The fibronectin-binding MSCRAMM FnbpA of Staphylococcus aureus is a bifunctional protein that also binds to fibrinogen

Staphylococcus aureus is an important pathogen capable of causing a wide spectrum of diseases in humans and animals. This bacterium expresses a variety of virulence factors that participate in the process of infection. These include MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) that mediate the adherence of the bacteria to host extracellular matrix components, such as collagen, fibronectin (Fn), and fibrinogen (Fg). Two Fn-binding MSCRAMMs, FnbpA and FnbpB, have been previously identified. The Fn binding activity has been localized to the approximately 40-amino acid residue D repeats in the C-terminal part of these proteins. However, no biological activity has yet been attributed to the N-terminal A regions of these proteins. These regions exhibit substantial amino acid sequence identity to the A regions of other staphylococcal MSCRAMMs, including ClfA, ClfB, and SdrG (Fbe), all of which bind Fg. This raises the question of whether the Fn-binding MSCRAMMs can also bind specifically to Fg. In this report, we show that a recombinant form of the A region of FnbpA does specifically recognize Fg. We localize the binding site in Fg for recombinant FnbpA to the gamma-chain, in particular to the C-terminal residues of this polypeptide, the site also recognized by ClfA. In addition, we demonstrate that recombinant FnbpA can compete with ClfA for binding to both immobilized and soluble Fg. By the use of surface plasmon resonance spectroscopy and fluorescence polarization, we determine the dissociation equilibrium constant for the interaction of recombinant FnbpA with intact immobilized Fg and with a synthetic C-terminal gamma-chain peptide, respectively. Finally, by overexpressing FnbpA in a mutant strain of S. aureus that lacks the expression of both ClfA and ClfB, we show that native FnbpA can mediate the interaction of S. aureus with soluble Fg.     

Binding of Efb from Staphylococcus aureus to fibrinogen blocks neutrophil adherence

In addition to its pivotal role in hemostasis, fibrinogen (Fg) and provisional fibrin matrices play important roles in inflammation and regulate innate immune responses by interacting with leukocytes. Efb (the extracellular fibrinogen-binding protein) is a secreted Staphylococcus aureus protein that engages host Fg and complement C3. However, the molecular details underlying the Efb-Fg interaction and the biological relevance of this interaction have not been determined. In the present study, we characterize the interaction of Efb with Fg. We demonstrate that the Fg binding activity is located within the intrinsically disordered N-terminal half of Efb (Efb-N) and that the D fragment of Fg is the region that mediates Efb-N binding. More detailed studies of the Efb-N-Fg interactions using ELISA and surface plasmon resonance analyses revealed that Efb-N exhibits a much higher affinity for Fg than typically observed with Fg-binding MSCRAMMs (microbial surface components recognizing adhesive matrix molecules), and data obtained from ELISA analyses using truncated Efb-N constructs demonstrate that Efb-N contains two binding sites located within residues 30-67 and 68-98, respectively. Efb-N inhibits neutrophil adhesion to immobilized Fg by binding to Fg and blocking the interaction of the protein with the leukocyte integrin receptor, α(M)β(2). A motif in the Fg γ chain previously shown to be central to the α(M)β(2) interaction was shown to be functionally distinguishable from the Efb-N binding site, suggesting that the Fg-Efb interaction indirectly impedes Fg engagement by α(M)β(2). Taken together, these studies provide insights into how Efb interacts with Fg and suggest that Efb may support bacterial virulence at least in part by impeding Fg-driven leukocyte adhesion events.     

Crystal structures of Bbp from Staphylococcus aureus reveal the ligand binding mechanism with Fibrinogen α

