Effect of different asthma treatments on risk of cold-related exacerbations

Common colds often trigger asthma exacerbations. The present study compared cold-related severe exacerbations during budesonide/formoterol maintenance and reliever therapy, and different regimens of maintenance inhaled corticosteroids (ICS), with or without long-acting β(2)-agonists (LABA), and with as-needed short-acting β(2)-agonists (SABA) or LABA. Reported colds and severe exacerbations (defined by oral corticosteroid use and/or hospitalisation/emergency room visit) were assessed for 12,507 patients during 6-12 months of double-blind treatment. Exacerbations occurring ≤14 days after onset of reported colds were analysed by a Poisson model. The incidence of colds was similar across treatments. Asthma symptoms and reliever use increased during colds. Budesonide/formoterol maintenance and reliever therapy reduced severe cold-related exacerbations by 36% versus pooled comparators plus SABA (rate ratio (RR) 0.64; p=0.002), and for individual treatment comparisons, by 52% versus the same maintenance dose of ICS/LABA (RR 0.48; p<0.001); there were nonsignificant reductions versus higher maintenance doses of ICS or ICS/LABA (RR 0.83 and 0.72, respectively). As-needed LABA did not reduce cold-related exacerbations versus as-needed SABA (RR 0.96). Severe cold-related exacerbations were reduced by budesonide/formoterol maintenance and reliever therapy compared with ICS with or without LABA and with as-needed SABA. Subanalyses suggested the importance of the ICS component in reducing cold-related exacerbations. Future studies should document the cause of exacerbations, in order to allow identification of different treatment effects.

Group A streptococcal phagocytosis resistance is independent of complement factor H and factor H-like protein 1 binding

Factor H (FH) and factor H-like protein 1 (FHL-1) regulate complement activation through the alternative pathway. Several extracellular bacterial pathogens, prime targets for the complement system, bind FH and FHL-1, thereby acquiring a potential mechanism for minimizing complement deposition on their surface. For group A streptococci (GAS), surface-bound antiphagocytic M proteins mediate the interaction. To study the role of the FH-FHL-1 interaction for complement deposition and opsonophagocytosis of GAS, we first constructed a set of truncated M5 protein variants and expressed them on the surface of a homologous M-negative GAS strain. Binding experiments with the resulting strains demonstrated that the major FH-FHL-1 binding is located in a 42-amino-acid region within the N-terminal third of M5. Measurement of bacteria-bound complement factor C3 after incubation in plasma showed that the presence of this region had little impact upon complement deposition through the alternative pathway. Moreover, streptococci expressing M5 proteins lacking the major FH and FHL-1 binding sequence resisted phagocytosis in human blood as efficiently as bacteria expressing the wild-type protein. Consequently, the data suggest that the binding of the regulators of the alternative pathway is of limited importance for GAS phagocytosis resistance.

Staphylococcus aureus induces release of bradykinin in human plasma

Staphylococcus aureus is a prominent human pathogen. Here we report that intact S. aureus bacteria activate the contact system in human plasma in vitro, resulting in a massive release of the potent proinflammatory and vasoactive peptide bradykinin. In contrast, no such effect was recorded with Streptococcus pneumoniae. In the activation of the contact system, blood coagulation factor XII and plasma kallikrein play central roles, and a specific inhibitor of these serine proteinases inhibited the release of bradykinin by S. aureus in human plasma. Furthermore, fragments of the cofactor H-kininogen of the contact system efficiently blocked bradykinin release. The results suggest that activation of the contact system at the surface of S. aureus and the subsequent release of bradykinin could contribute to the hypovolemic hypotension seen in patients with severe S. aureus sepsis. The data also suggest that the contact system could be used as a target in the treatment of S. aureus infections.

