The ParB-CTP cycle activates phase separation in bacterial DNA segregation

Area of Science:

• Cell biology
• Biophysics
• Molecular mechanisms of intracellular organization

Background:

• Membraneless organelles form via phase transitions, crucial for cell function but poorly understood.
• The bacterial ParABS system uses ParB proteins for chromosome segregation, with ParB binding CTP.
• Existing models fail to explain ParB recruitment and phase separation dynamics.

Purpose of the Study:

• To reconcile the ParB-CTP cycle with phase separation models.
• To elucidate the mechanism of ParB-mediated condensate formation.
• To understand the role of the ParB-CTP cycle in bacterial chromosome segregation.

Main Methods:

• Numerical simulations of droplet formation.
• Experimental analysis of ParB variants with disrupted CTP cycles.
• Investigating ParB-CTP interactions and phase transitions.

Main Results:

• The ParB-CTP cycle enhances ParB-ParB interactions, triggering phase transitions.
• ParB variants unable to complete the CTP cycle fail to undergo phase separation.
• A mechanistic framework for ParB-CTP-mediated phase transitions is established.

Conclusions:

• The ParB-CTP cycle is a molecular switch controlling phase separation.
• This mechanism explains the specific recruitment and assembly of ParB condensates.
• Findings offer insights into spatial control of intracellular condensate formation.