Phosphorylation sites are evolutionary checkpoints against liquid-solid transition in protein condensates

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Multivalent RNA-binding protein fused in sarcoma (FUS) forms intracellular condensates.
• These condensates can exist as functional liquid-like droplets or transition to less dynamic, potentially toxic amyloid and hydrogel states.
• Understanding mechanisms preventing toxic transitions is crucial for cellular health.

Purpose of the Study:

• To investigate how posttranslational phosphorylation regulates the liquid-solid phase transition of FUS condensates.
• To determine the role of phosphorylation site number and arrangement in preventing amyloid formation.
• To explore the evolutionary strategies employed by mammalian FUS to balance phase separation and condensate stability.

Main Methods:

• Residue-specific coarse-grained simulations of 85 mammalian FUS sequences.
• All-atom simulations of amyloid-prone FUS fragments.
• Detailed evolutionary analysis of FUS prion-like domains (PLDs).

Main Results:

• The number and spatial arrangement of phosphorylation sites significantly influence intracluster dynamics, preventing FUS condensate conversion to amyloids.
• Phosphorylation effectively reduces the β-sheet propensity in amyloid-prone FUS fragments.
• Mammalian FUS PLDs are enriched in amyloid-prone regions and strategically positioned phosphosites, suggesting an evolved mechanism for controlled self-assembly and phase separation.

Conclusions:

• Posttranslational phosphorylation acts as a critical 'handle' to prevent the liquid-solid transition of intracellular FUS condensates.
• Evolution has harnessed amyloid-prone sequences for enhanced phase separation while utilizing nearby phosphorylation sites to safeguard against detrimental liquid-solid transitions.
• This mechanism ensures the functional liquid-like state of FUS condensates, preventing the formation of toxic aggregates.