title: Complementary DNA and derived amino acid sequence of the alpha subunit of human complement protein C8: evidence for the existence of a separate alpha subunit messenger RNA.
abstract: The entire amino acid sequence of the alpha subunit (Mr 64,000) of the eighth component of complement (C8) was determined by characterizing cDNA clones isolated from a human liver cDNA library. Two clones with overlapping inserts of net length 2.44 kilobases (kb) were isolated and found to contain the entire alpha coding region [1659 base pairs (bp)]. The 5' end consists of an untranslated region and a leader sequence of 30 amino acids. This sequence contains an apparent initiation Met, signal peptide, and propeptide which ends with an arginine-rich sequence that is characteristic of proteolytic processing sites found in the pro form of protein precursors. The 3' untranslated region contains two polyadenylation signals and a poly(A) sequence. RNA blot analysis of total cellular RNA from the human hepatoma cell line HepG2 revealed a message size of approximately 2.5 kb. Features of the 5' and 3' sequences and the message size suggest that a separate mRNA codes for alpha and argues against the occurrence of a single-chain precursor form of the disulfide-linked alpha-gamma subunit found in mature C8. Analysis of the derived amino acid sequence revealed several membrane surface seeking domains and a possible transmembrane domain. These occur in a cysteine-free region of the subunit and may constitute the structural basis for alpha interaction with target membranes. Analysis of the carbohydrate composition indicates 1 or 2 asparagine-linked but no O-linked oligosaccharide chains, a result consistent with predictions from the amino acid sequence.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Chemistry and School of Medicine, University of South Carolina, Columbia 29208.
authors.forename: A G
authors.initials: AG
authors.lastname: Rao
authors.forename: O M
authors.initials: OM
authors.lastname: Howard
authors.forename: S C
authors.initials: SC
authors.lastname: Ng
authors.forename: A S
authors.initials: AS
authors.lastname: Whitehead
authors.forename: H R
authors.initials: HR
authors.lastname: Colten
authors.forename: J M
authors.initials: JM
authors.lastname: Sodetz
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002241:Carbohydrates / Q000032:analysis; D003185:Complement C8 / Q000235:genetics*; D004247:DNA / Q000032:analysis*; D004262:DNA Restriction Enzymes; D006133:Growth Substances / Q000235:genetics; D006801:Humans; D046911:Macromolecular Substances; D008970:Molecular Weight; D011487:Protein Conformation; D012333:RNA, Messenger / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid; D014158:Transcription, Genetic
chemical_list: D002241:Carbohydrates; D003185:Complement C8; D006133:Growth Substances; D046911:Macromolecular Substances; D012333:RNA, Messenger; D004247:DNA; D004262:DNA Restriction Enzymes