SC5b-9 complex of complement: Formation of the dimeric membrane attack complex by removal of S-protein

E. R. Podack, H. J. Muller-Eberhard

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SC5b-9 produced in serum by complement (C) activation is the hydrophilic analogue of the amphiphilic membrane attack complex (MAC) extracted from the membranes of C-lysed cells. Treatment of SC5b-9 with 145 mM desoxycholate (DOC) selectively dissociated the S-protein from the complex and produced C5b-9, which expressed amphiphilic properties, as evidenced by aggregation of the complex in the absence of detergents or phospholipids. When DOC was used at the critical micellar concentration (3.3 mM), C5b-9 formed dimers that corresponded in ultrastructure to the dimeric MAC. After recombination with phospholipids, C5b-9 formed aggregates of anular structures that were covered with a lipid monolayer and that resembled in size and shape C lesions. Proteolysis of SC5b-9 by 10% (w/w) trypsin and 10% (w/w) chymotrypsin in presence of 6 mM DOC also led to dissociation of the S-protein. In addition, some C6 antigen was released from the complex. Although other subunits of C5b-9 were cleaved by this treatment, the complex remained otherwise intact. It is concluded that strong noncovalent and covalent interactions within the complex render it virtually resistant to dissociation by proteolysis or high concentration of DOC. These studies suggest that S-protein binds to the forming C5b-9 complex at its membrane binding site and at sites involved in dimerization, thereby preventing membrane attack and MAC formation. They also indicate that C5b-9 in monomeric SC5b-9 and in the dimeric MAC are closely related in structure.

Original languageEnglish (US)
Pages (from-to)1779-1783
Number of pages5
JournalJournal of Immunology
Issue number4
StatePublished - Jan 1 1980

ASJC Scopus subject areas

  • Immunology and Allergy
  • Immunology


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