Protein interactions with peptides generally have low thermodynamic and mechanical stability. Streptococcus pyogenes fibronectin-binding protein FbaB contains a domain with a spontaneous isopeptide bond between Lys and Asp. By splitting this domain and rational engineering of the fragments, we obtained a peptide (SpyTag) which formed an amide bond to its protein partner (SpyCatcher) in minutes. Reaction occurred in high yield simply upon mixing and amidst diverse conditions of pH, temperature, and buffer. SpyTag could be fused at either terminus or internally and reacted specifically at the mammalian cell surface. Peptide binding was not reversed by boiling or competing peptide. Single-molecule dynamic force spectroscopy showed that SpyTag did not separate from SpyCatcher until the force exceeded 1 nN, where covalent bonds snap. The robust reaction conditions and irreversible linkage of SpyTag shed light on spontaneous isopeptide bond formation and should provide a targetable lock in cells and a stable module for new protein architectures. agging with peptides (e.g., HA, myc, FLAG, His 6 ) is one of the most common ways to detect, purify, or immobilize proteins (1-4). Peptides are very useful minimally disruptive probes (5) but they are also "slippery"-antibodies or other proteins typically bind peptides with low affinity and poor mechanical strength (6-9). We sought to form a rapid covalent bond to a peptide tag without the use of chemical modification, artificial amino acids, or cysteines (disulfide bond formation is reversible and restricted to particular cellular locations).It has recently been found that Streptococcus pyogenes, like many other Gram-positive bacteria, contains extracellular proteins stabilized by spontaneous intramolecular isopeptide bonds (10). Here we explored the second immunoglobulin-like collagen adhesin domain (CnaB2) from the fibronectin binding protein FbaB, found in invasive strains of S. pyogenes (11,12) and essential for phagocytosis-like uptake of the bacteria by endothelial cells (13). CnaB2 contains a single isopeptide bond conferring exceptional stability: CnaB2 remains folded even at pH 2 or up to 100°C (12). By splitting CnaB2 into peptide and protein fragments, followed by rational modification of the parts, we developed a peptide tag of 13 amino acids that rapidly formed a covalent bond with its protein partner (138 amino acids, 15 kDa) and characterized the conditions for reaction, cellular specificity of bond formation, and resilience of the reacted product.

Staphylococcus aureus and Streptococcus pyogenes, two important human pathogens, target host fibronectin (Fn) in their adhesion to and invasion of host cells. Fibronectin-binding proteins (FnBPs), anchored in the bacterial cell wall, have multiple Fn-binding repeats in an unfolded region of the protein. The bacterium-binding site in the amino-terminal domain (1-5F1) of Fn contains five sequential Fn type 1 (F1) modules. Here we show the structure of a streptococcal (S. dysgalactiae) FnBP peptide (B3) in complex with the module pair 1F12F1. This identifies 1F1- and 2F1-binding motifs in B3 that form additional antiparallel beta-strands on sequential F1 modules-the first example of a tandem beta-zipper. Sequence analyses of larger regions of FnBPs from S. pyogenes and S. aureus reveal a repeating pattern of F1-binding motifs that match the pattern of F1 modules in 1-5F1 of Fn. In the process of Fn-mediated invasion of host cells, therefore, the bacterial proteins seem to exploit the modular structure of Fn by forming extended tandem beta-zippers. This work is a vital step forward in explaining the full mechanism of the integrin-dependent FnBP-mediated invasion of host cells.

SummaryMany pathogenic Gram-positive bacteria produce cell wall-anchored proteins that bind to components of the extracellular matrix (ECM) of the host. These bacterial MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) are thought to play a critical role in infection. One group of MSCRAMMs, produced by staphylococci and streptococci, targets fibronectin (Fn, a glycoprotein found in the ECM and body fluids of vertebrates) using repeats in the C-terminal region of the bacterial protein. These bacterial Fn-binding proteins (FnBPs) mediate adhesion to host tissue and bacterial uptake into nonphagocytic host cells. Recent studies on interactions between the host and bacterial proteins at the residue-specific level and on the mechanism of host cell invasion are providing a much clearer picture of these processes.

Staphylococcus aureus can adhere to and invade endothelial cells by binding to the human protein fibronectin (Fn). FnBPA and FnBPB, cell wall-attached proteins from S. aureus, have multiple, intrinsically disordered, high-affinity binding repeats (FnBRs) for Fn. Here, 30 years after the first report of S. aureus/Fn interactions, we present four crystal structures that together comprise the structures of two complete FnBRs, each in complex with four of the N-terminal modules of Fn. Each Ϸ40-residue FnBR forms antiparallel strands along the triple-stranded ␤-sheets of four sequential F1 modules ( 2-5 F1) with each FnBR/ 2-5 F1 interface burying a total surface area of Ϸ4,300 Å 2 . The structures reveal the roles of residues conserved between S. aureus and Streptococcus pyogenes FnBRs and show that there are few linker residues between FnBRs. The ability to form large intermolecular interfaces with relatively few residues has been proposed to be a feature of disordered proteins, and S. aureus/Fn interactions provide an unusual illustration of this efficiency.intrinsic disorder ͉ tandem ␤-zipper ͉ host-pathogen interaction S taphylococcus aureus is a dangerous human pathogen that causes a wide range of debilitating and life-threatening infections (1). Incidence of S. aureus resistance to antibiotics (2) makes the understanding of its mechanisms of pathogenesis imperative. S. aureus/Fn interactions were first reported 30 years ago, and an S. aureus Fn-binding protein was isolated and characterized Ϸ20 years ago (3). Our recent work has dissected the 363-residue C-terminal region of FnBPA into 11 FnBRs (4) (FnBPA1-11; Fig. 1 A and B), six of which bind the NTD (N-terminal domain) of Fn (comprising modules 1-5 F1) with dissociation constants in the nanomolar range (5). The Cterminal region of FnBPB, a second S. aureus Fn-binding protein, is very similar to FnBPA but lacks one of the shorter FnBRs (5). In FnBPA, which also binds fibrinogen, the fibrinogen-and Fn-binding regions (Fig. 1 A) appear to cooperate in disease progression, with the FnBR region being particularly associated with persistence of infection (6). FnBPA/Fn interactions both mediate S. aureus invasion of (7) and activate endothelial cells, evoking both the proinflammatory and procoagulant responses typical of infective endocarditis (8). FnBPAs ability to mediate platelet activation, a key step in thrombus formation, is also likely to play a role in cardiovascular disease (9) and FnBPA has been implicated in cardiac device infections through its ability to mediate S. aureus attachment to implanted prosthetic materials (10). We previously predicted that in Fn-BPA each FnBR binds a string of three or four F1 modules in the NTD of Fn through a longer version of the tandem ␤-zipper mechanism that we discovered in Streptococcus dysgalactiae interactions with 1 F1 2 F1 (4). Results and DiscussionCrystal Structure of FnBPA-1/ 2-5 F1. Fig. 1C shows two F1 module pair/peptide structures that together comprise the structure of the most N-terminal S. aureus FnBR (...

Counting backwards: The cyanobactin class of heterocyclases, exemplified by TruD, possess an almost unique combination of processivity, specificity, chemical versatility, and promiscuity. TruD is shown by biochemical assay to be an adenylase, and processes cysteines in a defined order. The entire substrate leader can be removed and TruD will process a single specific cysteine residue; however the role of leader is to permit processivity through a balance of recognition. ATP/AMP=adenosine tri/monophosphate; PPi=pyrophosphate.