Mechanisms for Active Regulation of Biomolecular Condensates

Abstract

Liquid-liquid phase separation is a key organizational principle in eukaryotic cells, on par with intracellular membranes. It allows cells to concentrate specific proteins into condensates, increasing reaction rates and achieving switch-like regulation. However, it is unclear how cells trigger condensate formation or dissolution and regulate their sizes. We predict from first principles two mechanisms of active regulation by post-translational modifications such as phosphorylation: In enrichment-inhibition, the regulating modifying enzyme enriches in condensates and the modifications of proteins inhibit their interactions. Stress granules, Cajal bodies, P granules, splicing speckles, and synapsin condensates obey this model. In localization-induction, condensates form around an immobilized modifying enzyme, whose modifications strengthen protein interactions. Spatially targeted condensates formed during transmembrane signaling, microtubule assembly, and actin polymerization conform to this model. The two models make testable predictions that can guide studies into the many emerging roles of biomolecular condensates.Eukaryotic cells contain numerous types of membraneless organelles, which contain between a few and thousands of protein and RNA species that are highly enriched in comparison to the surrounding nucleoplasm or cytoplasm. These biomolecular condensates are held together by weak, multivalent and highly collaborative interactions, often between intrinsically disordered regions of their constituent proteins (Banani et al., 2017;Shin and Brangwynne, 2017).In contrast to membrane-bound organelles, cells can regulate the formation and size of condensates by posttranslational modifications of one or a few key proteins, most prominently by phosphorylation. The modifications usually lie within intrinsically disordered regions and modulate the strength of attractive interactions with other condensate components (Bah and Forman-Kay, 2016; Fung et al., 2018). Due to the highly cooperative nature of phase transitions, small changes in interaction strengths can result in the formation or dissolution of condensates, and this switch-like, dynamic nature makes them ideal for regulation.For instance the nucleolus, Cajal bodies, splicing speckles, paraspeckles, and PML bodies in the nucleus and P-bodies in the cytoplasm have to be dissolved during mitosis and reformed afterwards to ensure a balanced distribution of their content to daughter cells (Rai et al., 2018;Dundr and Misteli, 2010). Stress granules form upon cellular stress and are dissolved when the stress ceases (Wippich et al., 2013).Whereas these long-known, floating droplet or-ganelles are large enough to be visible using simpler 31 light microscopic techniques, in the past years liquid-32 liquid phase separation has been implicated in mul-33 tifarious processes in which -often sub-micrometer-34 sized -condensates are formed at particular sites in the 35 cell: at sites of DNA repair foci (Altmeyer et al., 2015), 36 Polycomb-mediated chromatin silencing (Tatavo...

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