Cells cluster biomolecules to form membrane-less organelles known as biological condensates most notably through liquid-liquid phase separation (LLPS). Unlike simple systems which undergo LLPS, such as oil in water, cells are far from simple; for example, they contain a vast diversity in components and are influenced by active processes. I am interested in the important principles dictating the form of and the relevant functional consequences from biological condensates in cells. During my graduate work, I explored the principles of stress-triggered phase separation of PABP from yeast which drive its LLPS (Riback et al., (2017) Cell). During my postdoc, I have begun elucidating the thermodynamics of LLPS in live cells; so far, I have found that endogenous LLPS lacks a fixed critical concentration instead being driven by heterotypic interactions between biomolecules (Riback et al., (2020) Nature, Riback and Brangwynne (2020) Science).

Biological condensates are frequently the location where various biomolecules assemble into specific and stable tertiary structure (e.g. RNA-protein complexes) where upon they are expelled. The physics governing this recruitment of substrates and expulsion of products along with how condensates impact such assembly reactions are not well understood. During my postdoc, I have begun to explore these questions in the formation of ribosomal subunits; some of my initial results link the expulsion of assembled ribosomal subunits and strong recruitment of nascent rRNA to heterotypic-drive LLPS thermodynamics (Riback et al., (2020) Nature).

Intrinsically disordered protein regions (IDRs) have been implicated in LLPS. IDRs can be viewed through the lens of polymer physics where the solvent quality for a polymer will dictate its tendency to collapse and undergo LLPS. During my graduate work, I studied the P-domain, a conserved hydrophobic and low change IDR of PABP and its role in PABP condensation in vitro and in vivo in yeast (Riback et al., (2017) Cell). Mutations that varied the P-domain's propensity to collapse correlated with PABP's propensity to LLPS in agreement with expectations from polymer physics. Additionally, my graduate work developed innovative methods to measure the solvent quality of polymers including IDPs from a single SAXS experiment (Riback et al., (2017) Science , Riback et al (2019) PNAS). When applied to unfolded state (or DSE) of foldable-proteins they showed slight contraction of the chain preventing collapse which in contrast to the P domain maybe due to their high fraction of charged residues. Based on polymer physics and the results on the P domain, we conjecture that the unfolded state only slightly contracts to minimize misfolding and aggregation of proteins during their synthesis.