Characterization of C1 Inhibitor-Ta

Dysfunctional C1 inhibitor (C1INH)-Ta is a naturally occurring mutant from a patient with type II hereditary angioedema. This mutant has a deletion of the codon for Lys-251, which is located in the connecting strand between helix F and strand 3A, overlying β sheet A. Deletion of this Lys modifies the amino acid sequence at this position from Asn-Lys-Ile-Ser to Asn-Ile-Ser and creates a new glycosylation site. To further characterize the mechanism of dysfunction, we have analyzed the recombinant normal and Ta proteins expressed by COS cells in addition to the proteins in serum and isolated from serum. Recombinant C1INH-Ta revealed an intermediate thermal stability in comparison with the intact and reactive center cleaved normal proteins. Analysis of the reactivity of this recombinant protein with target proteases demonstrated no complex formation with C1s, C1r, or kallikrein. Inefficient complex formation was, however, clearly detectable with β-factor XIIa. Each protease produced partial cleavage of the recombinant mutant inhibitor. Recombinant C1INH-Ta, on 7.5% SDS-polyacrylamide gel electrophoresis and by size fractionation on Superose 12, showed a higher molecular weight fraction that was compatible in size with dimer formation. However, no multimerization of C1INH-Ta isolated from serum or of C1INH-Ta in serum, was observed. The C1INH-Ta dimer expressed the epitopes that normally are expressed only on the protease complexed or the cleaved inhibitor. These epitopes were not expressed on the monomeric inhibitor. The data suggest that the mutation in C1INH-Ta results in a folding abnormality that behaves as if it consists of two populations of molecules, one of which is susceptible to multimerization and one of which is converted to a substrate, but which retains residual inhibitory activity.