title: Novel fluorescent prothrombin analogs as probes of staphylocoagulase-prothrombin interactions.
abstract: Staphylocoagulase (SC) is a potent nonproteolytic prothrombin (ProT) activator and the prototype of a newly established zymogen activator and adhesion protein family. The staphylocoagulase fragment containing residues 1-325 (SC-(1-325)) represents a new type of nonproteolytic activator with a unique fold consisting of two three-helix bundle domains. The N-terminal, domain 1 of SC (D1, residues 1-146) interacts with the 148 loop of thrombin and prethrombin 2 and the south rim of the catalytic site, whereas domain 2 of SC (D2, residues 147-325) occupies (pro)exosite I, the fibrinogen (Fbg) recognition exosite. Reversible conformational activation of ProT by SC-(1-325) was used to create novel analogs of ProT covalently labeled at the catalytic site with fluorescence probes. Analogs selected from screening 10 such derivatives were used to characterize quantitatively equilibrium binding of SC-(1-325) to ProT, competitive binding with native ProT, and SC domain interactions. The results support the conclusion that SC-(1-325) binds to a single site on fluorescein-labeled and native ProT with indistinguishable dissociation constants of 17-72 pM. The results obtained for isolated SC domains indicate that D2 binds ProT with approximately 130-fold greater affinity than D1, yet D1 binding accounts for the majority of the fluorescence enhancement that accompanies SC-(1-325) binding. The SC-(1-325).(pro)thrombin complexes and free thrombin showed little difference in substrate specificity for tripeptide substrates or with their natural substrate, Fbg. Lack of a significant effect of blockage of (pro)exosite I of (pro)thrombin by SC-(1-325) on Fbg cleavage indicates that a new Fbg substrate recognition exosite is expressed on the SC-(1-325).(pro)thrombin complexes. Our results provide new insight into the mechanism that mediates zymogen activation by this prototypical bacterial activator.
authors.affiliation: Department of Pathology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Panizzi
authors.forename: Rainer
authors.initials: R
authors.lastname: Friedrich
authors.forename: Pablo
authors.initials: P
authors.lastname: Fuentes-Prior
authors.forename: Heather K
authors.initials: HK
authors.lastname: Kroh
authors.forename: Judy
authors.initials: J
authors.lastname: Briggs
authors.forename: Guido
authors.initials: G
authors.lastname: Tans
authors.forename: Wolfram
authors.initials: W
authors.lastname: Bode
authors.forename: Paul E
authors.initials: PE
authors.lastname: Bock
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D020134:Catalytic Domain; D002448:Cell Adhesion; D003030:Coagulase / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D005337:Fibrin / Q000737:chemistry; D005456:Fluorescent Dyes / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D008856:Microscopy, Fluorescence; D008956:Models, Chemical; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011516:Prothrombin / Q000737:chemistry*; D011994:Recombinant Proteins / Q000737:chemistry; D013379:Substrate Specificity; D013997:Time Factors
chemical_list: D003030:Coagulase; D005456:Fluorescent Dyes; D010455:Peptides; D011994:Recombinant Proteins; D011516:Prothrombin; D005337:Fibrin