A covalent dimer of complement C4b serves as a subunit of a novel C5 convertase that involves no C3 derivatives.
A C intermediate, LAC14, was prepared from TNP-aminocaproyl liposomes sensitized with anti-TNP antibody (Ab) and purified human C1 and C4. LAC14, containing radiolabeled C4, was analyzed by SDS-PAGE followed by autoradiography, and yielded a 210-kDa band and a predominant 400-kDa band. The 210-kDa band consisted of monomeric C4b bound to low molecular mass acceptors. The 400-kDa band was comprised of a 200-kDa moiety, as well as beta- and gamma-chains of C4. The 200-kDa moiety contained neither C1 nor sensitizing Ab, but it was largely decreased by treatment with NH2OH to the 90-kDa moiety with the mobility corresponding to the alpha'-chain of C4b. A covalent dimer of C4b, therefore, is the predominant form of C4b deposited on liposomes sensitized with antibody. The C4b-C4b dimer formed rapidly (within 5 min) followed by slow dissociation into monomers. The LAC14 bearing the C4b dimer but not the monomer was lysed, although with relatively low efficiency, by the addition of oxyC2 and EDTA-supplemented C3-deficient serum (C3DS), and, furthermore, LAC142 possessed the ability to convert C5 into C5a and C5b. Moreover, lysis was inhibited not by anti-C3 Ab but by anti-C4 Ab. In other experiments, the dimer served as an element of C3 convertase, as well. These findings imply that the C4b dimer, when complexed with C2, expresses C3/C5 convertase activity without participation of C3, and may provide a molecular mechanism whereby sera from patients with complete C3 deficiency retain the ability to induce C-mediated cytolysis.
Masaki
T
T
Department of Immunology, Center for Adult Diseases, Osaka, Japan.
Matsumoto
M
M
Yasuda
R
R
Levine
R P
RP
Kitamura
H
H
Seya
T
T
D001345:Autoradiography; D003176:Complement C3 / Q000378:metabolism; D050577:Complement C3-C5 Convertases / Q000096:biosynthesis* / Q000502:physiology; D015935:Complement C4b / Q000378:metabolism*; D003170:Complement Pathway, Alternative; D003171:Complement Pathway, Classical; D015180:Electrophoresis, Gel, Two-Dimensional; D006801:Humans; D008081:Liposomes / Q000276:immunology
D003176:Complement C3; D008081:Liposomes; D015935:Complement C4b; D050577:Complement C3-C5 Convertases