Designed IDPs phase separate and mix or demix according to sequence designed parameters

Biomolecular condensates, self-assembled clusters of proteins and nucleic acids, form through liquid phase separation, creating membraneless organelles for compartmentalization. The composition of these condensates largely consists of multivalent proteins and nucleic acids, many of which contain intrinsically disordered regions (IDRs).

Traditionally, the relationship between IDRs and phase separation has been studied individually for single-component condensates. However, the co-phase separation and mixing of two or more IDRs in a condensate is a more intricate process, as the self- and cross-interactions of the sequences can vary widely, and their relative abundance in solution differs.

In a recent study, researchers explored the effects of IDR sequence and composition on phase separation and mixing using 18 LAF-1 RGG variants with the same length and amino-acid composition but varying residue order. These variants focused on altering charge patterning and, to a lesser extent, hydropathy patterning while keeping the protein composition constant. Each IDR sequence adopts a more extended conformation in single-component condensed phases due to a more favorable, self-solvated environment.

The study discovered that mixing two IDR sequences into a condensate disrupts the universal scaling behavior observed in single-component phases. This disruption is due to the relative interactions between the two components and the overall composition of the condensate. Two distinct qualitative behaviors were observed: cooperative co-condensation when both sequences are subcritical, and scaffold-client behavior when one sequence is supercritical.

Scaffold-client systems generally exhibit a high degree of demixing, while co-condensing systems are generally quite well-mixed in the dense phase.

Interestingly, even when IDR sequences have significantly different patterning parameters, they can still mix. This finding suggests that the behavior of a sequence alone can indicate whether it will mix or separate from a second component, but it does not fully determine the outcome. Thus, predicting the behavior of a single sequence can provide insights into its interaction with another sequence in a condensate, but it is not a complete predictor of the overall mixing or phase separation behavior.