Assembly of the functional membrane attack complex of human complement: formation of disulfide-linked C9 dimers

The 158,000 Mr protein, previously designated C5c, present in fully assembled complement (C) membrane attack complexes (MC5b-9) has been identified as a disulfide-bonded dimer of C9. This conclusion was based on the observations that: (i) a portion of the 125I-radiolabeled precursor C9 incorporated into MC5b-9 complexes comigrated with the 158,000 Mr protein band in NaDodSO4/polyacrylamide slab gels; (ii) monospecific antisera produced against native C9 and the 158,000 Mr protein immunologically crossreacted with monomeric native C9 by double immunodiffusion and with monomeric C9 and the 158,000 Mr protein on immunoreplication procedures; and (iii) two-dimensional NaDodSO4/polyacrylamide slab gel electrophoresis, in which the second dimension was conducted under reducing conditions, revealed that the 158,000 Mr protein contained two identical 71,000 Mr subunits which comigrated with monomeric C9. Molar ratio estimates indicated that 1 mol of C5b, C9 dimer, C6, C7, and C8 and 3-4 mol of C9 monomer were present per MC5b-9 complex. Each fully assembled membrane-bound MC5b-9 complex would therefore have a calculated Mr of 982,000. The presence of C9 dimers in the hemolytically active 29S dimeric form of the MC5b-9 complex and the absence of C9 dimers in the hemolytically inactive 23S monomeric form of the fluid phase SC5b-9 complex strongly suggest an important role for C9 dimer formation in MC5b-9 complex structure and function. The most probable function of C9 dimers would be the formation of intercomplex disulfide crosslinks which would provide a mechanism to stabilize the assembly of MC5b-9 into aggregates of increasing size on the target membrane surface which would thus be responsible for the observed pore size heterogeneity of functional C lesions.