Multiple forms of complement C3 in trout that differ in binding to complement activators.

In all other species analyzed to date, the functionally active form of complement component C3 exists as the product of a single gene. We have now identified and characterized three functional C3 proteins (C3-1, C3-3, and C3-4) in trout that are the products of at least two distinct C3 genes. All three proteins are composed of an alpha-and a beta-chain and contain a thioester bond in the alpha-chain. However, they differ in their electrophoretic mobility, glycosylation, reactivity with monospecific C3 antibodies, and relative ability to bind to various surfaces (zymosan, Escherichia coli, erythrocytes). A comparison of the partial amino acid sequences of the three proteins showed that the amino acid sequence identity/similarity of C3-3 to C3-4 is 87/91%, while that of C3-3 and C3-4 to C3-1 is 51.5/65.5% and 60/73% respectively. Thus, trout possess multiple forms of functional C3 that represent the products of several distinct genes and differ in their ability to bind covalently to various complement activators.