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.
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Rao AG, Howard OM, Ng SC, Whitehead AS, Colten HR, Sodetz JM
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.
Biochemistry. 1987 Jun 16;26(12):3556-64.
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- 2820471 [ View in PubMed]
- 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)