Bone sialoprotein-binding protein (Bbp), a MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules) family protein expressed on the surface of Staphylococcus aureus (S. aureus), mediates adherence to fibrinogen α (Fg α), a component in the extracellular matrix of the host cell and is important for infection and pathogenesis. In this study, we solved the crystal structures of apo-Bbp^273−598 and Bbp^273−598-Fg α^561−575 complex at a resolution of 2.03 Å and 1.45 Å, respectively. Apo-Bbp^273−598 contained the ligand binding region N2 and N3 domains, both of which followed a DE variant IgG fold characterized by an additional D1 strand in N2 domain and D1′ and D2′ strands in N3 domain. The peptide mapped to the Fg α^561−575 bond to Bbp^273−598 on the open groove between the N2 and N3 domains. Strikingly, the disordered C-terminus in the apo-form reorganized into a highly-ordered loop and a β-strand G′′ covering the ligand upon ligand binding. Bbp^{Ala298−Gly301} in the N2 domain of the Bbp^273−598-Fg α^561−575 complex, which is a loop in the apo-form, formed a short α-helix to interact tightly with the peptide. In addition, Bbp^{Ser547−Gln561} in the N3 domain moved toward the binding groove to make contact directly with the peptide, while Bbp^{Asp338−Gly355} and Bbp^{Thr365−Tyr387} in N2 domain shifted their configurations to stabilize the reorganized C-terminus mainly through strong hydrogen bonds. Altogether, our results revealed the molecular basis for Bbp-ligand interaction and advanced our understanding of S. aureus infection process.     

Staphylococcus aureus adhesion to bone matrix and bone-associated biomaterials

Staphylococcus aureus is a frequent cause of orthopedic infections in humans. The bacterium expresses several adhesins that facilitate bacterial binding to the bone matrix and to bone implant biomaterials coated with host plasma constituents. The relevant S. aureus adhesins are termed microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) and specific MSCRAMMs are involved in bone and joint infections.     

Binding of a peptide from a Streptococcus dysgalactiae MSCRAMM to the N-terminal F1 module pair of human fibronectin involves both modules

Host invasion by a number of pathogenic bacteria such as staphylococci and streptococci involves binding to fibronectin, a ubiquitous extracellular matrix protein. On the bacterial side, host extracellular matrix adherence is mediated by MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) which, in some cases, have been identified to be important virulence factors. In this study we used nuclear magnetic resonance spectroscopy to characterize the interaction of B3, a synthetic peptide derived from an adhesin of Streptococcus dysgalactiae, with the N-terminal module pair 1F12F1 of human fibronectin. 1F12F1 chemical shift changes occurring on formation of the 1F12F1/B3 complex indicate that both modules bind to the peptide and that a similar region of each module is involved. A similar surface of the 4F15F1 module pair had previously been identified as the binding site for a fibronectin-binding peptide from Staphylococcus aureus.     

Role of fibronectin-binding MSCRAMMs in bacterial adherence and entry into mammalian cells

Most bacterial infections are initiated by the adherence of microorganisms to host tissues. This process involves the interaction of specific bacterial surface structures, called adhesins, with host components. In this review, we discuss a group of microbial adhesins known as Microbial Surface Components Recognizing Adhesive Matrix Molecules (MSCRAMMs) which recognize and bind FN. The interaction of bacteria with FN is believed to contribute significantly to the virulence of a number of microorganisms, including staphylococci and streptococci. Several FN-binding MSCRAMMs of staphylococci and streptococci exhibit a similar structural organization and mechanism of ligand recognition. The ligand-binding domain consists of tandem repeats of a ∼45 amino acid long unit which bind to the 29-kDa N-terminal region of FN. The binding mechanism is unusual in that the repeat units are unstructured and appear to undergo a conformational change upon ligand binding. Apart from supporting bacterial adherence, FN is also involved in bacterial entry into non-phagocytic mammalian cells. A sandwich model has been proposed in which FN forms a molecular bridge between MSCRAMMs on the bacterial surface and integrins on the host cell. However, the precise mechanism of bacterial invasion and the roles of FN and integrins in this process have yet to be fully elucidated.     

Crystal Structures Reveal the Multi-Ligand Binding Mechanism of Staphylococcus aureus ClfB

