Molecular Basis for Bordetella pertussis Interference with Complement, Coagulation, Fibrinolytic, and Contact Activation Systems: the Cryo-EM Structure of the Vag8-C1 Inhibitor Complex.

Arun Dhillon,Justin C Deme,Dorina Roem,Steven Johnson,Emily Furlong,Ilse Jongerius,Susan M Lea,Wayne A Hendrickson

doi:10.1128/mbio.02823-20

Abstract

Complement, contact activation, coagulation, and fibrinolysis are serum protein cascades that need strict regulation to maintain human health. Serum glycoprotein, a C1 inhibitor (C1-INH), is a key regulator (inhibitor) of serine proteases of all the above-mentioned pathways. Recently, an autotransporter protein, virulence-associated gene 8 (Vag8), produced by the whooping cough pathogen, Bordetella pertussis, was shown to bind to C1-INH and interfere with its function. Here, we present the structure of the Vag8-C1-INH complex determined using cryo-electron microscopy at a 3.6-Å resolution. The structure shows a unique mechanism of C1-INH inhibition not employed by other pathogens, where Vag8 sequesters the reactive center loop of C1-INH, preventing its interaction with the target proteases.IMPORTANCE The structure of a 10-kDa protein complex is one of the smallest to be determined using cryo-electron microscopy at high resolution. The structure reveals that C1-INH is sequestered in an inactivated state by burial of the reactive center loop in Vag8. By so doing, the bacterium is able to simultaneously perturb the many pathways regulated by C1-INH. Virulence mechanisms such as the one described here assume more importance given the emerging evidence about dysregulation of contact activation, coagulation, and fibrinolysis leading to COVID-19 pneumonia.

Highlights

Complement, contact activation, coagulation, and fibrinolysis are serum protein cascades that need strict regulation to maintain human health
C1 inhibitor (C1-INH) has been implicated in regulation of fibrinolysis via action against tissue-type plasminogen activator and plasmin, study of this is complicated by the fact that both these enzymes cleave C1-INH [14, 15], highlighting the view that the serpin mechanism is a balancing act between trapping the enzyme in a nonfunctional complex with the inhibitor and cleavage of the inhibitor by the target enzyme
This virulence-associated gene 8 (Vag8)–C1-INH complex was concentrated to 0.5 mg/ml and applied to Quantifoil R1.2/1.3 carbon-coated grids before blotting using a Mark IV Vitrobot and plunge freezing in liquid ethane

Summary

Introduction

Complement, contact activation, coagulation, and fibrinolysis are serum protein cascades that need strict regulation to maintain human health. Protein cascades coordinate key processes for health within human serum, in particular immune and inflammatory responses (complement and contact activation) and control of clotting (contact activation, coagulation, and fibrinolysis) [1,2,3] They are independent processes, coordination between the pathways occurs by shared regulation, by C1 inhibitor (C1-INH) [4]. B. pertussis employs a range of virulence factors to colonize the human host and evade immune responses [17] Some of these factors, e.g., virulence-associated gene 8 (Vag8), Bordetella resistance to killing A (BrkA), filamentous hemagglutinin (FHA), and B. pertussis autotransporter protein C (BapC), have been implicated in evasion of the complement system [18,19,20,21]. Deletion of the gene encoding Vag predisposes B. pertussis to complement-mediated killing [18, 22]

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