title: A model system for the study of the assembly and regulation of human complement C3 convertase (classical pathway).
abstract: The formation of classical C3 convertase of complement and its regulation by C4b-binding protein (C4bp) were studied using two different approaches: (a) the analysis was first carried out in fluid phase; a soluble stabilized C3 proconvertase could be assembled from C4b (or C4b-like C4) and iodine-treated C2 in the presence of Ni2+ ions. Upon activation of this complex by C1s, a C3 convertase C4b(C4b-like C4)-C2a was generated which was able to cleave purified C3. C4bp dissociated both C3 proconvertase and C3 convertase, but its effect was more important on C3 convertase. (b) A model system of phospholipid vesicles has been developed to study the assembly of the C3 convertase on a membrane. Among different phospholipid mixtures tested, P-glycerol/P-choline vesicles were found most effective for C4b binding. Optimal conditions were determined for C4b fixation on these vesicles; bound C4b participated in the formation of a functional membrane-associated C3 convertase. C4bp was found to bind to phospholipid vesicles with a higher affinity than C4b; it was able to dissociate the vesicle-associated C3 convertase.
mesh_terms: D002021:Buffers; D002352:Carrier Proteins / Q000378:metabolism / Q000502:physiology; D003166:Complement Activating Enzymes / Q000378:metabolism*; D003167:Complement Activation; D050577:Complement C3-C5 Convertases / Q000378:metabolism*; D003181:Complement C4 / Q000378:metabolism / Q000502:physiology; D015935:Complement C4b; D003171:Complement Pathway, Classical; D006801:Humans; D016167:Integrin alphaXbeta2; D008081:Liposomes / Q000032:analysis; D008954:Models, Biological; D010713:Phosphatidylcholines / Q000378:metabolism; D010715:Phosphatidylglycerols / Q000378:metabolism; D011951:Receptors, Complement / Q000032:analysis
chemical_list: D002021:Buffers; D002352:Carrier Proteins; D003181:Complement C4; D016167:Integrin alphaXbeta2; D008081:Liposomes; D010713:Phosphatidylcholines; D010715:Phosphatidylglycerols; D011951:Receptors, Complement; D015935:Complement C4b; D003166:Complement Activating Enzymes; D050577:Complement C3-C5 Convertases