title: Post-translational modification of the fourth component of complement. Sulfation of the alpha-chain.
abstract: The fourth component of complement (C4) was found to incorporate radiolabel from [35S]O4 during synthesis in murine peritoneal macrophages and the human hepatoma-derived cell line, HepG2. The sulfate label was localized to the COOH-terminal autolytic fragment of the C4 alpha-chain. No label was seen associated with intracellular pro-C4. The structurally similar third and fifth components of complement, and alpha 2-macroglobulin, did not incorporate labeled sulfate. Tryptic peptides from [35S]O4-labeled C4 alpha-chain were analyzed by reversed phase liquid chromatography and found to elute in a single, homogeneous peak, suggesting a unique sulfation site in the C4 alpha-chain. Thin layer chromatography of a base hydrolysate of [35S]O4-labeled C4 alpha-chains, or C4 isolated from human plasma, revealed the presence of tyrosine-O-sulfate. The possible significance of this unusual amino acid modification for the function of C4 is unknown.
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D003181:Complement C4 / Q000235:genetics* / Q000302:isolation & purification; D046911:Macromolecular Substances; D008264:Macrophages / Q000378:metabolism*; D051379:Mice; D010446:Peptide Fragments / Q000032:analysis; D011499:Protein Processing, Post-Translational; D013431:Sulfates / Q000378:metabolism; D013462:Sulfur Radioisotopes; D014357:Trypsin
chemical_list: D003181:Complement C4; D046911:Macromolecular Substances; D010446:Peptide Fragments; D013431:Sulfates; D013462:Sulfur Radioisotopes; D014357:Trypsin