Murine complement component C4 and sex-limited protein: identification of amino acid residues essential for C4 function

Two clusters of acidic amino acids near the NH2 terminus of complement component C4 alpha'-chain are important for C2 binding

Previous work has indicated a role for the NH2-terminal segment of the C3 alpha'-chain in the binding interactions of C3b with a number of its protein ligands. In particular, we have identified two clusters of acidic residues, namely, E736 and E737 and to a lesser extent D730 and E731, as being important in the binding of C3b to factor B and complement receptor 1 and the binding of iC3b to complement receptor 3. Whereas human C3 and C4 have an overall sequence identity of 29%, over a segment near the NH2 termini of their respective alpha'-chains the sequence identity is 56% (70% chemical similarity). Given the functional similarity between the C4b-C2 and C3b-B interactions in the respective formation of the classical and alternative pathway C3 convertases, as well as the sequence conservation of two acidic clusters, we hypothesized that residues 744EED and 749DEDD within the NH2-terminal segment of the C4 alpha'-chain would mediate in part the binding of C2 to C4b. We tested this hypothesis using three independent approaches. Site-directed mutagenesis experiments revealed that replacing subsets of the charged residues by their isosteric amides within either acidic cluster resulted in molecules having reduced C2 binding activity. Moreover, a synthetic peptide (C4 residues 740-756) encompassing the two acidic clusters was a specific inhibitor of the binding of C2 to red cell-associated C4b. Finally, Ab raised against the above peptide was able to block the interaction between red cell-associated C4b and fluid phase C2. Taken together, these results strongly suggest that the NH2-terminal acidic residue-rich segment of C4 alpha'-chain contributes importantly to the interaction of C4b with C2.

Distal recognition site for classical pathway convertase located in the C345C/netrin module of complement component C5

Previous studies focused on indels in the complement C345 protein family identified a number of potential protein-protein interaction sites in components C3 and C5. Here, one of these sites in C5, near the alpha-chain C terminus, was examined by alanine-scanning mutagenesis at 16 of the 18 non-alanine residues in the sequence KEALQIKYNFSF RYIYPLD. Alanine substitutions affected activities in the highly variable manner characteristic of binding sites. Substitutions at the lysine or either phenylalanine residue in the central KYNFSF sequence had the greatest effects, yielding mutants with <20% of the normal activity. These three mutants were also resistant to the classical pathway (CP) C5 convertase, with sensitivities roughly proportional to their hemolytic activities, but had normal susceptibilities to the cobra venom factor (CVF)-dependent convertase. Synthetic peptide MGKEALQIKYNFS-NH2 was found similarly to inhibit CP but not CVF convertase activation, and the effects of alanine substitutions in this peptide largely reflected those of the equivalent mutations in C5. These results indicate that residues KYNFSF form a novel, distal binding site for the CP, but not CVF convertase. This site lies approximately 880 residues downstream of the convertase cleavage site within a module that has been independently named C345C and NTR; this module is found in diverse proteins including netrins and tissue inhibitors of metalloproteinases.

Sex-limited protein: in vitro and in vivo functions

Mouse complement component C4 exists in two isoforms, C4 and a protein with expression restricted to male animals called sex-limited protein (Slp). Although Slp is about 95% homologous to C4, it is generally believed to be non-functional, at least in conventional haemolytic complement assays. In a previous study, however, we showed that Slp is haemolytically active in a C1-inhibitor (C1INH)-regulated, EDTA-resistant mouse complement activation pathway. To study other possible implications of this finding, we generated constitutively expressing Slp-transgenic mice. The transgene was crossed into otherwise Slp-deficient C57Bl/6J and NZB mice. Members of the third backcross generation of C57Bl/6J mice were tested for functional Slp and classical and alternative complement pathway activities (CH50 and AP50 levels, respectively). Slp-transgenic C57Bl/6J mice showed enhanced CH50, but normal AP50 levels when compared with non-transgenic littermates. To discover a possible protective role for Slp in spontaneous systemic lupus erythematosus (SLE) in NZBxNZW (NZBxW) mice, the third backcross generation of Slp-transgenic NZB mice was mated with NZW mice and the development of SLE in the female offspring was followed. In these introductory experiments, Slp-transgenic NZBxW animals presented with a significantly extended life span. Our results imply that Slp is a mouse complement component with functions which partially resemble some of those of human C4A.

