title: Biosynthesis and postsynthetic processing of human C3b/C4b inactivator (factor I) in three hepatoma cell lines.
abstract: Human factor I is a two-chain plasma glycoprotein composed of disulfide-linked 50,000- and 38,000-dalton subunits. Analysis of its biosynthesis and postsynthetic processing demonstrated that factor I is synthesized as a single chain precursor (pro-I) that undergoes glycosylation and limited proteolysis to generate the native protein. One of three human hepatoma cell lines, HepG2 , secreted factor I predominantly (70-90%) in a single chain pro-I form. The other cell lines secrete factor I predominantly in its two chain native form. The defect in conversion of pro-I to I in HepG2 was protein specific since other multichain proteins, derived from single chain precursors, the third, fourth, and fifth components of complement were processed normally. Further analysis of the inefficient pro-I to I conversion by HepG2 revealed that Xenopus oocytes injected with HepG2 mRNA secreted factor I in a predominantly two-chain form. In addition, the apparent sizes of native factor I, transferrin, and alpha-1-antitrypsin secreted by the three hepatoma lines differed due to differences in postsynthetic processing.
mesh_terms: D000818:Animals; D001798:Blood Proteins / Q000096:biosynthesis* / Q000235:genetics; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002460:Cell Line; D003180:Complement C3b Inactivator Proteins / Q000096:biosynthesis* / Q000235:genetics; D015935:Complement C4b; D005260:Female; D005340:Fibrinogen / Q000302:isolation & purification; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms / Q000378:metabolism*; D046911:Macromolecular Substances; D008970:Molecular Weight; D009865:Oocytes / Q000378:metabolism; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics; D014981:Xenopus
chemical_list: D001798:Blood Proteins; D003180:Complement C3b Inactivator Proteins; D046911:Macromolecular Substances; D012333:RNA, Messenger; D015935:Complement C4b; D005340:Fibrinogen