Multiphase condensates from a kinetically arrested phase transition

Abstract

The formation of biomolecular condensates through liquid-liquid phase separation from proteins and nucleic acids is emerging as a spatial organisational principle used by living cells. Many such biomolecular condensates are not, however, homogeneous fluids, but contain an internal structure consisting of distinct sub-compartments with different compositions. In many instances, such compartments inside the condensate are depleted in the biopolymers that make up the condensate. Here, we describe that this multiphase structure arises from a kinetically arrested phase transition. The combination of a change in composition coupled with a slow response to this change can lead to the spontaneous formation of multiple emulsions consisting of several inner cores within a polymer-rich middle phase. In the case of liquid-like biomolecular condensates, the slow diffusion of biopolymers causes nucleation of biopolymer-poor liquid inside of the condensate to achieve the new equilibrium composition. This framework shows that multiphase condensates can be a result of kinetic trapping, rather than thermodynamic stability, and provides and avenue to understand and control the internal structure of condensates in vitro and in vivo.

Protein structural transitions critically transform the network connectivity and viscoelasticity of RNA-binding protein condensates but RNA can prevent it

“…Such a severe imbalance in inter-molecular forces due to ageing—strong enough to drive the progressive dynamical arrest of proteins within droplets—contributes to rationalizing the physicochemical and molecular factors behind the intricate process of condensate ageing. Indeed, imbalance of inter-molecular forces has been shown to drive FUS single-component condensates to display multiphase architectures upon ageing [36, 124] or upon phosphorylation [85]. Furthermore, this imbalance is consistent with the formation of amorphous condensates observed in LARKS-containing proteins [57, 60, 62] such as hnRNPA1 [21], FUS [25], TDP-43 [125], or NUP-98 [33, 41].…”

RNAs undergo phase transitions with lower critical solution temperatures

“…Nevertheless, poly(rU) chains were previously shown to have access to both UCST and LCST-type transitions in the presence of multivalent ions 22 . Further, recent studies have shown that in monovalent salts, and in the presence of polyethylene glycol, RNA homopolymers exhibit UCST phase behavior 94 . Therefore, segregative transitions, i.e., phase separation, can be either LCST- or UCST-type depending on the solution conditions and types of solution ions, which directly influence the temperature dependent free energies of solvation.…”

Protein structural transitions critically transform the network connectivity and viscoelasticity of RNA-binding protein condensates but RNA can prevent it

“…Such a severe imbalance in inter-molecular forces due to ageing-strong enough to drive the progressive dynamical arrest of proteins within droplets-contributes to rationalizing the physicochemical and molecular factors behind the intricate process of condensate ageing. Indeed, imbalance of inter-molecular forces has been shown to drive FUS single-component condensates to display multiphase architectures upon ageing 36,125 or upon phosphorylation 86 . Furthermore, this imbalance is consistent with the formation of amorphous condensates observed in LARKS-containing proteins 58,61,63 such as hnRNPA1 21 , FUS 25 , TDP-43 126 , or NUP-98 33,41 .…”