L-bodies are RNA–protein condensates driving RNA localization in Xenopus oocytes

Abstract

RNP granules are membrane-less compartments within cells, formed by phase separation, that function as regulatory hubs for diverse biological processes. However, the mechanisms by which RNAs and proteins interact to promote RNP granule structure and function in vivo remain unclear. In Xenopus laevis oocytes, maternal mRNAs are localized as large RNPs to the vegetal hemisphere of the developing oocyte, where local translation is critical for proper embryonic patterning. Here, we demonstrate that RNPs containing vegetally localized RNAs represent a new class of cytoplasmic RNP granule, termed Localization-bodies (L-bodies). We show that L-bodies contain a dynamic protein-containing phase surrounding a non-dynamic RNA-containing phase. Our results support a role for RNA as a critical component within these RNP granules and suggest that cis-elements within localized mRNAs may drive subcellular RNA localization through control over phase behavior.

Viscoelastic phase separation in biological cells

“…Nevertheless, we argue that the selective inclusion of large-size polymeric components such as RNA and DNA into one of the phases is responsible for causing a significant dynamic asymmetry between the two phases and inducing VPS. We demonstrate that this scenario can well explain recent experimental findings in mesh-like or network-like structure formation in TIS granules network 13,14 , RNP (ribonucleoprotein) granules 15 , localisation bodies 16 , FXR1 condensates 17 , NLRP6 18 and centrosome assemblies 21,23,38 .…”

“…These aggregates lose their coalescence ability and tend to form network-like structures. Such networklike or irregular-droplet morphology has also been discovered in TIS (TPA (12-0-tetradecanoyl-phorbol-13-acetate)-inducible sequence protein) granules 13,14 , RNP (ribonucleoprotein) granules 15 , localisation bodies 16 , FXR1 (fragile X mental retardation syndrome-related protein 1) condensates 17 and NLRP6 (nucleotide-binding oligomerization domain-like (NOD-like) receptor family pyrin domain containing 6) 18 (see also recent reviews 6,19 ). This type of morphology provides a large interface area with a bicontinuous nature, crucial for biological reactions and transport.…”

“…It was speculated that the network structure is formed by gelation 13 . However, it has recently been found by FRAP (fluorescence recovery after photobleaching) experiments 14 that TIS granules have dynamic protein components, similarly to L-bodies in frog oocytes 16 . This finding apparently indicates that the network still retains the liquid-like nature.…”

“…Physical mechanism of mesh-like structure formation in biological systems. Recent studies on phase separation forming mesh-like structures in TIS granules network 13,14 (see above), RNP (ribonucleoprotein) granules 15 , localisation bodies 16 , FXR1 condensates 17 , and NLRP6 18 all indicate the importance of unstructured RNAs promoting intermolecular RNA-RNA interactions in producing networks. Longer RNAs have more sites for intermolecular RNA-RNA interactions 65 .…”

Multivalent interactions with RNA drive recruitment and dynamics in biomolecular condensates in Xenopus oocytes

“…In vitro , low concentrations of RNA often promote liquid-liquid phase separation (LLPS) of proteins ( Burke et al., 2015 ; Lin et al., 2015 ; Molliex et al., 2015 ; Patel et al., 2015 ), whereas high concentrations can inhibit LLPS ( Banerjee et al., 2017 ; Maharana et al., 2018 ). Conversely, many RNAs are capable of protein-free self-assembly ( Jain and Vale, 2017 ; Langdon et al., 2018 ; Neil et al., 2021 ; Van Treeck et al., 2018 ). In vivo , recent studies have shown that RNA may be acting to form a non-dynamic or structural component within a variety of biomolecular condensates ( Clemson et al., 2009 ; Neil et al., 2021 ; Niepielko et al., 2018 ; Trcek et al., 2020 ; Van Treeck et al., 2018 ).…”