in linker 1, impaired IGF2BP1 condensate formation in vitro and in cells, whereas a Y396E mutant in the second linker increased condensate size and dynamics. Structural approaches showed that the first linker binds RNAs nonspecifically through its RGG/RG motif, an interaction weakened in the S181E mutant. Notably, linker 2 interacts with IGF2BP1s folded domains and these interactions were partially impaired in the Y396E mutant. Our data reveal how phosphorylation modulates low affinity interaction networks in disordered linkers to regulate RNP condensate formation.

biochemistry↗

bioRxiv2023

Stress-induced clustering of the UPR sensor IRE1 is driven by disordered regions within its ER lumenal domain

Upon accumulation of unfolded proteins at the endoplasmic reticulum (ER), IRE1 activates the unfolded protein response (UPR) to restore protein-folding homeostasis. During ER stress, IRE1s ER lumenal domain (LD) drives its clustering on the ER membrane to initiate signaling. How IRE1s LD assembles into high-order oligomers remains largely unknown. By in vitro reconstitution experiments we show that human IRE1 LD forms dynamic biomolecular condensates. IRE1 LD condensates were stabilized when IRE1 LD was tethered to model membranes and upon binding of unfolded polypeptide ligands. Molecular dynamics simulations suggested that weak multivalent interactions are involved in IRE1 LD assemblies. Mutagenesis showed that disordered regions in IRE1 LD control its clustering in vitro and in cells. Importantly, dysregulated clustering led to defects in IRE1 signaling. Our results reveal that membranes and unfolded polypeptides act as scaffolds to assemble dynamic IRE1 condensates into stable, signaling competent clusters.

cell biology↗