Enzyme kinetic and binding studies identify determinants of specificity for the immunomodulatory enzyme ScpA, a C5a inactivating bacterial protease

C5a peptidase (ScpA) activity towards human type II and type III interferons

C5a peptidase, also known as ScpA, is a surface associated serine protease derived from Streptococcus pyogenes and has been described as an important factor in streptococcus virulence, capable of cleaving complement components C5a, C3 and C3a. Although the interactions of ScpA with complement components is well studied, extensive screening of ScpA activity against other pro-inflammatory cytokines is lacking. Here, ScpA's ability to cleave human pro-inflammatory cytokines was tested, revealing its ability to cleave human IFNγ, IFNλ1, IFNλ2, C5, IL-37 but with significantly reduced activities. The functional consequence of ScpA's cleavage of IFNγ in its signalling through the Jak-Stat pathway has also been evaluated in an in vitro RPE1 cell model. These newly identified targets for ScpA highlight the complexity of streptococcus infections and indeed, the potential for ScpA to have a therapeutic role in the progression of inflammatory diseases involving these cytokines.

The 1.7 Å crystal structure of the C5a peptidase from Streptococcus agalactiae (ScpB) reveals an active site competent for catalysis

A 1.7 Å structure is presented for an active form of the virulence factor ScpB, the C5a peptidase from Streptococcus agalactiae. The previously reported structure of the ScpB active site mutant exhibited a large separation (~20 Å) between the catalytic His and Ser residues. Significant differences are observed in the catalytic domain between the current and mutant ScpB structures resulting with a high RMSDCα (4.6 Å). The fold of the active form of ScpB is nearly identical to ScpA (RMSDCα 0.2 Å), the C5a-peptidase from Streptococcus pyogenes. Both ScpA and ScpB have comparable activity against human C5a, indicating neither enzyme require host proteins for C5a-ase activity. These studies are a first step in resolving reported differences in the specificities of these enzymes.

The protease associated (PA) domain in ScpA from Streptococcus pyogenes plays a role in substrate recruitment

Annually, over 18 million disease cases and half a million deaths worldwide are estimated to be caused by Group A Streptococcus. ScpA (or C5a peptidase) is a well characterised member of the cell enveleope protease family, which possess a S8 subtilisin-like catalytic domain and a shared multi-domain architecture. ScpA cleaves complement factors C5a and C3a, impairing the function of these critical anaphylatoxins and disrupts complement-mediated innate immunity. Although the high resolution structure of ScpA is known, the details of how it recognises its substrate are only just emerging. Previous studies have identified a distant exosite on the 2nd fibronectin domain that plays an important role in recruitment via an interaction with the substrate core. Here, using a combination of solution NMR spectroscopy, mutagenesis with functional assays and computational approaches we identify a second exosite within the protease-associated (PA) domain. We propose a model in which the PA domain assists optimal delivery of the substrate's C terminus to the active site for cleavage.

Comparative Structure Analysis of the Multi-Domain, Cell Envelope Proteases of Lactic Acid Bacteria

Lactic acid bacteria (LAB) have an extracellular proteolytic system that includes a multi-domain, cell envelope protease (CEP) with a subtilisin homologous protease domain. These CEPs have different proteolytic activities despite having similar protein sequences. Structural characterization has previously been limited to CEP homologs of dairy- and human-derived LAB strains, excluding CEPs of plant-derived LAB strains. CEP structures are a challenge to determine experimentally due to their large size and attachment to the cell envelope. This study aims to clarify the prevalence and structural diversity of CEPs by using the structure prediction software AlphaFold 2. Domain boundaries are clarified based on a comparative analysis of 21 three-dimensional structures, revealing novel domain architectures of CEP homologs that are not necessarily restricted to specific LAB species or ecological niches. The C-terminal flanking region of the protease domain is divided into fibronectin type-III-like domains with various structural traits. The analysis also emphasizes the existence of two distinct domains for cell envelope attachment that are preceded by an intrinsically disordered cell wall spanning domain. The domain variants and their combinations provide CEPs with different stability, proteolytic activity, and potentially adhesive properties, making CEPs targets for steering proteolytic activity with relevance for both food development and human health.

Impact of Propeptide Cleavage on the Stability and Activity of a Streptococcal Immunomodulatory C5a Peptidase for Biopharmaceutical Development

Posttranslational modifications of proteins can impact their therapeutic efficacy, stability, and potential for pharmaceutical development. The Group AStreptococcus pyogenesC5a peptidase (ScpA) is a multi-domain protein composed of an N-terminal signal peptide, a catalytic domain (including propeptide), three fibronectin domains, and cell membrane-associated domains. It is one of several proteins produced by Group AS. pyogenesknown to cleave components of the human complement system. After signal peptide removal, ScpA undergoes autoproteolysis and cleaves its propeptide for full maturation. The exact location and mechanism of the propeptide cleavage, and the impact of this cleavage on stability and activity, are not clearly understood, and the exact primary sequence of the final enzyme is not known. A form of ScpA with no autoproteolysis fragments of propeptide present may be more desirable for pharmaceutical development from a regulatory and a biocompatibility in the body perspective. The current study describes an in-depth structural and functional characterization of propeptide truncated variants of ScpA expressed inEscherichia colicells. All three purified ScpA variants, ScpA, 79ΔPro, and 92ΔPro, starting with N32, D79, and A92 positions, respectively, showed similar activity against C5a, which suggests a propeptide-independent activity profile of ScpA. CE-SDS and MALDI top-down sequencing analyses highlight a time-dependent propeptide autoproteolysis of ScpA at 37 °C with a distinct end point at A92 and/or D93. In comparison, all three variants of ScpA exhibit similar stability, melting temperatures, and secondary structure orientation. In summary, this work not only highlights propeptide localization but also provides a strategy to recombinantly produce a final mature and active form of ScpA without any propeptide-related fragments.

Exosite binding modulates the specificity of the immunomodulatory enzyme ScpA, a C5a inactivating bacterial protease

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C5a, the human complement protein is implicated in COVID-19 and sepsis.

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Proteolytic inactivation of C5a by ScpA involves interactions with a novel exosite.

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High affinity binding of C5a is shown to involve D783 of the ScpA Fn2 domain.

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Activity and affinity of ScpA is tuned by residues approx 50 Å from the active site.

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The data supports a complex dynamic process in ScpA substrate recognition.

The C5a peptidase from Streptococcus pyogenes (ScpA) is a highly specific enzyme with potential therapeutic value. ScpA is a good model for studying determinants of specificity in the multidomain immunomodulatory enzymes (IMEs), which comprise a large family of bacterial surface proteases. The surface exposed region of ScpA has 5 main domains which includes 3 C-terminal Fn3-like domains (Fn1, Fn2 and Fn3) (Kagawa et al. 2009). Progressive deletion of the Fn3-like domains from the C-ter resulted in loss of enzyme activity and showed an important role for the Fn2 domain in enzyme function. Functional investigation of specific acidic residues on the Fn2 domain identified 3 residues 30–50 Å from the catalytic site (D783, E864 and D889) which impacted to differing degrees on binding and on catalysis, supporting the presence of an exosite on the Fn2. In particular, residue D783 was observed to impact on both substrate binding affinity and the activity of ScpA. A double mutant cycle analysis showed energetic coupling between the targeted ScpA residues and residues in the core portion (residues 1–67) of the C5a substrate. The data supports the presence of a communication network between the active site and the exosite on Fn2. These findings provide a basis for rational engineering of this important enzyme family to enhance stability, activity and/or specificity.