Zinc-dependent conformational changes in domain D5 of high molecular mass kininogen modulate contact activation

Human high molecular mass kininogen (HK) participates as nonenzymatic cofactor in the contact system. Here, we show that recombinant domain D5 of HK (rD5) prolongs the clotting time of the intrinsic pathway of coagulation and attenuates the generation of bradykinin. Further studies indicate that a correct fold of domain D5 within HK is required for the activation of the contact system. The folding of rD5 seems to be modulated by the metal ions Zn2+, Ni2+, and Cu2+ as a specific antibody directed against the zinc-binding site in HK binds to HK and rD5 in a metal ion concentration dependent manner. The finding that these three metal ions specifically affect contact activation suggests that they regulate the accessibility of rD5 for negatively charged surfaces. Support for the assumption that the observed phenomena are due to conformational changes was obtained by fluorescence spectroscopy of rD5, demonstrating that its fluorescence spectrum was changed in the presence of ZnCl2. Moreover, negative staining electron microscopy experiments suggest that the zinc-induced changes in D5 also affect the conformation of the entire HK protein. The present data emphasize the role of zinc and other metal ions in the regulation of contact activation.

Role for a secreted cysteine proteinase in the establishment of host tissue tropism by group A streptococci

Primary infection of the human host by group A streptococci (GAS) most often involves either the epidermis of the skin or the oropharyngeal mucosa. A humanized in vivo model for impetigo was used to investigate the basis for host tissue tropism among GAS. Disruption of the speB gene (encoding for a secreted cysteine proteinase) led to a loss of virulence for two impetigo-derived strains (M-types 33 and 53), as evidenced by a diminution in tissue damage and a lack of reproductive growth. The level of cysteine proteinase activity in overnight cultures was associated with the extent of gross pathological changes induced by strains displaying varied degrees of virulence in the impetigo model. Moreover, high levels of secreted cysteine proteinase activity correlated with a genetic marker for preferred tissue site of infection at the skin (emm pattern D). The addition of exogenous SpeB to a speB mutant (emm pattern D) or to an avirulent throat-like strain (emm pattern A) led to increased bacterial reproduction at the skin. The data provide both experimental and epidemiological evidence for a critical role of a secreted bacterial protease in promoting host tissue-specific infection.

A role for the fibrinogen-binding regions of streptococcal M proteins in phagocytosis resistance

All virulent group A streptococcal isolates bind fibrinogen, a property that is closely linked to expression of type-specific antiphagocytic surface molecules designated M proteins. Here we show that although the M proteins from two different strains, M1 and M5, both bind fibrinogen with high affinity, they interact with different regions in the ligand. Moreover, mapping experiments demonstrated that the fibrinogen-binding regions in the M1 and M5 proteins are quite dissimilar at the amino acid sequence level and that they bind to different regions in the plasma protein. In spite of these differences, the fibrinogen-binding regions of M1 and M5 could both be shown to contribute to streptococcal survival in human blood, providing evidence for the distinct function of a plasma protein interaction in bacterial pathogenesis.

Analysis of plasminogen-binding M proteins of Streptococcus pyogenes

Group A streptococci are common human pathogens that cause a variety of infections. They express M proteins which are important cell wall-bound type-specific virulence factors. We have found that a set of strains, associated primarily with skin infections, express M proteins that bind plasminogen and plasmin with high affinity. The binding is mediated by a 13-amino-acid internal repeated sequence located in the N-terminal surface-exposed portion of these M proteins. This sequence binds to kringle 2 in plasminogen, a domain that is not involved in the interaction with streptokinase, a potent group A streptococcal activator of plasminogen. It could be demonstrated that plasminogen, absorbed from plasma by growing group A streptococci expressing the plasminogen-binding M proteins, could be activated by exogenous and endogenous streptokinase, thereby providing the bacteria with a surface-associated enzyme that could act on the tissue barriers in the infected host.

Human complement regulators: a major target for pathogenic microorganisms

The C3 convertases of the human complement system are controlled by fluid-phase and membrane proteins in the RCA (regulators of complement activation) family. Accumulated data show that many pathogenic microorganisms interact with these complement regulators. Recent advances in this field include determination of the crystal structure of the binding domains in the measles virus receptor CD46 and identification of a CD46 transgenic mouse line that is sensitive to measles virus. Moreover, recent findings support the hypothesis that pathogenic bacteria binding fluid-phase RCA proteins exploit these proteins to escape complement attack. These studies provide novel insight into the interplay between pathogens and the innate immune system and may have implications for the plans to use animals expressing an RCA protein for xenotransplantation.

