A role for Q/N-rich aggregation-prone regions in P-body localization

Abstract

P-bodies are cytoplasmic foci that are sites of mRNA degradation and translational repression. It is not known what causes the accumulation of RNA-degradation factors in Pbodies, although RNA is required. The yeast Lsm1-7p complex (comprising Lsm1p to Lsm7p) is recruited to P-bodies under certain stress conditions. It is required for efficient decapping and degradation of mRNAs, but not for the assembly of Pbodies. Here we show that the Lsm4p subunit and its asparagine-rich C-terminus are prone to aggregation, and that this tendency to aggregate promotes efficient accumulation of Lsm1-7p in P-bodies. The presence of glutamine-and/or asparagine-rich (Q/N-rich) regions in other P-body components suggests a more general role for aggregation-prone residues in P-body localization and assembly. This is supported by reduced P-body accumulation of Ccr4p, Pop2p and Dhh1p after deletion of these domains, and by the observed aggregation of the Q/Nrich region from Ccr4p. Supplementary material available online at

mRNP granules

“…These domains are unusually common in proteins involved in RNA metabolism, and examples of proteins whose prion-like domain contributes to granule assembly include TIA-1/Pub1 in stress granules, 12,33 and Lsm4 in yeast P-bodies. 28 34 Recently, in vitro studies demonstrated that at sufficient concentrations, proteins containing prion-like domains such as FUS can assemble "hydrogel" structures, capable of interacting with other prion-like domains from a wide range of known mRNP granule proteins. 38 Taken together, prion-like domains may therefore be important in localization of factors to mRNP granules, as well as granule assembly.…”

“…16 However, studies in which assembly of P-bodies or stress granules is impaired typically reveal no significant change in the stability of the mRNAs examined. 12,28,34,130 It is therefore likely that mRNA decay can proceed on individual mRNPs outside of granules. However, the studies referred to above examined only a handful of mRNAs, thus it remains possible that specific mRNAs are reliant on assembly into P-bodies, or related structures, for efficient decay.…”

Long-term memory consolidation: The role of RNA-binding proteins with prion-like domains

“…186 Similarly, Q/N domains of the Lsm4 protein that is required for P body formation in yeast and GW182 that is required for P body assembly in human and Drosophila cells mediate RNA granule aggregation. [186][187][188][189] Although conventional high-affinity interactions also contribute to RNA granule assembly, 173 recent evidence suggest that prion-like domains mediate weak interactions among RBPs that drive a "liquid-liquid phase transition" from a dispersed soluble state in the cytoplasm to a condensed state that resemble "liquid droplets.." Consistent with such a model, RNA granules are dynamic in that, like liquid droplets, their component proteins exchange and move freely within the granule and also exchange with the cytoplasmic pool. The importance of such liquid-liquid demixing phase transition is best demonstrated in C.elegans P-body assembly, where the granules show typical liquid-droplet characteristics like wetting of membranes, fusion and dripping during assembly.…”

Prions, amyloids, and RNA: Pieces of a puzzle

“…117 In yeast, several P-body proteins contain Q/N-rich regions that can aggregate in vivo. 118,119 In mammals, P-bodies contain fibrils of TNRC6A (GW182) 120 and DDX6 (RCK, p54) 121 proteins; both of which also have CBRs / LCRs. 120,121 Even though the presence of multiple amyloidogenic proteins with CBRs / LCRs in P-bodies and stress granules led to the hypothesis that functional amyloids are implicated in the biogenesis of these RNPs, 45,111,118,119 so far there is no proof that these proteins are present in RNPs in the amyloid state.…”

“…118,119 In mammals, P-bodies contain fibrils of TNRC6A (GW182) 120 and DDX6 (RCK, p54) 121 proteins; both of which also have CBRs / LCRs. 120,121 Even though the presence of multiple amyloidogenic proteins with CBRs / LCRs in P-bodies and stress granules led to the hypothesis that functional amyloids are implicated in the biogenesis of these RNPs, 45,111,118,119 so far there is no proof that these proteins are present in RNPs in the amyloid state. 122 Indeed, it appears that initial formation of these RNPs is enthalpy driven, depends on multivalent interactions involving both CBRs/LCRs and RNAbinding domains of proteins and RNAs, and leads to the formation of large RNP complexes in extremely dynamic phase-separated liquidlike droplets.…”