Complement proteins C5b-9 induce transbilayer migration of membrane phospholipids.

Area of Science:

• Biochemistry
• Cell Biology
• Immunology

Background:

• The terminal complement pathway involves the assembly of the membrane attack complex (MAC), C5b-9.
• Understanding the interaction of complement proteins with cell membranes is crucial for elucidating their biological functions.

Purpose of the Study:

• To investigate transbilayer migration of membrane phospholipids induced by the terminal human complement proteins.
• To determine the specific complement protein complex responsible for initiating phospholipid movement.

Main Methods:

• Asymmetric vesicles with pyrene-labeled phosphatidylcholine (pyrenePC) in the inner monolayer were prepared.
• Vesicles were exposed to purified C5b-9 proteins, and transbilayer phospholipid exchange was monitored using pyrene excimer/monomer fluorescence.
• Complement protein C8 binding to the C5b67 complex was identified as the trigger for phospholipid migration.

Main Results:

• Membrane deposition of C5b67 complex did not alter pyrenePC fluorescence.
• Addition of C8 to C5b67-bound vesicles caused a dose-dependent decrease in the excimer/monomer ratio, indicating transbilayer phospholipid exchange.
• This effect was observed with or without C9, and not in control vesicles.

Conclusions:

• C8 binding to the C5b67 complex initiates transbilayer phospholipid exchange.
• Approximately 1% of total membrane phospholipid undergoes net transbilayer migration upon C8 binding.
• This phospholipid exchange may play a significant role in the biological activity of terminal complement proteins.