Subcytoplasmic location of translation controls protein output

The cytoplasm is highly compartmentalized, but the extent and consequences of subcytoplasmic mRNA localization in non-polarized cells are largely unknown. We determined mRNA enrichment in TIS granules (TGs) and the rough endoplasmic reticulum (ER) through particle sorting and isolated cytosolic mRNAs by digitonin extraction. When focusing on genes that encode non-membrane proteins, we observed that 52% have transcripts enriched in specific compartments. Compartment enrichment correlates with a combinatorial code based on mRNA length, exon length, and 3' UTR-bound RNA-binding proteins. Compartment-biased mRNAs differ in the functional classes of their encoded proteins: TG-enriched mRNAs encode low-abundance proteins with strong enrichment of transcription factors, whereas ER-enriched mRNAs encode large and highly expressed proteins. Compartment localization is an important determinant of mRNA and protein abundance, which is supported by reporter experiments showing that redirecting cytosolic mRNAs to the ER increases their protein expression. In summary, the cytoplasm is functionally compartmentalized by local translation environments.

The FXR1 network acts as signaling scaffold for actomyosin remodeling

It is currently not known that mRNAs fulfill structural roles in the cytoplasm. Here, we report the FXR1 network, an mRNA-protein (mRNP) network present throughout the cytoplasm: FXR1 packages exceptionally long mRNAs that serve as an underlying network scaffold and concentrate FXR1 molecules, which have multiple protein binding sites. The proximity of FXR1 molecules makes the FXR1 network a hub for transient interactions of proteins lacking RNA-binding domains. We show that the FXR1 network is necessary for RhoA signaling-induced actomyosin reorganization to provide spatial proximity between kinases and their substrates. A point mutation in FXR1, which is found in its FMR1 homolog and causes Fragile X syndrome, disrupts the network. FXR1 network disruption prevents actomyosin remodeling-an essential and ubiquitous process for the regulation of cell shape, migration, and synaptic function. These findings uncover a structural role for cytoplasmic mRNA and show how the FXR1 RNA-binding protein as part of the FXR1 network acts as organizer of signaling reactions.