Staphylococcus aureus (S. aureus) pathogenesis is a complex process involving a diverse array of extracellular and cell wall components. ClfB, an MSCRAMM (Microbial Surface Components Recognizing Adhesive Matrix Molecules) family surface protein, described as a fibrinogen-binding clumping factor, is a key determinant of S. aureus nasal colonization, but the molecular basis for ClfB-ligand recognition remains unknown. In this study, we solved the crystal structures of apo-ClfB and its complexes with fibrinogen α (Fg α) and cytokeratin 10 (CK10) peptides. Structural comparison revealed a conserved glycine-serine-rich (GSR) ClfB binding motif (GSSGXGXXG) within the ligands, which was also found in other human proteins such as Engrailed protein, TCF20 and Dermokine proteins. Interaction between Dermokine and ClfB was confirmed by subsequent binding assays. The crystal structure of ClfB complexed with a 15-residue peptide derived from Dermokine revealed the same peptide binding mode of ClfB as identified in the crystal structures of ClfB-Fg α and ClfB-CK10. The results presented here highlight the multi-ligand binding property of ClfB, which is very distinct from other characterized MSCRAMMs to-date. The adherence of multiple peptides carrying the GSR motif into the same pocket in ClfB is reminiscent of MHC molecules. Our results provide a template for the identification of other molecules targeted by S. aureus during its colonization and infection. We propose that other MSCRAMMs like ClfA and SdrG also possess multi-ligand binding properties.     

Structural and biochemical characterization of Staphylococcus aureus clumping factor B/ligand interactions

Clumping factor B (ClfB) from Staphylococcus aureus is a bifunctional protein that binds to human cytokeratin 10 (K10) and fibrinogen (Fg). ClfB has been implicated in S. aureus colonization of nasal epithelium and is therefore a key virulence factor. People colonized with S. aureus are at an increased risk for invasive staphylococcal disease. In this study, we have determined the crystal structures of the ligand-binding region of ClfB in an apo-form and in complex with human K10 and Fg α-chain-derived peptides, respectively. We have determined the structures of MSCRAMM binding to two ligands with different sequences in the same site showing the versatile nature of the ligand recognition mode of microbial surface components recognizing adhesive matrix molecules. Both ligands bind ClfB by parallel β-sheet complementation as observed for the clumping factor A·γ-chain peptide complex. The β-sheet complementation is shorter in the ClfB·Fg α-chain peptide complex. The structures show that several residues in ClfB are important for binding to both ligands, whereas others only make contact with one of the ligands. A common motif GSSGXG found in both ligands is part of the ClfB-binding site. This motif is found in many human proteins thus raising the possibility that ClfB recognizes additional ligands.     

Isopeptide bond in collagen- and fibrinogen-binding MSCRAMMs

The internal isopeptide bonds are amide bonds formed autocatalytically between the side chains of Lys and Asn/Asp residues and have been discovered recently. These bonds are well conserved in Gram-positive bacterial pilin proteins and are also observed over a wide range of Gram-positive bacterial surface proteins. The presence of these bonds confers the pilus subunits with remarkable properties in terms of thermal stability and resistance to proteases. Like pili, microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) are also surface proteins found only in Gram-positive bacteria. They specifically interact with the extracellular matrix (ECM) molecules like collagen, fibrinogen, fibronectin, laminin, etc. Many biophysical and biochemical studies have been carried out to characterize the isopeptide bonds in pili proteins from Gram-positive bacteria, but no attempts have been made to study the isopeptide bonds in MSCRAMMs. This short review aims to study the significance of the isopeptide bonds in relation to their function, by analyzing the crystal structures of collagen- and fibrinogen-binding MSCRAMMs. In this analysis, interestingly, we observed that the putative isopeptide bonds are restricted to the collagen-binding MSCRAMMs. Based on analogy with bacterial pilus subunits, we hypothesize that the collagen-binding MSCRAMMs possessing putative isopeptide bonds exhibit similar structural properties, which could help the bacteria in colonizing the host and provide resistance against host–defense mechanisms.     

Identification of residues in the Staphylococcus aureus fibrinogen-binding MSCRAMM clumping factor A (ClfA) that are important for ligand binding

