Activation of the SspA Serine Protease Zymogen of Staphylococcus aureus Proceeds through Unique Variations of a Trypsinogen-like Mechanism and Is Dependent on Both Autocatalytic and Metalloprotease-specific Processing

Abstract

The serine and cysteine proteases SspA and SspB of Staphylococcus aureus are secreted as inactive zymogens, zSspA and zSspB. Mature SspA is a trypsin-like glutamyl endopeptidase and is required to activate zSspB. Although a metalloprotease Aureolysin (Aur) is in turn thought to contribute to activation of zSspA, a specific role has not been demonstrated. We found that pre-zSspA is processed by signal peptidase at ANA 29 Binding of glutamate within the active site of zSspA is energetically unfavorable, but glutamine fits into the primary specificity pocket and is predicted to hydrogen bond to Thr 232 proximal to Ser 237 , permitting autocatalytic cleavage of the glutamine-rich propeptide segment. These and other observations suggest that zSspA is activated through a trypsinogen-like mechanism where supplementary features of the propeptide must be sequentially processed in the correct order to allow efficient activation.Staphylococcus aureus can colonize and infect virtually every tissue and organ system of the body (1, 2). A key factor in these defining traits is its ability to sustain bacteremia and adhere to tissues. Consequently it has adapted to growth in blood by subversion of plasma proteins (3) and the tissue extracellular matrix (4). Members of the MSCRAMM (microbial extracellular matrix-binding protein) family of adhesion proteins bind extracellular matrix ligands such as collagen, fibronectin, and fibrinogen (4) of which the latter two are also abundant in plasma. Manipulation of other plasma proteins such as IgG and von Willebrand factor is facilitated by staphylococcal Protein A (5), whereas coagulase promotes fibrin clot formation by activation of prothrombin (6), and staphylokinase activates plasminogen (7) to facilitate fibrin clot dissolution. Our studies on secreted proteases are also supportive of S. aureus as a paradigm for the manipulation of plasma and coagulation proteins.The SspA 4 glutamyl endopeptidase, also known as V8 protease, moderates adhesion of S. aureus to fibronectin by degrading cell surface fibronectin-binding proteins in which there is a high glutamic acid content, including a conserved motif that is essential for ligand binding (8, 9). SspA is expressed in an operon with a cysteine protease, SspB, and activates the zSspB zymogen by removing its N-terminal propeptide (10). Mature 20-kDa SspB mimics plasma serine proteases as noted by its ability to (i) convert high molecular weight kininogen into heavy and light chains, (ii) hydrolyze the Bz-Pro-Phe-Arg substrate of kallikrein, a serine protease that processes single chain kininogen by excision of the vasoactive peptide bradykinin (RPPGFSPFR), (iii) process the N terminus of the fibrinogen ␤-chain at the same site as plasmin, and (iv) remove the N-terminal domain of fibronectin with a specificity equivalent to plasminogen activator (10). We have also found that antibodies to SspB are produced when mice are challenged with hypervirulent strains of community-acquired methicillin-resistant S. aureus and that SspA and SspB are r...

Proteolytic Cleavage Inactivates the Staphylococcus aureus Lipoteichoic Acid Synthase

“…In several instances, this processing leads to enzyme activation, for instance, in the case of the S. aureus extracellular proteases themselves. These proteins are secreted as inactive zymogens and cleaved by signal peptidase to release them from the membrane, and subsequent additional processing steps at defined sites finally lead to the full activation of these enzymes (33). Through previous work, it has been established that the S. aureus LTA synthase enzyme LtaS is efficiently processed at a defined site during bacterial growth (32,35,46).…”

Staphylococcus aureus Metalloprotease Aureolysin Cleaves Complement C3 To Mediate Immune Evasion

“…However, aureolysin seems to act in synergy with other factors from the bacterial supernatant to degrade C3 fully. Because previous studies indicated that aureolysin is required for activation of the V8 protease (53), this suggests that the V8 protease may act as a cofactor for aureolysin-mediated FIGURE 8. Aureolysin is required and sufficient for cleavage of C3 by S. aureus supernatant.…”

Staphylococcal SplB Serine Protease Utilizes a Novel Molecular Mechanism of Activation

“…The inhibitory profragment of V8 protease is 39 residues long and is removed in a multistep process (29,30). The newly formed N terminus becomes directly a part of the S1 specificity pocket contrary to the N terminus of chymotrypsin, which only influences the folding of the polypeptide chain constituting the S1 pocket.…”