Liquid-liquid phase separation, biomolecular condensates, puncta, non-stoichiometric supramolecular assemblies, membraneless organelles, and bacterial chemotaxis are best understood as emergent phenomena with switch-like behaviour

TL;DR

This paper explains how liquid-liquid phase separation (LLPS) in cells exhibits switch-like behavior, and demonstrates that small molecular assemblies can produce such responses without forming large visible condensates.

Contribution

It introduces the use of the Hill equation as a simple model for cooperative, switch-like behavior in LLPS, highlighting that small assemblies can generate strong responses.

Findings

Small molecular assemblies (around ten molecules) can produce switch-like responses.

Large, visible condensates are not necessary for switch-like biological functions.

The Hill equation effectively models cooperative behavior in LLPS.

Abstract

Liquid-liquid phase separation (LLPS) is currently of great interest in cell biology. LLPS is an example of what is called an emergent phenomenon -- an idea that comes from condensed-matter physics. Emergent phenomena have the characteristic feature of having a switch-like response. I show that the Hill equation of biochemistry can be used as a simple model of strongly cooperative, switch-like, behaviour. One result is that a switch-like response requires relatively few molecules, even ten gives a strongly switch-like response. Thus if a biological function enabled by LLPS relies on LLPS to provide a switch-like response to a stimulus, then condensates large enough to be visible in optical microscopy are not needed.