Phosphorylation-Mediated Clearance of Amyloid-like Assemblies in Meiosis

LC Domain-Mediated Coalescence Is Essential for Otu Enzymatic Activity to Extend Drosophila Lifespan

In eukaryotic cells, RNA-binding proteins (RBPs) interact with RNAs to form ribonucleoprotein complexes (RNA granules) that have long been thought to regulate RNA fate or activity. Emerging evidence suggests that some RBPs not only bind RNA but also possess enzymatic activity related to ubiquitin regulation, raising important questions of whether these RBP-formed RNA granules regulate ubiquitin signaling and related biological functions. Here, we show that Drosophila Otu binds RNAs and coalesces to membrane-less biomolecular condensates via its intrinsically disordered low-complexity domain, and coalescence represents a functional state for Otu exerting deubiquitinase activity. Notably, coalescence-mediated enzymatic activity of Otu is positively regulated by its bound RNAs and co-partner Bam. Further genetic analysis reveals that the Otu/Bam deubiquitinase complex and dTraf6 constitute a feedback loop to maintain intestinal immune homeostasis during aging, thereby controlling longevity. Thus, regulated biomolecular condensates may represent a mechanism that controls dynamic enzymatic activities and related biological processes.

Friend or foe-Post-translational modifications as regulators of phase separation and RNP granule dynamics

Ribonucleoprotein (RNP) granules are membrane-less organelles consisting of RNA-binding proteins (RBPs) and RNA. RNA granules form through liquid-liquid phase separation (LLPS), whereby weak promiscuous interactions among RBPs and/or RNAs create a dense network of interacting macromolecules and drive the phase separation. Post-translational modifications (PTMs) of RBPs have emerged as important regulators of LLPS and RNP granule dynamics, as they can directly weaken or enhance the multivalent interactions between phase-separating macromolecules or can recruit or exclude certain macromolecules into or from condensates. Here, we review recent insights into how PTMs regulate phase separation and RNP granule dynamics, in particular arginine (Arg)-methylation and phosphorylation. We discuss how these PTMs regulate the phase behavior of prototypical RBPs and how, as "friend or foe," they might influence the assembly, disassembly, or material properties of cellular RNP granules, such as stress granules or amyloid-like condensates. We particularly highlight how PTMs control the phase separation and aggregation behavior of disease-linked RBPs. We also review how disruptions of PTMs might be involved in aberrant phase transitions and the formation of amyloid-like protein aggregates as observed in neurodegenerative diseases.

RNA Recognition-like Motifs Activate a Mitogen-Activated Protein Kinase

Smk1 is a mitogen-activated protein kinase (MAPK) family member in the yeast Saccharomyces cerevisiae that controls the postmeiotic program of spore formation. Ssp2 is a meiosis-specific protein that activates Smk1 and triggers the autophosphorylation of its activation loop. A fragment of Ssp2 that is sufficient to activate Smk1 contains two segments that resemble RNA recognition motifs (RRMs). Mutations in either of these motifs eliminated Ssp2's ability to activate Smk1. In contrast, deletions and insertions within the segment linking the RRM-like motifs only partially reduced the activity of Ssp2. Moreover, when the two RRM-like motifs were expressed as separate proteins in bacteria, they activated Smk1. We also find that both motifs can be cross-linked to Smk1 and that at least one of the motifs binds near the ATP-binding pocket of the MAPK. These findings demonstrate that motifs related to RRMs can directly activate protein kinases.

Developmental regulation of an organelle tether coordinates mitochondrial remodeling in meiosis

Cellular differentiation involves remodeling cellular architecture to transform one cell type to another. By investigating mitochondrial dynamics during meiotic differentiation in budding yeast, we sought to understand how organelle morphogenesis is developmentally controlled in a system where regulators of differentiation and organelle architecture are known, but the interface between them remains unexplored. We analyzed the regulation of mitochondrial detachment from the cell cortex, a known meiotic alteration to mitochondrial morphology. We found that mitochondrial detachment is enabled by the programmed destruction of the mitochondria-endoplasmic reticulum-cortex anchor (MECA), an organelle tether that bridges mitochondria and the plasma membrane. MECA regulation is governed by a meiotic transcription factor, Ndt80, which promotes the activation of a conserved kinase, Ime2. We further present evidence for Ime2-dependent phosphorylation and degradation of MECA in a temporally controlled manner. Our study defines a key mechanism that coordinates mitochondrial morphogenesis with the landmark events of meiosis and demonstrates that cells can developmentally regulate tethering to induce organelle remodeling.

The molecular language of membraneless organelles

Eukaryotic cells organize their intracellular components into organelles that can be membrane-bound or membraneless. A large number of membraneless organelles, including nucleoli, Cajal bodies, P-bodies, and stress granules, exist as liquid droplets within the cell and arise from the condensation of cellular material in a process termed liquid-liquid phase separation (LLPS). Beyond a mere organizational tool, concentrating cellular components into membraneless organelles tunes biochemical reactions and improves cellular fitness during stress. In this review, we provide an overview of the molecular underpinnings of the formation and regulation of these membraneless organelles. This molecular understanding explains emergent properties of these membraneless organelles and shines new light on neurodegenerative diseases, which may originate from disturbances in LLPS and membraneless organelles.

Phosphorylation Leads the Way for Protein Aggregate Disassembly

Protein aggregation can be beneficial, with important biological functions, but must be somehow controlled. In this issue of Developmental Cell, Carpenter et al. (2018) uncover how a solid-like supermolecular protein assembly that regulates yeast meiosis is disassembled through phosphorylation of a disordered prion-like domain to control the timing of meiotic progression.