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Complement proteins C5b-9 induce transbilayer migration of membrane phospholipids

B W Van der Meer1, R D Fugate, P J Sims

  • 1Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation, Oklahoma City 73104.

Biophysical Journal
|November 1, 1989
PubMed

Insights

The terminal complement proteins C5b-9 initiate phospholipid transbilayer exchange upon C8 binding. This process, involving approximately 1% of membrane phospholipids, is crucial for complement-mediated biological activities.

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.

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