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The membrane attack complex of complement: C5b-8 complex as accelerator of C9 polymerization
Insights
The complement protein C5b-8 complex significantly accelerates C9 polymerization, forming tubular structures within minutes. The ratio of C9 to C5b-8 influences the formation and structure of the final C5b-9 complex.
Area of Science:
- Immunology
- Complement System Biology
Background:
- Complement protein C9 polymerization is a crucial step in the formation of the membrane attack complex (MAC).
- Spontaneous C9 polymerization is slow, requiring days to complete.
- The tetramolecular complex C5b-8 is known to interact with C9.
Purpose of the Study:
- To investigate the role of the C5b-8 complex in accelerating C9 polymerization.
- To determine how the molar ratio of C9 to C5b-8 affects the formation of tubular poly C9.
- To examine C9 polymerization in the context of whole serum and its effect on C5b-9 complex ultrastructure.
Main Methods:
- Investigated spontaneous C9 polymerization kinetics at 37°C.
- Studied C9 polymerization induced by the C5b-8 complex using phospholipid vesicles.
- Analyzed the formation of tubular and non-tubular poly C9 using SDS-resistance assays.
- Generated C5b-9 complexes on rabbit erythrocytes using human serum and assessed C9 polymerization.
Main Results:
- C5b-8 dramatically accelerated C9 polymerization, completing it within 10 minutes compared to over 3 days spontaneously.
- The molar ratio of C9:C5b-8 was critical; ratios of 9:1 or 12:1 resulted in 72% SDS-resistant tubular poly C9.
- Lower ratios favored non-tubular poly C9 formation.
- In erythrocytes, high serum concentration or C9-supplemented serum led to up to 40% SDS-resistant tubular poly C9, with incomplete polymerization observed otherwise.
Conclusions:
- The C5b-8 complex acts as a potent accelerator for C9 polymerization.
- The relative concentrations of C5b-8 and C9 dictate the ultrastructure of the C5b-9 complex, specifically the formation of tubular poly C9.
- Understanding these dynamics is key to comprehending complement-mediated cell lysis.
Abstract:
Polymerization of C9 occurs spontaneously or can be induced by the tetramolecular complex C5b-8. Spontaneous C9 (0.15 mg/ml) polymerization required more than 3 days at 37 degrees C. In the presence of C5b-8, C9 polymerization was complete within 10 min. The molar C9:C5b-8 ratio determined the extent of tubular poly C9 formation by C5b-8-bearing phospholipid vesicles. When this ratio was 9:1 or 12:1, 72% of complex-bound C9 was present as SDS resistant tubular poly C9 (Mr = 1.1 X 10(6]. At lower C9:C5b-8 ratios, poly C9 was bound primarily in nontubular form. Tubular poly C9, as part of C5b-9, could also be generated on rabbit erythrocytes by using whole human serum as a complement source. At limiting serum concentration (molar C9 to C8 ratio approximately 2), no SDS-resistant tubular poly C9 was detected. At high serum concentration or when using serum that was supplemented with C9, up to 40% of the C9 was SDS-resistant tubular poly C9, and the rest was poly C9, which was incompletely polymerized. It is suggested that the C5b-8 complex acts as an accelerator of C9 polymerization, and that its relative concentration to C9 determines the ultrastructure of the C5b-9 complex.