Strain-specific restriction of the antiphagocytic property of group A streptococcal M proteins

Group A streptococcal M proteins are type-specific virulence factors that inhibit phagocytosis. We used two M proteins, M5 and Emm22, to analyze the influence of genetic background on the properties of M proteins. Mutant strains, engineered to lack these M proteins, were complemented with genes encoding the homologous or heterologous M protein, and the complemented strains were analyzed for phagocytosis resistance. Neither the M5 nor the Emm22 protein conferred phagocytosis resistance in the heterologous background, but they did do so in the homologous background. This was not due to lack of surface expression in the heterologous background. Moreover, the M5 and Emm22 proteins expressed in heterologous background appeared to have normal structure, since they were not affected in their ability to bind different human plasma proteins. In particular, M5 or Emm22 had normal ability to bind human complement inhibitors, a property that has been implicated in phagocytosis resistance. Results similar to those obtained with M5 and Emm22 were obtained in experiments with the M6 and Emm4 proteins. Together, these data suggest that the surface expression of M protein alone may not be sufficient to confer phagocytosis resistance and consequently that strain-specific factors other than M and Emm proteins may contribute to the ability of group A streptococci to resist phagocytosis.

Selective distribution of a high-affinity plasminogen-binding site among group A streptococci associated with impetigo

Group A streptococci can be classified according to their tendency to cause either impetigo, pharyngitis, or both types of infection. Genotypic markers for tissue site preference lie within emm genes, which encode fibrillar surface proteins that play a key role in virulence. emm gene products (M and M-like proteins) display an extensive array of binding activities for tissue and plasma proteins of the human host. In a previous study, a high-affinity binding site for human plasmin(ogen) was mapped to the emm53 gene product. In this report, a structurally similar plasminogen-binding domain is found to be widely and selectively distributed among group A streptococci harboring the emm gene marker for the skin as the preferred tissue site for infection. The findings are highly suggestive of a central role for bacterial modulation of host plasmin(ogen) during localized infection at the epidermis.

Protein LA, a novel hybrid protein with unique single-chain Fv antibody- and Fab-binding properties

Existing Ig-binding proteins all suffer from limitations in their binding spectrum. In the pursuit of the ultimate, non-restricted, Ig-binding protein, we have constructed the hybrid protein LA, by fusing four of the Ig kappa light-chain-binding domains of peptostreptococcal protein L with four of the IgGFc- and Fab-binding regions of staphylococcal protein A. Ligand-blot experiments demonstrated that the L and the A components were both functional in the hybrid, as the protein was shown to bind purified kappa light chains and IgGFc. Protein LA bound human Ig of different classes and IgG from a wide range of mammalian species. IgG, IgM and IgA were purified from human serum and saliva by affinity chromatography on protein LA agarose. Similarly, single-chain Fv (scFv) antibodies carrying the kappa light-chain variable domain or expressing the V(H)III (variable domain of the heavy chain of Ig) determinant, were efficiently purified on immobilized protein LA. As judged by surface plasmon resonance (SPR), protein LA showed enhanced affinity for all tested ligands, including several scFv antibodies, compared with proteins L and A alone. SPR analysis also demonstrated that binding of a ligand to one of the components in protein LA did not affect the ability of the hybrid protein to interact simultaneously with a ligand for the other component. The antigen-binding capacity of a kappa-expressing scFv antibody was unaffected by the interaction with protein LA, whereas the binding of a V(H)III-expressing scFv antibody to its antigen was, unexpectedly, blocked by protein A and protein LA. Together, these data demonstrate that protein LA represents a highly versatile Ig-binding molecule.

Role of the hypervariable region in streptococcal M proteins: binding of a human complement inhibitor

Antigenic variation allows pathogenic microorganisms to evade the immune system of the infected host. The variable structure must play an important role in pathogenesis, but its function is in most cases unknown. Here, we identify a function for the surface-exposed hypervariable region of streptococcal M5 protein, a virulence factor that inhibits phagocytosis. The hypervariable region of M5 was found to bind the human complement inhibitor FHL-1 (factor H-like protein 1), a 42-kDa plasma protein. Plasma absorption experiments with M5-expressing bacteria showed that the interaction with FHL-1 occurs also under physiologic conditions. Studies of another extensively characterized M protein, M6, indicated that this protein also has a binding site for FHL-1 in the hypervariable region. The complement-inhibitory function of FHL-1 was retained after binding to streptococci, suggesting that bound FHL-1 protects bacteria against complement attack. All available data now indicate that FHL-1, or another human complement inhibitor, binds to the hypervariable region of M proteins. These findings provide insights into the forces that drive antigenic variation and may explain why the hypervariable region of M protein is essential for phagocytosis resistance. Moreover, these data add to a growing body of evidence that human complement inhibitors are major targets for pathogenic microorganisms.