Clumping factor A (ClfA) is a cell surface-associated protein of Staphylococcus aureus that promotes binding of this pathogen to both soluble and immobilized fibrinogen (Fg). Previous studies have localized the Fg-binding activity of ClfA to residues 221-559 within the A region of this protein. In addition, the C-terminal part of the A region (residues 484-550) has been implicated as being important for Fg binding. In this study, we further investigate the involvement of this part of ClfA in the interaction of this protein with Fg. Polyclonal antibodies generated against a recombinant protein encompassing residues 500-559 of the A region inhibited the interaction of both S. aureus and recombinant ClfA with immobilized Fg in a dose-dependent manner. Using site-directed mutagenesis, two adjacent residues, Glu(526) and Val(527), were identified as being important for the activity of ClfA. S. aureus expressing ClfA containing either the E526A or V527S substitution exhibited a reduced ability to bind to soluble Fg and to adhere to immobilized Fg. Furthermore, bacteria expressing ClfA containing both substitutions were almost completely defective in Fg binding. The E526A and V527S substitutions were also introduced into recombinant ClfA (rClfA-(221-559)) expressed in Escherichia coli. The single mutant rClfA-(221-559) proteins showed a significant reduction in affinity for both immobilized Fg and a synthetic fluorescein-labeled C-terminal gamma-chain peptide compared with the wild-type protein, whereas the double mutant rClfA-(221-559) protein was almost completely defective in binding to either species. Substitution of Glu(526) and/or Val(527) did not appear to alter the secondary structure of rClfA-(221-559) as determined by far-UV circular dichroism spectroscopy. These data suggest that the C terminus of the A region may contain at least part of the Fg-binding site of ClfA and that Glu(526) and Val(527) may be involved in ligand recognition.     

Inhibition of the Staphylococcus aureus sortase transpeptidase SrtA by phosphinic peptidomimetics

During pathogenesis, Gram-positive bacteria utilize surface protein virulence factors such as the MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) to aid the initiation and propagation of infection through adherence to host endothelial tissue and immune system evasion. These virulence-associated proteins generally contain a C-terminal LPXTG motif that becomes covalently anchored to the peptidoglycan biosynthesis intermediate lipid II. In Staphylococcus aureus, deletion of the sortase isoform SrtA results in marked reduction in virulence and infection potential, making it an important antivirulence target. Here we describe the chemical synthesis and kinetic characterization of a nonhydrolyzable phosphinic peptidomimetic inhibitor of SrtA derived from the LPXTG substrate sequence.     

SdrG, a fibrinogen-binding bacterial adhesin of the microbial surface components recognizing adhesive matrix molecules subfamily from Staphylococcus epidermidis, targets the thrombin cleavage site in the Bbeta chain

Staphylococcus epidermidis is an important opportunistic pathogen and is a major cause of foreign body infections. We have characterized the ligand binding activity of SdrG, a fibrinogen-binding microbial surface component recognizing adhesive matrix molecules from S. epidermidis. Western ligand blot analysis showed that a recombinant form of the N-terminal A region of SdrG bound to the native Bbeta chain of fibrinogen (Fg) and to a recombinant form of the Bbeta chain expressed in Escherichia coli. By analyzing recombinant truncates and synthetic peptide mimetics of the Fg Bbeta chain, the binding site for SdrG was localized to residues 6-20 of this polypeptide. Recombinant SdrG bound to a synthetic 25-amino acid peptide (beta1-25) representing the N terminus of the Fg Bbeta chain with a KD of 1.4 x 10(-7) m as determined by fluorescence polarization experiments. This was similar to the apparent K(D) (0.9 x 10(-7) m) calculated from an enzyme-linked immunosorbent assay where SdrG bound immobilized Fg in a concentration-dependent manner. SdrG could recognize fibrinopeptide B (residues 1-14), but with a substantially lower affinity than that observed for SdrG binding to synthetic peptides beta1-25 and beta6-20. However, SdrG does not bind to thrombin-digested Fg. Thus, SdrG appears to target the thrombin cleavage site in the Fg Bbeta chain. In fact, SdrG was found to inhibit thrombin-induced fibrinogen clotting by interfering with fibrinopeptide B release.     

Collagen adhesin–nanoparticle interaction impairs adhesin's ligand binding mechanism

Background

Pathogenic bacteria specifically recognize extracellular matrix (ECM) molecules of the host (e.g. collagen, fibrinogen and fibronectin) through their surface proteins known as MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules) and initiate colonization. On implantation, biomaterials easily get coated with these ECM molecules and the MSCRAMMs mediate bacterial adherence to biomaterials. With the rapid rise in antibiotic resistance, designing alternative strategies to reduce/eliminate bacterial colonization is absolutely essential.

