Biomolecular condensate

Biomolecular partitioning

The term biomolecular condensates was introduced in the context of intracellular assemblies as a convenient and non-exclusionary term to describe non-stoichiometric assemblies of biomolecules.[15] The choice of language here is specific and important. It has been proposed that many biomolecular condensates form through liquid-liquid phase separation (LLPS), liquid-gel phase separation or liquid-solid phase separation;[27] however, unequivocally demonstrating that a cellular body forms through phase separation is challenging.[28][29][30][31] Similarly, while much attention has been paid to the formation of LLPS assemblies, different material states (liquid vs. gel vs. solid) are not always easy to distinguish in living cells.[32][33]

The term "biomolecular condensate" directly address both of these challenges by making no assumption regarding either the physical mechanism through which assembly is achieved, nor the material state of the resulting assembly. Consequently, cellular bodies that form through phase separation are a subset of biomolecular condensates, as are those where the physical origins of assembly are unknown. Historically, many cellular non-membrane bound compartments identified microscopically fall under the broad umbrella of biomolecular condensates.

Further, it appears that intrinsically disordered proteins might play a role in distinguishing between liquid phase states, though how they play this role remains unknown.[27]As of 2019, a light activated system had been developed to experimentally induce condensation.[27]

Formation and examples of nuclear membraneless compartments

Stress granule dynamics

Many examples of biomolecular condensates have been characterized in the cytoplasm and the nucleus.

Cytoplasmic biomolecular condensates thought to arise by either liquid-liquid, liquid-gel or liquid-solid phase separation:

• Lewy bodies
• Stress granule
• P-body
• Germline P-granules
• Lipid droplets
• Starch granules
• Glycogen granules
• Corneal lens formation and cataracts
• Other cytoplasmic inclusions such as pigment granules or cytoplasmic crystals
• Misfolded protein aggregation such as amyloid fibrils or mutant Haemoglobin S (HbS) fibres in sickle cell disease
• Membrane protein, or membrane-associated protein, clustering at neurological synapses, cell-cell junctions, or other membrane domains.
• Other large multiprotein complexes or supramolecular assemblies in the Wnt signaling pathway.[34][35]

It can also be argued that cytoskeletal filaments form by a polymerisation process similar to phase separation, except ordered into filamentous networks instead of amorphous droplets or granules.

Nuclear biomolecular condensates:

• Nucleolus [36]
• Cajal body
• Paraspeckle
• Synaptonemal complex

Other nuclear structures including heterochromatin and DNA condensation in condensed mitosis chromosomes form by mechanisms similar to phase separation, so can also be classified as biomolecular condensates.

One of the first discovered examples of highly dynamic biomolecular condensate were the supramolecular complexes formed by components of the Wnt signaling pathway.[37][38] The Dishevelled (Dsh) protein undergoes clustering in the cytoplasm via its DIX domain, which mediates protein polymerisation, and is important for signal transduction. The Dsh protein functions both in planar polarity and Wnt signalling, where it recruits another supramolecular complex (the Axin complex) to Wnt receptors at the plasma membrane. The formation of these Dishevelled and Axin containing droplets is conserved across metazoans, including in Drosophila, Xenopus, and human cells.

Another example of liquid droplets in cells are the germline P granules in Caenorhabditis elegans.[27][29] These granules separate out from the cytoplasm and form droplets, as oil does from water. Both the granules and the surrounding cytoplasm are liquid in the sense that they flow in response to forces, and two of the granules can coalesce when they come in contact. When (some of) the molecules in the granules are studied (via fluorescence recovery after photobleaching), they are found to rapidly turnover in the droplets, meaning that molecules diffuse into and out of the granules, just as expected in a liquid droplet. The droplets can also grow to be many molecules across (micrometres)[29] Studies of droplets of the Caenorhabditis elegans protein LAF-1 in vitro[39] also show liquid-like behaviour, with an apparent viscosity ${\displaystyle \eta \sim 10}$Pa s. This is about a ten thousand times that of water at room temperature, but it is small enough to enable the LAF-1 droplets to flow like a liquid.