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Disease-causing mutations in the serpin antithrombin reveal a key domain critical for inhibiting protease activities.

ABSTRACT: Antithrombin mainly inhibits factor Xa and thrombin. The reactive center loop (RCL) is crucial for its interactions with its protease targets and is fully inserted into the A-sheet after its cleavage, causing translocation of the covalently linked protease to the opposite end of the A-sheet. Antithrombin variants with altered RCL hinge residues behave as substrates rather than inhibitors, resulting in stoichiometries of inhibition greater than one. Other antithrombin residues have been suggested to interfere with RCL insertion or the stability of the antithrombin-protease complex, but available crystal structures or mutagenesis studies have failed to identify such residues. Here, we characterized two mutations, S365L and I207T, present in individuals with type II antithrombin deficiency and identified a new antithrombin functional domain. S365L did not form stable complexes with thrombin or factor Xa, and the I207T/I207A variants inhibited both proteases with elevated stoichiometries of inhibition. Close proximity of Ile-207 and Ser-365 to the inserted RCL suggested that the preferred reaction of these mutants as protease substrates reflects an effect on the rate of the RCL insertion and protease translocation. However, both residues lie within the final docking site for the protease in the antithrombin-protease complex, supporting the idea that the enhanced substrate reactions may result from an increased dissociation of the final complexes. Our findings demonstrate that the distal end of the antithrombin A-sheet is crucial for the last steps of protease inhibition either by affecting the rate of RCL insertion or through critical interactions with proteases at the end of the A-sheet.

SUBMITTER: Aguila S

PROVIDER: S-EPMC5633112 | biostudies-literature | 2017 Oct

REPOSITORIES: biostudies-literature

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Disease-causing mutations in the serpin antithrombin reveal a key domain critical for inhibiting protease activities.

Águila Sonia S Izaguirre Gonzalo G Martínez-Martínez Irene I Vicente Vicente V Olson Steven T ST Corral Javier J

The Journal of biological chemistry 20170725 40

Antithrombin mainly inhibits factor Xa and thrombin. The reactive center loop (RCL) is crucial for its interactions with its protease targets and is fully inserted into the A-sheet after its cleavage, causing translocation of the covalently linked protease to the opposite end of the A-sheet. Antithrombin variants with altered RCL hinge residues behave as substrates rather than inhibitors, resulting in stoichiometries of inhibition greater than one. Other antithrombin residues have been suggested ...[more]

PMID: 28743742

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Project description:BackgroundAntithrombin (AT) is the main physiological anticoagulant involved in hemostasis. Hereditary AT deficiency is a rare autosomal dominant thrombotic disease mainly caused by mutations in SERPINC1, which was usually manifested as venous thrombosis and pulmonary embolism. In this study, we analyzed the clinical characteristics and screened for mutant genes in two pedigrees with hereditary AT deficiency, and the functional effects of the pathogenic mutations were evaluated.MethodsCandidate gene variants were analyzed by next-generation sequencing to screen pathogenic mutations in probands, followed by segregation analysis in families by Sanger sequencing. Mutant and wild-type plasmids were constructed and transfected into HEK293T cells to observe protein expression and cellular localization of SERPINC1. The structure and function of the mutations were analyzed by bioinformatic analyses.ResultsThe proband of pedigree A with AT deficiency carried a heterozygous frameshift mutation c.1377delC (p.Asn460Thrfs*20) in SERPINC1 (NM000488.3), a 1377C base deletion in exon 7 resulting in a backward shift of the open reading frame, with termination after translation of 20 residues, and a different residue sequence translated after the frameshift. Bioinformatics analysis suggests that the missing amino acid sequence caused by the frameshift mutation might disrupt the disulfide bond between Cys279 and Cys462 and affect the structural function of the protein. This newly discovered variant is not currently included in the ClinVar and HGMD databases. p.Arg229* resulted in a premature stop codon in exon 4, and bioinformatics analysis suggests that the truncated protein structure lost its domain of interaction with factor IX (Ala414 site) after the deletion of nonsense mutations. However, considering the AT truncation protein resulting from the p.Arg229* variant loss a great proportion of the molecule, we speculate the variant may affect two functional domains HBS and RCL and lack of the corresponding function. The thrombophilia and decreased-AT-activity phenotypes of the two pedigrees were separated from their genetic variants. After lentiviral plasmid transfection into HEK293T cells, the expression level of AT protein decreased in the constructed c.1377delC mutant cells compared to that in the wild-type, which was not only reduced in c.685C > T mutant cells but also showed a significant band at 35 kDa, suggesting a truncated protein. Immunofluorescence localization showed no significant differences in protein localization before and after the mutation.ConclusionsThe p.Asn460Thrfs*20 and p.Arg229* variants of SERPINC1 were responsible for the two hereditary AT deficiency pedigrees, which led to AT deficiency by different mechanisms. The p.Asn460Thrfs*20 variant is reported for the first time.

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Disease-causing mutations in the serpin antithrombin reveal a key domain critical for inhibiting protease activities.

Publications

Disease-causing mutations in the serpin antithrombin reveal a key domain critical for inhibiting protease activities.

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