The purification and characterization of subcomponent C1s of the first component of bovine complement

Genomic and cDNA cloning of the human C1 inhibitor. Intron-exon junctions and comparison with other serpins

Amino acid sequencing of trypsin fragments of C1 inhibitor gave regions of low codon degeneracy that were used for oligonucleotide probes. Human liver cDNA libraries gave clones containing most of the protein sequence, showing that the inhibitory domain belongs to the 'serpin' class of protein inhibitors. Fragments of these cDNA clones were used to probe human genomic cosmid libraries. The genomic sequence was found to be about 17 X 10(3) base pairs, with a coding sequence of approximately 1800 base pairs containing introns at amino acid positions--6, 162, 207, 275, 321, 395, and one in the 5' non-coding region. There is very little similarity of intron position amongst the serpin genes. All but one of the intron positions in the C1 inhibitor structural gene correspond to surface residues if C1 inhibitor is considered to have a structure similar to the cleaved form of alpha 1-antiproteinase. The serine and threonine residues in the N-terminal 100 amino acids of the sequence thought to carry complex carbohydrates are found in a single exon.

Molecular cloning of cDNA for human complement component C1s. The complete amino acid sequence

The complete amino acid sequence (673 residues plus 15 residues of leader sequence) of human complement component C1s has been determined by nucleotide sequencing of cDNA clones from a human liver library probed with synthetic oligonucleotides. Much of the sequence is supported by independent amino acid sequence information. The cDNA sequence contains an anomalous "intron-like" sequence, including a stop codon, that can be discounted because of the amino acid sequence evidence. The N-terminal chain (422 residues) of C1s, like that of C1r with which it is broadly homologous, contains five domains: domains I and III are homologous to one another and to similar regions in C1r, domain II is homologous to the epidermal growth factor sequence found in C1r and several other proteins, and domains IV and V are homologous to one another and to the 60-residue repeating sequence found in C1r, C2, factor B, C4-binding protein and some apparently unrelated proteins. The sequence of the C-terminal chain (251 residues) agrees with that already established to be the "serine protease" domain of C1s.

The serine proteinase chain of human complement component C1s. Cyanogen bromide cleavage and N-terminal sequences of the fragments

Human complement component C1s was purified from fresh blood by conventional methods of precipitation and chromatography. The single-chain zymogen form was activated by treatment with C1r. Reduction and carboxymethylation then allowed the light chain and heavy chain to be separated on DEAE-Sepharose CL-6B in 8 M-urea. Liquid-phase sequencing of the light chain determined 50 residues from the N-terminus. CNBr-cleavage fragments of the light chain were separated by high-pressure liquid chromatography on gel-permeation and reverse-phase columns. N-Terminal sequencing of these fragments determined the order of a further 138 residues, giving a total of 188 residues or about 75% of the light chain. Seven of these eight sequences could be readily aligned with the amino acid sequences of other serine proteinases. The typical serine proteinase active-site residues are clearly conserved in C1s, and the specificity-related side chain of the substrate-binding pocket is aspartic acid, as in trypsin, consistent with the proteolytic action of C1s on C4 at an arginine residue. Somewhat surprisingly, when the C1s sequence is compared with that of complement subcomponent C1r, the percentage difference (59%) is approximately the same as that found between the other mammalian serine proteinases (56-71%).

Primary structure of bovine complement activation fragment C4a, the third anaphylatoxin. Purification and complete amino acid sequence

Purification of C4a from heat-activated bovine plasma by elution from CM-Sephadex C-50 at pH 7.4 and gel filtration on Sephadex G-50 gives a 20% yield of pure C4a. The complete amino acid sequence of bovine C4a has been determined by automatic sequencer degradation of CNBr and enzymic fragments, and by carboxypeptidase digestion. The 77-residue bovine sequence shows 12 differences from the human sequence with five of these differences occurring in the C-terminal 11 residues. The sequence of C4a confirms earlier suggestions of homology with C3a and C5a: the three sequences show an almost equal number of identities with each other. The six cysteine residues of the 'disulphide knot' are conserved as well as seven other residues including the C-terminal arginine.

A study on the structure and interactions of the C1 sub-components C1r and C1s in the fluid phase

1. Both proenzyme and activated C1r, which are dimers at pH 7.4, dissociated into monomers at pH 5.0 (C1r) and 4.0 (C1r), as shown by the decrease of apparent molecular weight and of sedimentation coefficient, which was shifted from 7.1 S (dimer) to 5.0 S (monomer). 125I-labelling of C1r in the presence of lactoperoxidase occurred, for the dimer, 16-20% in the A chain and 80-84% in the B chain, whereas the distribution was 67.5% and 32.5%, respectively, for the monomer. It appears likely that the two monomers of C1r interact through their A chain and that the A and B chains are relatively independent from each other. 2. 125I-labelling of C1s in the presence of lactoperoxidase confirmed the calcium-dependent dimerization of this subcomponent. In the monomer, the B chain appears to be embedded in the A chain, as shown by the 125I- distribution in these chains, which was 5% and 95%, respectively. This changed after dimerization to 25% and 75%, respectively, which suggests that interactions occur through the A chain of each monomer and lead to an unfolding of the B chain. 3. C1r dimer and C1s monomer were found to interact in the absence of calcium to form a C1r2-C1s complex (7.7 S), whereas in the presence of calcium the two sub-components were associated into a C1r2-C1s2 complex (8.7S). It appears likely that the formation of this tetrameric complex involves both calcium-dependent, and calcium-independent binding forces, and that C1r and C1s interact through their respective A chain which, in the case of C1s, is hidden upon association.

The isolation and characterization of bovine C4a, an activation fragment of the fourth component of complement

The fourth component of bovine complement, C4, was cleaved specifically by subcomponent C1s to produce two fragments, C4a and C4b. The smaller, C4a, was isolated in pure form and is a peptide of 9500 mol.wt. containing approx. 84 amino acids and no detectable carbohydrate. C4a has an amino acid composition that is comparable with the anaphylatoxins C3a and C5a, containing six cysteine residues/mol and a high proportion of basic residues. The amino acid sequence of the first thirteen residues shows four identities with the porcine C3a sequence. There is almost complete identity between the C4a sequence and that of the alpha-chain of human C4, indicating that this region is highly conserved. This evidence also clearly establishes that C4a is cleaved from the N-terminal of the alpha-chain of C4.