Methods

The Rhusiopathiae surface protein B (RspB) is a collagen‐binding MSCRAMM of Erysipelothrix rhusiopathiae. It also binds to abiotic surfaces. The crystal structure of the collagen‐binding region of RspB (rRspB_31–348) reported here revealed that RspB also binds collagen by a unique ligand binding mechanism called “Collagen Hug” which is a common theme for collagen‐binding MSCRAMMs of many Gram-positive bacteria. Here, we report the interaction studies between rRspB_31–348 and silver nanoparticles using methods like gel shift assay, gel permeation chromatography and circular dichroism spectroscopy.

Results

The “Collagen Hug” mechanism was inhibited in the presence of silver nanoparticles as rRspB_31–348 was unable to bind to collagen. The total loss of binding was likely because of rRspB_31–348 and silver nanoparticle protein corona formation and not due to the loss of the structural integrity of rRspB_31–348 on binding with nanoparticles as observed from circular dichroism experiments.

General significance

Interaction of rRspB_31–348 with silver nanoparticle impaired its ligand binding mechanism. Details of this inhibition mechanism may be useful for the development of antimicrobial materials and antiadhesion drugs.     

The molecular basis of fibronectin-mediated bacterial adherence to host cells

Many 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 non-phagocytic 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.     

The N-terminal A domain of Staphylococcus aureus fibronectin-binding protein A binds to tropoelastin

Staphylococcus aureus is an important human pathogen. Its virulence factors include a variety of MSCRAMMs (microbial surface component recognizing adhesive matrix molecules), each capable of binding specifically to the host extracellular matrix. The fibronectin-binding protein, FnBPA, has been shown previously to bind immobilized fibronectin, fibrinogen, and alpha-elastin peptides. Here we show that region A of FnBPA (rAFnBPA) binds to recombinant human tropoelastin. Binding occurs to three separate truncates of tropoelastin, encompassing domains 2-18, 17-27, and 27-36, signifying that the interaction occurs at multiple sites. The greatest affinity was for the N-terminal truncate. We observed a pH dependency for the rAFnBPA-tropoelastin interaction with strong, nonsaturable binding at low pH. The interaction ceased at higher pH. These data support a model of surface-surface interactions between the negative charges present on rAFnBPA and the positive lysines of tropoelastin. A protein lacking the negatively charged C-terminal fibronectin-binding motif of the A domain of FnBPA and another construct lacking subdomain N1 were both capable of binding immobilized tropoelastin with a lower affinity. The binding properties of five site-directed mutants of rAFnBPA were compared with wild-type rAFnBPA. There was no decreased affinity for immobilized tropoelastin, in contrast to the defective binding of these mutants to alpha-elastin and fibrinogen. The data indicate novel interactions between tropoelastin and FnBPA that include the use of surface charges. These results demonstrate that FnBPA is capable of directly binding tropoelastin prior to its incorporation into elastin.     

A Structural Model of the Staphylococcus aureus ClfA–Fibrinogen Interaction Opens New Avenues for the Design of Anti-Staphylococcal Therapeutics

The fibrinogen (Fg) binding MSCRAMM Clumping factor A (ClfA) from Staphylococcus aureus interacts with the C-terminal region of the fibrinogen (Fg) γ-chain. ClfA is the major virulence factor responsible for the observed clumping of S. aureus in blood plasma and has been implicated as a virulence factor in a mouse model of septic arthritis and in rabbit and rat models of infective endocarditis. We report here a high-resolution crystal structure of the ClfA ligand binding segment in complex with a synthetic peptide mimicking the binding site in Fg. The residues in Fg required for binding to ClfA are identified from this structure and from complementing biochemical studies. Furthermore, the platelet integrin α_IIbβ₃ and ClfA bind to the same segment in the Fg γ-chain but the two cellular binding proteins recognize different residues in the common targeted Fg segment. Based on these differences, we have identified peptides that selectively antagonize the ClfA-Fg interaction. The ClfA-Fg binding mechanism is a variant of the “Dock, Lock and Latch” mechanism previously described for the Staphylococcus epidermidis SdrG–Fg interaction. The structural insights gained from analyzing the ClfANFg peptide complex and identifications of peptides that selectively recognize ClfA but not α_IIbβ₃ may allow the design of novel anti-staphylococcal agents. Our results also suggest that different MSCRAMMs with similar structural organization may have originated from a common ancestor but have evolved to accommodate specific ligand structures.     

