title: Molecular composition of the terminal membrane and fluid-phase C5b-9 complexes of rabbit complement. Absence of disulphide-bonded C9 dimers in the membrane complex.
abstract: The terminal membrane C5b-9(m) and fluid-phase SC5b-9 complexes of rabbit complement were isolated from target sheep erythrocyte membranes and from inulin-activated rabbit serum respectively. In the electron microscope, rabbit C5b-9(m) was observed as a hollow protein cylinder, a structure identical with that of human C5b-9(m). Monodispersed rabbit C5b-9(m) exhibited an apparent sedimentation coefficient of 29 S in deoxycholate-containing sucrose density gradients, corresponding to a composite protein-detergent molecular-weight of approx. 1.4 X 10(6). Protein subunits corresponding to human C5b-C9 were found on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. By densitometry, there were consistently six molecules of monomeric C9 present for each monomeric C5b-8 complex. Fluid-phase rabbit SC5b-9 was a hydrophilic 23 S ma macromolecule that differed in subunit composition from its membrane counterpart in that it contained S-protein and only two to three molecules of C9 per monomer complex. The data are in accord with the previous report on human C5b-9 that C5b-9(m) contains more C9 molecules than SC5b-9 [Ware & Kolb (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6426-6430]. They corroborate the previous molecular-weight estimate of approx. 10(6) for C5b-9(m) and thus support the concept that the fully assembled, unit lesion of complement is a C5b-9 monomer [Bhakdi & Tranum-Jensen (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 1818-1822]. They also show that C9 dimer formation is not required for assembly of the rabbit C5b-9(m) protein cylinder, or for expression of its membrane-damaging function.
mesh_terms: D000818:Animals; D002499:Centrifugation, Density Gradient; D003186:Complement C9 / Q000032:analysis*; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins / Q000302:isolation & purification; D003720:Densitometry; D004220:Disulfides / Q000032:analysis; D004591:Electrophoresis, Polyacrylamide Gel; D004910:Erythrocyte Membrane / Q000032:analysis; D006801:Humans; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008970:Molecular Weight; D010455:Peptides / Q000032:analysis; D011817:Rabbits; D012756:Sheep
chemical_list: D003186:Complement C9; D015938:Complement Membrane Attack Complex; D004220:Disulfides; D046911:Macromolecular Substances; D010455:Peptides; D003165:Complement System Proteins