A stickers-and-spacers framework explains the viscoelastic and aging properties of biomolecular condensates.

Abstract

Prion-like low complexity domains (PLCDs) in RNA-binding proteins drive phase separation and contribute to stress granule assembly. Time-dependent changes, also known as aging, of biomolecular condensates, are implicated as a mechanism underlying the pathogenesis of neurodegenerative diseases. The sequence grammar encoding the driving forces for phase separation has come into focus through the development and application of a stickers-and-spacers model that classifies stickers as the segments of the polymers that form physical crosslinks with one another and spacers as the segments connecting the stickers and influencing the solubility of PLCDs. While condensates are often described as liquids, growing evidence suggests that they have viscoelastic properties, consistent with a network fluid structure deriving from the stickers-and-spacers architecture of PLCDs. Here we focus on determining how sequence features encode viscoelasticity and the time-dependent changes of condensates. We systematically titrated the interaction strength of the aromatic stickers in the PLCD of hnRNPA1. We used passive microrheology with optical tweezers to determine the sticker-dependent effects and find that viscosity, and elasticity scale with the inter-sticker interaction strength. We also find that increasing the sticker strength in the condensates lengthens the timescales where their rheological behavior is dominated by an elastic response. In aging condensates, an increasing fraction of molecules show sub-diffusive behavior, even in the absence of fibrillization. Increasing the effective solvation volume of spacers accelerates aging, while increasing sticker strength delays it. Our results reveal for the first time the sequence grammar underlying material properties and aging of PLCD condensates and provide new insights into the mechanisms of condensate aging.