Surface protein adhesins of Staphylococcus aureus

Staphylococcus aureus can colonize the host to initiate infection by adhering to components of the extracellular matrix. Adherence is mediated by surface protein adhesins (MSCRAMMs). Ligand binding by these fibronectin-, fibrinogen- and collagen-binding proteins occurs by distinct mechanisms that are being investigated at the molecular level.     

Proposed lead molecules against Hemagglutinin of avian influenza virus (H5N1)

Human infection with avian influenza H5N1 is an emerging infectious disease characterized by respiratory symptoms and a high fatality rate. Hemagglutinin and neuraminidase are the two surface proteins responsible for infection by influenza virus. Till date, neuraminidase has been the major target for antiviral drugs. In the present study we chose hemagglutinin protein as it mediates the binding of the virus to target cells through sialic acid residues on the host cell-surface. Hemagglutinin of H5 avian influenza (PDB ID: 1JSN) was used as the receptor protein. Ligands were generated by structure-based de novo approach and virtual screening of ZINC database. A total of 11,104 conformers were generated and docked into the receptor binding site using 'High Throughput Virtual Screening'. We proposed potential lead molecules against the receptor binding site of hemagglutinin based on the results obtained from in silico docking and hydrogen bond interaction between the ligand and the 1JSN protein molecule. We found sialic acid derivative 1 to be the lead molecules amongst the ligands generated by structure based de novo approach. However the molecules obtained from ZINC database were showing better docking scores as well as conserved hydrogen bond interactions. Thus we proposed ZINC00487720 and ZINC00046810 as potential lead molecules that could be used as an inhibitor to the receptor binding site of hemagglutinin. They could now be studied in vivo to validate the in silico results.     

The Fibrinogen-binding M1 Protein Reduces Pharyngeal Cell Adherence and Colonization Phenotypes of M1T1 Group A Streptococcus

Group A Streptococcus (GAS) is a leading human pathogen producing a diverse array of infections from simple pharyngitis (“strep throat”) to invasive conditions, including necrotizing fasciitis and toxic shock syndrome. The surface-anchored GAS M1 protein is a classical virulence factor that promotes phagocyte resistance and exaggerated inflammation by binding host fibrinogen (Fg) to form supramolecular networks. In this study, we used a virulent WT M1T1 GAS strain and its isogenic M1-deficient mutant to examine the role of M1-Fg binding in a proximal step in GAS infection-interaction with the pharyngeal epithelium. Expression of the M1 protein reduced GAS adherence to human pharyngeal keratinocytes by 2-fold, and this difference was increased to 4-fold in the presence of Fg. In stationary phase, surface M1 protein cleavage by the GAS cysteine protease SpeB eliminated Fg binding and relieved its inhibitory effect on GAS pharyngeal cell adherence. In a mouse model of GAS colonization of nasal-associated lymphoid tissue, M1 protein expression was associated with an average 6-fold decreased GAS recovery in isogenic strain competition assays. Thus, GAS M1 protein-Fg binding reduces GAS pharyngeal cell adherence and colonization in a fashion that is counterbalanced by SpeB. Inactivation of SpeB during the shift to invasive GAS disease allows M1-Fg binding, increasing pathogen phagocyte resistance and proinflammatory activities.     

Structural biology of Gram-positive bacterial adhesins

The structural biology of Gram-positive cell surface adhesins is an emerging field of research, whereas Gram-negative pilus assembly and anchoring have been extensively investigated and are well understood. Gram-positive surface proteins known as MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) and individual proteins that assemble into long, hair-like organelles known as pili have similar features at the primary sequence level as well as at the tertiary structural level. Some of these conserved features are essential for their transportation from the cytoplasm and for cell wall anchoring. More importantly, the MSCRAMMs and the individual pilins are assembled with building blocks that are variants of structural modules used for human immunoglobulins. MSCRAMMs target the host's extracellular matrix proteins, such as collagen, fibrinogen, and fibronectin, and they have received considerable attention from structural biologists in the last decade, who have primarily been interested in understanding their interactions with host tissue. The recent focus is on the newly discovered pili of Gram-positive bacteria, and in this review, we highlight the advances in understanding of the individual pilus constituents and their associations and stress the similarities between the individual pilins and surface proteins.