The first component of complement from the bullfrog, Rana catesbeiana: functional properties of C1- and isolation of subcomponent C1q.

Abstract We have found that a euglobulin fraction of bullfrog (Rana catesbeiana) serum can replace a similar fraction of guinea pig serum in the standard assay for C1̄, the activated first component of complement (C). In this assay, sheep erythrocytes sensitized with rabbit antibodies and coated with guinea pig C4 (EAC4gp) are reacted with the C1̄-containing test sample and then with guinea pig C2 and C3-9. This finding, together with earlier observations that some frog antibodies interact with guinea pig C and C1̄, suggested that the C1 components in frogs and in mammals might be similar in structure. CI is a complex of three subcomponents; one of these, C1q, binds to immunoglobulins and activates C1r, which, in turn, activates C1s, thereby triggering the classical C cascade. Clq, isolated from human serum, is an unusual protein, having both globular and collagen-like regions. To determine whether frog Clq has a similar structure, we isolated this protein from frog serum by methods similar to those used in the isolation of human Clq. The activity of frog Clq was measured by substituting it for human Clq in an assay utilizing human C1̄r̄ and C1̄s̄, EAC4gp, and guinea pig C2 and C3-9. By gel filtration, frog C1q seems to be similar in overall size to human C1q. As judged by polyacrylamide gel electrophoresis, the subunit structure of the two proteins also appears to be similar, with pairs of polypeptide chains cross-linked by disulfide bridges. Like the human protein, frog C1q contains the unusual amino acids, hydroxyproline and hydroxylysine, and is rich in glycine, suggesting that it also has a collagen-like domain. The combined evidence indicates that frog and human C1q are very similar, although not identical, in structure. It seems that the characteristic features of this protein, which forms a link between the immune and C systems, have been preserved in evolution, at least since the appearance of the class Amphibia.