The return of the rings: evolutionary role of aromatic residues in liquid-liquid phase separation

Abstract

Aromatic residues appeared relatively late in the evolution of protein sequences. They stabilize the hydrophobic core of globular proteins and are typically absent from intrinsically disordered regions (IDRs). However, recent advances in protein liquid-liquid phase separation (LLPS) studies have shown that aromatic residues in IDRs often act as important "stickers", promoting multivalent interactions and the formation of higher-order oligomers. To reconcile this apparent contradiction, we compared levels of sequence disorder in RNA binding proteins and the human proteome and found that aromatic residues appear more frequently than expected in the IDRs of RNA binding proteins, which are often found to undergo LLPS. Phylogenetic analysis shows that aromatic residues are highly conserved among chordates, highlighting their importance in LLPS-driven functional assembly. These results suggest therefore that aromatic residues have contributed twice to evolution: in stabilizing structured proteins and in the assembly of biomolecular condensates.