Component C3 of hagfish complement has a unique structure: identification of native C3 and its degradation products

A protein from hagfish serum that cross-reacted with the third component of hagfish complement (C3) was purified to homogeneity and its structural properties were compared with those of C3 which has a two-subunit chain structure (115 and 72 kDa). This protein (designated C3b), when purified from plasma, consisted of three disulfide-linked polypeptide chains (77, 72 and 30 kDa). On immunoelectrophoresis, purified C3b migrated more rapidly towards the anode than the beta mobility of C3. However, immunochemical analysis revealed that C3b, after the first step in its purification, consisted of two disulfide-linked polypeptide chains (105 and 72 kDa). Treatment of C3b with methylamine, prior to spectrophotometric titration of the free sulfhydryl groups, did not significantly affect the end-point of the titration, suggesting the absence of a thioester bond in this molecule. Analysis of the amino acid sequences of the amino-termini of the subunits of C3b revealed that 77 amino acid residues at the amino-terminus of the native alpha chain were missing from both the 77-kDa and the 105-kDa polypeptides from C3b. These results indicate that the C3b in this study was analogous to mammalian C3b. Furthermore, amino acid sequencing data indicated that most of the native C3 from hagfish serum has an irregular two-subunit (alpha+gamma and beta)-linked structure, as a result of one-sided processing of putative hagfish pro-C3 at the beta-alpha processing site exclusively. Moreover, it appears that only the molecular features of degenerated hagfish C3 (C3b) are altered during its purification to generate a three-chain structure.