The functional integrity of the serpin domain of C1-inhibitor depends on the unique N-terminal domain, as revealed by a pathological mutant

C1-inhibitor (C1-Inh) is a serine protease inhibitor ( serpin) with a unique, non-conserved N-terminal domain of unknown function. Genetic deficiency of C1-Inh causes hereditary angioedema. A novel type of mutation (Delta3) in exon 3 of the C1-Inh gene, resulting in deletion of Asp(62)-Thr(116) in this unique domain, was encountered in a hereditary angioedema pedigree. Because the domain is supposedly not essential for inhibitory activity, the unexpected loss-of-function of this deletion mutant was further investigated. The Delta3 mutant and three additional mutants starting at Pro(76), Gly(98), and Ser(115), lacking increasing parts of the N-terminal domain, were produced recombinantly. C1-Inh(76) and C1-Inh(98) retained normal conformation and interaction kinetics with target proteases. In contrast, C1-Inh(115) and Delta3, which both lack the connection between the serpin and the nonserpin domain via two disulfide bridges, were completely non-functional because of a complex-like and multimeric conformation, as demonstrated by several criteria. The Delta3 mutant also circulated in multimeric form in plasma from affected family members. The C1-Inh mutant reported here is unique in that deletion of an entire amino acid stretch from a domain not shared by other serpins leads to a loss-of-function. The deletion in the unique N-terminal domain results in a "multimerization phenotype" of C1-Inh, because of diminished stability of the central beta-sheet. This phenotype, as well as the location of the disulfide bridges between the serpin and the non-serpin domain of C1-Inh, suggests that the function of the N-terminal region may be similar to one of the effects of heparin in antithrombin III, maintenance of the metastable serpin conformation.