  Evidence accumulated over the past decade provides support for liquid-liquid
phase separation as the mechanism underlying the formation of biomolecular
condensates, which include not only membraneless organelles such as nucleoli
and RNA granules, but additional assemblies involved in transcription,
translation and signaling. Understanding the molecular mechanisms of condensate
function requires knowledge of the structures of their constituents. Current
knowledge suggests that structures formed via multivalent domain-motif
interactions remain largely unchanged within condensates. Two different
viewpoints exist regarding structures of disordered low-complexity domains
within condensates; one argues that low-complexity domains remain largely
disordered in condensates and their multivalency is encoded in short motifs
called stickers, while the other argues that the sequences form cross-beta
structures resembling amyloid fibrils. We review these viewpoints and highlight
outstanding questions that will inform structure-function relationships for
biomolecular condensates.
