Mechanistic insights into the promotion or inhibition of phase separation by polyubiquitin chains of different linkages

Abstract

ALS-linked ubiquitin-binding shuttle protein UBQLN2 mediates crosstalk among several protein quality control pathways, including proteasomal degradation and autophagy that are likely mediated by interactions with K48-linked and K63-linked polyubiquitin chains, respectively. We recently showed that UBQLN2 is recruited to stress granules in cells and undergoes liquid-liquid phase separation (LLPS) in vitro. However, interactions with ubiquitin or multivalent K48-linked chains eliminate LLPS. Surprisingly, other multivalent chains of longer lengths, including K63- and M1-linked chains and a designed tetrameric ubiquitin construct, significantly enhanced UBQLN2 LLPS by increasing the volume, but not the concentration, of the dense phase.