Phase separation in biology; functional organization of a higher order

Abstract

Inside eukaryotic cells, macromolecules are partitioned into membrane-bounded compartments and, within these, some are further organized into non-membrane-bounded structures termed membrane-less organelles. The latter structures are comprised of heterogeneous mixtures of proteins and nucleic acids and assemble through a phase separation phenomenon similar to polymer condensation. Membrane-less organelles are dynamic structures maintained through multivalent interactions that mediate diverse biological processes, many involved in RNA metabolism. They rapidly exchange components with the cellular milieu and their properties are readily altered in response to environmental cues, often implicating membrane-less organelles in responses to stress signaling. In this review, we discuss: (1) the functional roles of membrane-less organelles, (2) unifying structural and mechanistic principles that underlie their assembly and disassembly, and (3) established and emerging methods used in structural investigations of membrane-less organelles.

Tau protein liquid–liquid phase separation can initiate tau aggregation

“…Several disordered proteins with LCDs have been shown to undergo LLPS leading to liquid droplet formation in aqueous solution upon increasing the molecular crowding in the solution (Mitrea et al , 2016). Cells use LLPS to reversibly assemble membrane‐less organelles such as stress granules, p‐granules, or nucleoli (reviewed in Mitrea et al , 2016; Hyman et al , 2014).…”

“…Cells use LLPS to reversibly assemble membrane‐less organelles such as stress granules, p‐granules, or nucleoli (reviewed in Mitrea et al , 2016; Hyman et al , 2014). In a number of LLPS proteins, including Ddx4 (Nott et al , 2014), domains of low amino acid complexity introduce an unequal charge distribution along the protein backbone resulting in multivalency of these polypeptides with alternating patches of high positive or negative charge densities (Toretsky & Wright, 2014).…”

“…For many LLPS proteins such as the helicases Ddx4 and LAF‐1 and hnRNPA1, arginine‐rich positively charged sequences (e.g., RGG boxes, RRMs, SR repeats) drive their LLPS in a ionic strength dependent manner (reviewed in Mitrea et al , 2016). Interestingly, p‐tau441 droplet formation appeared to be insensitive to increasing salt conditions (≤ 1 M NaCl), suggesting that shielding of electrostatic interactions had a small effect on LLPS of p‐tau441.…”

“…Electrostatic forces are often predominant drivers for the LLPS of IDPs or protein domains (Mitrea et al , 2016). Phosphorylation introduces negative charges into proteins and changes the charge distribution and the electrostatic interactions along the polypeptide backbone, and hence also the LLPS characteristics of proteins.…”

“…Intracellular LLPS and the resulting functional membrane‐less organelles, which biophysically can be described as gels or polyelectrolyte brushes, are emerging concepts in cell biology, which are now implicated in manifold cellular functions and applications (Li et al , 2012; Aumiller et al , 2014; Elbaum‐Garfinkle et al , 2015; Jiang et al , 2015; Aguzzi & Altmeyer, 2016; Mitrea et al , 2016; Schmidt & Görlich, 2016; Schmidt & Rohatgi, 2016). For research in neurodegenerative diseases, in particular, the function and misfunction of LLPS by involved proteins—such as tau, FUS, TDP43, hnRNPA1, C9orf72 dipeptide repeats—introduces a novel exciting concept that may provide a common underlying mechanism in these diseases.…”