Composition tunes physical properties of multi-component biomolecular condensates

Biomolecular condensates are membrane-less compartments enriched in proteins and nucleic acids, and are thought to provide unique microenvironments for cellular biochemistry. While design principles are emerging to specify condensate properties through protein sequence, much less is known about how condensate microenvironment can be regulated when the molecular identities are fixed. In multi-component systems, one possible route is through modulation of condensate composition. To explore this, we employ quantitative phase imaging and thermodynamic tie-line measurements to measure the stoichiometry inside model multi-component condensates containing full-length FUS protein and RNA. By systematically tuning the ratio of the total protein and RNA concentrations, we find that stoichiometry inside the condensate is a non-linear function of the total stoichiometry and can be tuned over a wide range. As a probe of the local environment inside the condensate, we measure the fluorescence emission spectra of the solvatochromatic dye Nile Red. We observe significant stoichiometry-dependent shifts in the local emission spectra, suggesting that tuning local composition is sufficient to significantly alter the local microenvironment. We discuss implications for the regulation of biochemical reactions by condensate composition.