Spontaneous driving forces give rise to protein−RNA condensates with coexisting phases and complex material properties

Significance

Biomolecular condensates comprise multiple protein and RNA molecules that are typically organized into complex, multilayered structures. Although recent studies have shown that multilayered condensates can arise spontaneously, the interactions that drive the spontaneous transitions remain unclear. Further, can molecules within the same layer have different dynamical properties or do such complex features require inputs of energy? Here, we report results from in vitro studies, which show that coexisting liquid- and solid-like material properties and multilayered architectures can result from spontaneous, sequence-encoded driving forces. Our studies, which are directed at the simplest biologically relevant protein and RNA sequences, suggest that spontaneous processes make key contributions to the formation of condensates with complex morphologies and diverse material properties.

Abstract

Phase separation of multivalent protein and RNA molecules underlies the biogenesis of biomolecular condensates such as membraneless organelles. In vivo, these condensates encompass hundreds of distinct types of molecules that typically organize into multilayered structures supporting the differential partitioning of molecules into distinct regions with distinct material properties. The interplay between driven (active) versus spontaneous (passive) processes that are required for enabling the formation of condensates with coexisting layers of distinct material properties remains unclear. Here, we deploy systematic experiments and simulations based on coarse-grained models to show that the collective interactions among the simplest, biologically relevant proteins and archetypal RNA molecules are sufficient for driving the spontaneous emergence of multilayered condensates with distinct material properties. These studies yield a set of rules regarding homotypic and heterotypic interactions that are likely to be relevant for understanding the interplay between active and passive processes that control the formation of functional biomolecular condensates.