Micron-scale, liquid-liquid phase separation in ternary lipid membranes containing DPPE

Area of Science:

• Membrane biophysics
• Lipid phase behavior
• Cell membrane structure

Background:

• Micron-scale liquid-liquid phase separation is crucial for cell membrane function.
• Model membranes typically use phosphatidylcholines and sphingomyelins, but phosphatidylethanolamines (PEs) are underrepresented.
• PEs are abundant in biological membranes and influence protein function and membrane fusion.

Purpose of the Study:

• To investigate the role of phosphatidylethanolamines (PEs) in supporting liquid-liquid phase separation in model cell membranes.
• To map miscibility phase diagrams for ternary lipid mixtures containing PEs.
• To test the hypothesis that PEs promote phase separation due to their higher melting temperatures.

Main Methods:

• Substitution of a saturated PE lipid (DPPE) for its corresponding phosphatidylcholine (DPPC) in two established ternary lipid mixtures.
• Utilized fluorescence microscopy to map full ternary phase diagrams for giant vesicles.
• Analyzed phase diagrams across a range of temperatures to observe phase transitions and compositions.

Main Results:

• Contrary to expectations, no micron-scale liquid-liquid phase separation was observed in DOPC/DPPE/cholesterol mixtures.
• Only a limited region of liquid-liquid phase separation was found in DiPhyPC/DPPE/cholesterol mixtures.
• Widespread coexisting solid and liquid phases were observed, with DPPE enriching the solid phase.

Conclusions:

• Saturated PE lipids (DPPE) do not promote micron-scale liquid-liquid phase separation in these ternary model membranes.
• PE lipids appear to favor the formation of solid phases over liquid-ordered or liquid-disordered phases.
• The findings challenge the assumption that higher melting temperatures of PEs directly correlate with increased liquid-liquid phase separation.