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Viral Defense and Phase Separation

What is liquid-liquid phase separation (LLPS)?

Over the past few decades, there has been groundbreaking exploration within cell biology into the topic of the structures within the cell and how they organize and function, which ultimately has implications for disease states [1]. Within the cell, there are structures bound with a membrane like the mitochondria or the nucleus. Separate from these membrane-bound structures within the cell, there are areas of the cell that are phase separated from the membrane, like islands. They are referred to as liquid-like membrane-less organelles, or MLOs [2]. These MLOs are not defined by a membrane but tend to associate and dissociate depending on factors such as the chemical nature of the constituents, pH, and temperature [3]. In their higher functioning state, these clusters of self-organizing, ordered, and disordered liquid particles come together to perform functions like assisting in the generation of energy (ATP), stress response, gene expression, and even viral replication (see Figure 1) [3]. They are a viable, vital cellular repository of proteins, RNA, and nucleic acids, often working together for these biological functions [4,5]. Since they tend to be higher in protein with interactions that allow for rapid rates of reactions, they tend to take on characteristics of liquids [5]. Thus, they are often referred to as droplets or condensates [5].


Specifically, this liquid-liquid phase separation (LLPS), which may appear as organized random clusters, does not have a cell membrane, but is in equilibrium at a certain temperature and concentration of particles. Any change in cellular activity can alter their interaction. In some cases, the MLOs may become dysfunctional and result in the aggregation of proteins like amyloid and tau, thereby being implicated in neurodegenerative conditions (see Figure 1) [2, 3, 6]. Several conditions such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis have protein aggregation as part of their pathology; and, therefore, investigation of LLPS may be worthwhile to prevent or treat these conditions [5]. Along these lines, emerging research indicates that natural components, such as epigallocatechin gallate from green tea, may be able to regulate the LLPS of these proteins [6,7]. Moreover, there is literature to suggest that there would be clinical implications of LLPS in cancer and viral replication [5].

PS Image 1.png

Figure 1. Functions and dysfunctions of liquid–liquid phase separation (LLPS)-driven membrane-less organelles (MLOs).

Image Credit: Brocca, S.; Grandori, R.; Longhi, S.; Uversky, V. Liquid–Liquid Phase Separation by Intrinsically Disordered Protein Regions of Viruses: Roles in Viral Life Cycle and Control of Virus–Host Interactions. Int. J. Mol. Sci. 2020, 21, 9045. (

Even though there is clinical relevance to LLPS and understanding the intracellular dynamics of these MLOs, it is difficult to observe or assess as it would involve methods such as fluorescence-based or contrast-based microscopy or even macroscopy of cellular contents to look at optical density and light scattering, techniques that most non-research clinicians do not use or apply [3,5]. However, there is the increasing suggestion that these (dys)functional aggregates within the cell may be associated with the underlying pathology of various chronic diseases. Therefore, more scientific-clinical crosstalk needs to occur for better understanding and implementation of findings.


The connection to viruses

One of the areas of research as it relates to LLPS is the nature of proteins and viruses contained therein. There are types of proteins that do not follow traditional conformations that are referred to as “intrinsically disordered proteins” or IDPs (Figure 2). Viruses with these proteins or with regions where they collect “intrinsically disordered regions” are often infectious areas due to their ability to bind host proteins [3]. Moreover, viruses can overtake these regions to engage in viral replication and assembly to manufacture viral proteins, and in so doing, they can be somewhat protected within these areas from host immune defense [3,8]. As one could postulate, these areas of the cell could be potential targets for pharmaceutical and nutrient interventions. For example, there is research investigating specific viruses such as SARS-CoV2 and HIV-1 to understand their LLPS dynamics to formulate therapeutic targets [9].

Figure 2. Functions of intrinsically disordered proteins (IDPs).

Image Credit: Brocca, S.; Grandori, R.; Longhi, S.; Uversky, V. Liquid–Liquid Phase Separation by Intrinsically Disordered Protein Regions of Viruses: Roles in Viral Life Cycle and Control of Virus–Host Interactions. Int. J. Mol. Sci. 2020, 21, 9045. (

What is the role of melatonin in LLPS?

Melatonin may seem to be a likely candidate to affect LLPS, or at least viral activity related to early viral infection through LLPS and mitochondrial dynamics. Published research exists to tout the robust anti-viral action of melatonin [10-14]. Concerning LLPS, preliminary research compiled by Loh and Reiter [15] suggests that melatonin application in the early stages of infection, specific to SARS-CoV2, may help to prevent the formation of these viral “factories” through LLPS, thereby blocking several of their outputs to try to overtake the transcriptomic features of the cell. They would further contend that melatonin could be the key molecule to prevent viral dominance over the host genome during the early stages of acute infection [15]. 


Another aspect of melatonin that makes it beneficial in its anti-viral action is its role in the mitochondria. The mitochondria are viral targets to control cellular respiration and innate immune response [16]. Melatonin is a well-known mitochondrial protector and regulator of metabolism and mitophagy [17]. Furthermore, through its antioxidant potential, melatonin may be helpful in protection of the mitochondria if there were to be viral infection. While LLPS is an area in its infancy with potential to explore more extensively in relationship to melatonin, there exists established clinical literature to support the use of melatonin for immune health, neurodegenerative conditions, and cancer.



In conclusion, due to its multi-factorial characteristics, and potentially its role in LLPS, melatonin may be able to be instrumental in preventing the unhealthy buildup of proteins that could subsequently lead to neurodegeneration, along with preventing or mitigating viral infections through interfering with LLPS and subsequent viral takeover of the cell. While the mechanisms would seem to imply there is a plausible link here between melatonin and LLPS, further research is needed to understand specifics and to test hypotheses.

Author: Deanna Minich, Ph.D.

Reviewer: Kim Ross, DCN

Last updated: February 14, 2023




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