Staphylococcus aureus evasion proteins EapH1 and EapH2: Residue-level investigation of an alternative binding motif for human neutrophil elastase

Staphylococcal peroxidase inhibitor (SPIN): Residue-level investigation of the helical bundle domain

Myeloperoxidase is a critical component of the antibacterial arsenal of neutrophils, whereby it consumes H2O2 as an oxidant to convert halogen and pseudohalogen anions into cytotoxic hypohalous acids. Following phagocytosis by neutrophils, the human pathogen Staphylococcus aureus secretes a potent myeloperoxidase inhibitory protein, called SPIN, as part of its immune evasion repertoire. The matured S. aureus SPIN polypeptide consists of only 73 residues yet contains two functional domains: whereas the 60 residue C-terminal helical bundle domain is responsible for MPO binding, the 13 residue N-terminal domain is required to inhibit MPO. Previous studies have informed understanding of the SPIN N-terminal domain, but comparatively little is known about the helical domain insofar as the contribution of individual residues is concerned. To address this limitation, we carried out a residue-level structure/function investigation on the helical bundle domain of S. aureus SPIN. Using sequence conservation and existing structures of SPIN bound to human MPO as a guide, we selected residues L49, E50, H51, E52, Y55, and Y75 for interrogation by site-directed mutagenesis. We found that loss of L49 or E52 reduced SPIN activity by roughly an order of magnitude, but that loss of Y55 or H51 caused progressively greater loss of inhibitory potency. Direct binding studies by SPR showed that loss of inhibitory potency in these SPIN mutants resulted from a diminished initial interaction between the inhibitor and MPO. Together, our studies provide new insights into the structure/function relationships of SPIN and identify positions Y55 and H51 as critical determinants of SPIN function.

Simultaneous inhibition of two neutrophil serine proteases by the S. aureus innate immune evasion protein EapH2

Extracellular adherence protein domain (EAP) proteins are high-affinity, selective inhibitors of neutrophil serine proteases (NSP), including cathepsin-G (CG) and neutrophil elastase (NE). Most Staphylococcus aureus isolates encode for two EAPs, EapH1 and EapH2, that contain a single functional domain and share 43% identity with one another. Although structure/function investigations from our group have shown that EapH1 uses a globally similar binding mode to inhibit CG and NE, NSP inhibition by EapH2 is incompletely understood due to a lack of NSP/EapH2 cocrystal structures. To address this limitation, we further studied NSP inhibition by EapH2 in comparison with EapH1. Like its effects on NE, we found that EapH2 is a reversible, time-dependent, and low nanomolar affinity inhibitor of CG. We characterized an EapH2 mutant which suggested that the CG binding mode of EapH2 is comparable to EapH1. To test this directly, we used NMR chemical shift perturbation to study EapH1 and EapH2 binding to CG and NE in solution. Although we found that overlapping regions of EapH1 and EapH2 were involved in CG binding, we found that altogether distinct regions of EapH1 and EapH2 experienced changes upon binding to NE. An important implication of this observation is that EapH2 might be capable of binding and inhibiting CG and NE simultaneously. We confirmed this unexpected feature by solving crystal structures of the CG/EapH2/NE complex and demonstrating their functional relevance through enzyme inhibition assays. Together, our work defines a new mechanism of simultaneous inhibition of two serine proteases by a single EAP protein.

Characterization of two distinct neutrophil serine protease-binding modes within a Staphylococcus aureus innate immune evasion protein family

Extracellular adherence protein domain (EAPs) proteins are a class of innate immune evasion proteins secreted by the human pathogen Staphylococcus aureus. EAPs are potent and selective inhibitors of cathepsin-G (CG) and neutrophil elastase (NE), which are the two most abundant neutrophil serine proteases (NSPs). Previous work from our group has shown that the prototypical EAP, EapH1, relies on plasticity within a single inhibitory site to block the activities of CG and NE. However, whether other EAPs follow similar structure-function relationships is unclear. To address this question, we studied the inhibitory properties of the first (Eap1) and second (Eap2) domains of the modular extracellular adherence protein of S. aureus and determined their structures when bound to CG and NE, respectively. We observed that both Eap1 and Eap2 displayed time-dependent inhibition of CG (on the order of 10^-9 M) and of NE (on the order of 10^-10 M). We also found that whereas the structures of Eap1 and Eap2 bound to CG showed an overall inhibitory mode like that seen previously for EapH1, the structures of Eap1 and Eap2 bound to NE revealed a new inhibitory mode involving a distal region of the EAP domain. Using site-directed mutagenesis of Eap1 and Eap2, along with enzyme assays, we confirmed the roles of interfacial residues in NSP inhibition. Taken together, our work demonstrates that EAPs can form structurally divergent complexes with two closely related serine proteases and further suggests that certain EAPs may be capable of inhibiting two NSPs simultaneously.

Staphylococcus aureus and Neutrophil Extracellular Traps: The Master Manipulator Meets Its Match in Immunothrombosis

Over the past 10 years, neutrophil extracellular traps (NETs) have become widely accepted as an integral player in immunothrombosis, due to their complex interplay with both pathogens and components of the coagulation system. While the release of NETs is an attempt by neutrophils to trap pathogens and constrain infections, NETs can have bystander effects on the host by inducing uncontrolled thrombosis, inflammation, and tissue damage. From an evolutionary perspective, pathogens have adapted to bypass the host innate immune response. Staphylococcus aureus (S. aureus), in particular, proficiently overcomes NET formation using several virulence factors. Here we review mechanisms of NET formation and how these are intertwined with platelet activation, the release of endothelial von Willebrand factor, and the activation of the coagulation system. We discuss the unique ability of S. aureus to modulate NET formation and alter released NETs, which helps S. aureus to escape from the host's defense mechanisms. We then discuss how platelets and the coagulation system could play a role in NET formation in S. aureus-induced infective endocarditis, and we explain how targeting these complex cellular interactions could reveal novel therapies to treat this disease and other immunothrombotic disorders.

Local structural plasticity of the Staphylococcus aureus evasion protein EapH1 enables engagement with multiple neutrophil serine proteases

Members of the EAP family of Staphylococcus aureus immune evasion proteins potently inhibit the neutrophil serine proteases (NSPs) neutrophil elastase, cathepsin-G, and proteinase-3. Previously, we determined a 1.8 Å resolution crystal structure of the EAP family member EapH1 bound to neutrophil elastase. This structure revealed that EapH1 blocks access to the enzyme's active site by forming a noncovalent complex with this host protease. To determine how EapH1 inhibits other NSPs, we studied here the effects of EapH1 on cathepsin-G. We found that EapH1 inhibits cathepsin-G with a K_i of 9.8 ± 4.7 nm Although this K_i value is ∼466-fold weaker than the K_i for EapH1 inhibition of neutrophil elastase, the time dependence of inhibition was maintained. To define the physical basis for EapH1's inhibition of cathepsin-G, we crystallized EapH1 bound to this protease, solved the structure at 1.6 Å resolution, and refined the model to R_work and R_free values of 17.4% and 20.9%, respectively. This structure revealed a protease-binding mode for EapH1 with cathepsin-G that was globally similar to that seen in the previously determined EapH1-neutrophil elastase structure. The nature of the intermolecular interactions formed by EapH1 with cathepsin-G differed considerably from that with neutrophil elastase, however, with far greater contributions from the inhibitor backbone in the cathepsin-G-bound form. Together, these results reveal that EapH1's ability to form high-affinity interactions with multiple NSP targets is due to its remarkable level of local structural plasticity.