In vivo reconstitution finds multivalent RNA-RNA interactions as drivers of mesh-like condensates.

Weirui Ma,Wei Xie,Christine Mayr,Gang Zhen

doi:10.7554/elife.64252

Abstract

Liquid-like condensates have been thought to be sphere-like. Recently, various condensates with filamentous morphology have been observed in cells. One such condensate is the TIS granule network that shares a large surface area with the rough endoplasmic reticulum and is important for membrane protein trafficking. It has been unclear how condensates with mesh-like shapes but dynamic protein components are formed. In vitro and in vivo reconstitution experiments revealed that the minimal components are a multivalent RNA-binding protein that concentrates RNAs that are able to form extensive intermolecular mRNA-mRNA interactions. mRNAs with large unstructured regions have a high propensity to form a pervasive intermolecular interaction network that acts as condensate skeleton. The underlying RNA matrix prevents full fusion of spherical liquid-like condensates, thus driving the formation of irregularly shaped membraneless organelles. The resulting large surface area may promote interactions at the condensate surface and at the interface with other organelles.

Highlights

Despite lacking a surrounding lipid membrane, membraneless organelles are micron-sized structures that compartmentalize the subcellular space to organize biological reactions (Hyman et al, 2014; Banani et al, 2017)
When we introduced different point mutations to disrupt RNA binding (Figure 1—figure supplement 1B–C; Lai et al, 2000), the mesh-like assemblies were turned into sphere-like condensates that are no longer intertwined with the endoplasmic reticulum (ER) (Figure 1B, Figure 1—figure supplement 1D, E)
We showed that several chimeric proteins that consist of multivalent domains that are paired with RNA-binding domain (RBD) that bind to RNA matrix-forming RNAs can form mesh-like condensates in cells (Table 1)

Summary

Introduction

Despite lacking a surrounding lipid membrane, membraneless organelles are micron-sized structures that compartmentalize the subcellular space to organize biological reactions (Hyman et al, 2014; Banani et al, 2017). Several mesh-like condensates were found in cells that include the TIS granule network, FXR1 condensates, and localization bodies (Lbodies) (Ma and Mayr, 2018; Neil et al, 2020; Smith et al, 2020). TIS granules are formed through assembly of the RNA-binding protein TIS11B. TIS granules have tubule-like structures and generate a reticular meshwork that is intertwined with the rough endoplasmic reticulum (ER), one of the major sites of protein translation. The mesh-like morphology of the TIS granule network allows it to share a lot of surface area with the ER, generating a large condensate–organelle interface. Translation in the TIS granule–ER interface, the so-called TIGER domain, allows membrane proteins to form specific protein complexes, indicating that TIS granules are important for the trafficking of plasma membrane proteins (Ma and Mayr, 2018; Mayr, 2019)

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