Intrinsically disordered linkers determine the interplay between phase separation and gelation in multivalent proteins

Abstract

Phase transitions of linear multivalent proteins control the reversible formation of many intracellular membraneless bodies. Specific non-covalent crosslinks involving domains/motifs lead to system-spanning networks referred to as gels. Gelation transitions can occur with or without phase separation. In gelation driven by phase separation multivalent proteins and their ligands condense into dense droplets, and gels form within droplets. System spanning networks can also form without a condensation or demixing of proteins into droplets. Gelation driven by phase separation requires lower protein concentrations, and seems to be the biologically preferred mechanism for forming membraneless bodies. Here, we use coarse-grained computer simulations and the theory of associative polymers to uncover the physical properties of intrinsically disordered linkers that determine the extent to which gelation of linear multivalent proteins is driven by phase separation. Our findings are relevant for understanding how sequence-encoded information in disordered linkers influences phase transitions of multivalent proteins.

Protein Phase Separation: A New Phase in Cell Biology

“…IDPs and proteins undergoing phase separation are enriched in SLiMs and degenerate repeats, which can serve as primary protein-binding modules [51] (Figure 2A,B). Specificity may be related to additional features of IDPs, such as the number and spacing of repetitive binding motifs (multivalency), their post-translational modifications (PTMs), or the dynamics of the intervening linkers [44,78]. Nonspecific electrostatic interactions, especially with RNA, could be critical to nucleate droplet assembly, and different IDRs respond differently to changing ionic strength [10,49].…”

“…Here, valence refers to the effective numbers of adhesive domains/motifs that provide specificity in intra- as well intermolecular interactions. Recent computer simulations and adaptations of the theories of associative polymers show that multivalent proteins may be parsed into associative domains/motifs, so-called stickers, interspersed by spacers [78]. The stickers enable physical crosslinking, whereas the spacers or linkers determine whether or not gelation will be driven by phase separation.…”

Directed Growth of Biomimetic Microcompartments

“…[26] Importantly, weak multivalent interactions underlie the nucleation process. [19,23,27,28] For FET family proteins, the composition of interspacing residues (so-called spacers) that segregates the stickers plays an important role in modulating the interaction strength and thus the liquid-to-solid transition. However, in the test tube, droplet growth continues until the thermodynamic equilibrium has been reached.…”

Phospho‐regulation of intrinsically disordered proteins for actin assembly and endocytosis

“…Formation of liquid droplets is likely a more general phenomenon that can occur via an accumulation of transient and weak interactions between proteins with multiple interacting domains or motifs [41]. Furthermore, organization into a phase-separated liquid droplet may accompany the formation of a gel-like state (gelation) within the droplet in a process known as gelation [42]. Key determinants of whether a set of interacting proteins has the capacity for gelation include the number of interacting surfaces, the strength of individual interactions, and the intrinsic disorder of the connecting regions between the interacting motifs [42].…”