title: Ultrastructure of C4b-binding protein fragments formed by limited proteolysis using chymotrypsin.
abstract: C4b-binding protein is a regulator of the classical pathway of the complement system, acting as a cofactor to the serine protease factor I in the degradation of C4b. Its molecular weight is approximately 570,000 and it is composed of multiple, disulfide-linked 70-kDa subunits. Visualized by electron microscopy (Dahlbäck, B., Smith, C. A., and Muller-Eberhard, H. J. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 3641-3645), it has an unusual spider-like structure with multiple thin (30 A), elongated (330 A) tentacles. The number of tentacles was estimated to be seven. Limited proteolysis by chymotrypsin produces fragments of approximately 50- and 160-kDa, the latter composed of multiple, disulfide-linked, 25-kDa polypeptides. We now have isolated the undenatured C4b-binding protein fragments formed by treatment of the protein with chymotrypsin and have visualized them by electron microscopy. The 160-kDa fragment comprises the central portion of the C4b-binding protein, which appears as a ringlike structure with an inner diameter of 13 A and an outer diameter of 60 A and having attached an approximately 40-A long piece of each tentacle. The liberated 50-kDa fragment constitutes the major part (290-A long) of the tentacles. Chymotrypsin digestion of C4b-binding protein was also monitored as a function of time by polyacrylamide gel electrophoresis and the number of subunits cleaved was found to be seven, supporting our previous ultrastructural data which suggested that C4b-binding protein contains seven identical tentacle-like subunits.
mesh_terms: D000595:Amino Acid Sequence; D002352:Carrier Proteins / Q000378:metabolism*; D002918:Chymotrypsin / Q000378:metabolism*; D003169:Complement Inactivator Proteins; D004591:Electrophoresis, Polyacrylamide Gel; D006023:Glycoproteins; D006801:Humans; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008970:Molecular Weight; D010446:Peptide Fragments / Q000032:analysis*
chemical_list: D002352:Carrier Proteins; D003169:Complement Inactivator Proteins; D006023:Glycoproteins; D046911:Macromolecular Substances; D010446:Peptide Fragments; D002918:Chymotrypsin