Active Sites in Complement Component C3 Mapped by Mutations at Indels

Engineered mutants of human complement component C3 were used to test the idea that sites of length polymorphisms in protein families (indels) can guide a search for protein:protein interaction sites. Sequence changes were introduced at each of the 27 indels in the C3/4/5 protein family, and mutants at 26 indels were expressed by transiently transfected COS cells. Expressed proteins were assayed 1) for concentration, by ELISA and by autoradiography of radiolabeled protein; 2) for classical pathway hemolytic activity; 3) for susceptibility to proteolytic activation by the alternative pathway and cobra venom factor C3 convertases; and 4) for susceptibility to complement factor I in the presence of factor H. Most of the mutations did not appreciably alter expression or activity relative to wild-type C3, consistent with the idea that most indels occur at the protein surface. Mutations at four indels severely damaged C3 functional activity, but did not affect the stability or structure of the protein, as assessed by their effects on expression by COS cells and on susceptibility to cleavage by C3 convertases and factor I. These indels are therefore near functionally important amino acid residues; they represent good candidates for sites of protein:protein interactions. Mutation of the sequence at a fifth indel altered the equilibrium between the latent and reacted C3 conformations, and mutations at 4 other indels substantially decreased both protein activity and expression. The mutants provided an overview of the structural and functional roles played by different parts of C3.

Duplication of the MHC-linked Xenopus complement factor B gene

We have previously reported the molecular cloning of the mammalian major histocompatibility complex (MHC) class III gene, complement factor B (Bf) from Xenopus laevis, and linkage of the gene to the frog MHC. Here, we estimated the copy number of the Xenopus Bf gene by genomic Southern blotting analysis and demonstrated that Xenopus laevis has two copies of the Bf gene. Both genes co-segregated with the MHC-linked HSP70 genes among 19 offspring of an f/r x f/r cross, indicating a close linkage of the two Bf genes to the frog MHC. Both genes are transcribed and contain open reading frames. When compared with the previously determined cDNA sequence (Xenopus Bf A), the predicted amino acid sequence of the second cDNA species (Xenopus Bf B) shows 82% overall identity. Polymerase chain reaction analysis indicated that all of the partially inbred frogs with the f, r, g, and j MHC haplotypes, as well as 12 outbred frogs tested have both Bf genes, suggesting that the duplicated Bf genes are stable genetic traits in Xenopus laevis.

Slp is an essential component of an EDTA-resistant activation pathway of mouse complement

Slp (sex-limited protein) is a mouse serum protein encoded by a major histocompatibility complex class III gene. It is considered to be a product of a duplicated complement component C4 gene, but without functional activity. Originally it has been found expressed only in adult males with the S region of the H-2d or H-2s haplotype. In this report we present evidence that Slp is involved in a form of mouse complement activation that occurs after fractionation of serum by polyethylene glycol precipitation. This activation pathway is EDTA-resistant (i.e., independent of classical and alternative pathway activation), is regulated by C1 inhibitor, and leads to the generation of hemolytically active membrane attack complexes. A positive correlation between this EDTA-resistant mouse complement activity and reported Slp levels was found. Direct evidence for a functional role of Slp came from substitution experiments in which purified Slp induced hemolytic activity in polyethylene glycol-fractionated, Slp-deficient mouse serum. Selective depletion of other complement components suggested a role for C1s-, C2, and C5, but not C3, in the Slp-dependent complement activation. A model for this type of mouse complement activation is presented.

Mutants of complement component C3 cleaved by the C4-specific C1-s protease

To identify some of the structural features determining specific protease recognition of complement components C3 and C4, we used site-specific mutagenesis to construct mutants of murine C3 that are cleaved by the C4-specific C1-s protease. Insertion of three amino acid residues corresponding to residues at the C1-s cleavage site of human C4 into murine C3 at the analogous C3 convertase cleavage site was adequate to render the mutant protein susceptible to C1-s cleavage. In addition, insertion of C3-specific residues at the same site or introduction of the C4-specific residues as substitutions rather than as an insertion also rendered the site susceptible to cleavage, but with 10- to 50-fold lower efficiencies, and insertion of even a single amino acid residue affected recognition by C1-s. Finally, insertion of amino acid residues into mC3 partially inhibited cleavage by the alternative-pathway C3 convertase, with insertion of C3- or C4-specific residues giving about the same level of inhibition. A simple interpretation of these data is that C1-s cleavage is dependent primarily on steric accessibility and on recognition of specific amino acid residues at the cleavage site, whereas C3 convertase cleavage is dependent primarily on specific interactions distal to the cleavage site, with only relatively weak, non-C3-specific interactions at the cleavage site itself.