Kringle 2 mediates high affinity binding of plasminogen to an internal sequence in streptococcal surface protein PAM

Many cells express receptors for plasminogen (Pg), although the responsible molecules in most cases are poorly defined. In contrast, the group A streptococcal surface protein PAM contains a domain with two 13-amino acid residue long repeated sequences (a1 and a2) responsible for Pg binding. Here we identify the region in Pg that interacts with PAM. A radiolabeled proteolytic plasminogen fragment containing the first three kringles (K1-K3) interacted with streptococci expressing PAM or a chimeric surface protein harboring the a1a2 sequence. In contrast, plasminogen fragments containing kringle 4 or kringle 5 and the activable serine proteinase domain failed to bind to PAM-expressing group A streptococci. A synthetic and a recombinant polypeptide containing the a1a2 sequence both bound to immobilized recombinant K2 (rK2) but not to rK1 or rK3. The interaction between the a repeat region and rK2 was reversible, and rK2 completely blocked the binding of Pg to the a1a2 region. The binding of the a repeat containing polypeptide to K2 occurred with an equilibrium association constant of 4.5 x 10(7) M-1, as determined by surface plasmon resonance, a value close to that (1.6 x 10(7) M-1) calculated for the a1a2-Pg interaction. Inhibition experiments suggested involvement of the lysine-binding site of K2 in the interaction. These data demonstrate that K2 contains the major Pg-binding site for PAM, providing the first well defined example of an interaction between an internal Pg-binding region in a protein and a single kringle domain.

Identification of a domain in human factor H and factor H-like protein-1 required for the interaction with streptococcal M proteins

The plasma protein factor H (FH) inhibits the alternative pathway of complement activation. Previous work has shown that FH binds to group A streptococci and that the interaction does not interfere with the complement-inhibitory capacity of FH. In this work, we report a molecular analysis of this interaction. In absorption experiments with human plasma, M protein-expressing group A streptococci bound both FH and FH-like protein-1 (FHL-1), an active 42-kDa splice product of the FH-gene transcript comprising the first 7 of its 20 short consensus repeat (SCR) domains. rFHL-1 also bound to M protein-expressing streptococci, but rFH fragments containing SCR 1-5 or SCR 1-6 did not. rFHL-1 bound to purified M5 protein with an affinity that was higher than the value calculated for the interaction between FH and M5 protein. The binding of radiolabeled rFHL-1 to immobilized M5 was blocked completely by unlabeled rFHL-1, but was inhibited only partially by SCR 1-6, emphasizing the importance of SCR 7 for the interaction. In experiments with the FH-related proteins FHR-3 and FHR-4, only the former bound to M protein-expressing streptococci, again pointing to an involvement of SCR 7, since FHR-3, but not FHR-4, contains a domain that is similar to SCR 7. Finally, the interaction between rFHL-1 and purified M5 protein was inhibited by heparin, which binds FH via SCR 7. Together, these data indicate that the interaction between streptococcal M proteins and FH or FHL-1 requires SCR 7.

Molecular co-operation between protein PAM and streptokinase for plasmin acquisition by Streptococcus pyogenes

Bacterial surface-associated plasmin formation is believed to contribute to invasion, although the underlying molecular mechanisms are poorly understood. To define the components necessary for plasmin generation on group A streptococci we used strain AP53 which exposes an M-like protein ("PAM") that contains a plasminogen-binding sequence with two 13-amino acid residues long tandem repeats (a1 and a2). Utilizing an Escherichia coli-streptococcal shuttle vector, we replaced a 29-residue long sequence segment of Arp4, an M-like protein that does not bind plasminogen, with a single (a1) or the combined a1a2 repeats of PAM. When expressed in E. coli, the purified chimeric Arp/PAM proteins both bound plasminogen, as well as plasmin, and when used to transform group A streptococcal strains lacking the plasminogen-binding ability, transformants with the Arp/PAM constructs efficiently bound plasminogen. Moreover, when grown in the presence of plasminogen, both Arp/PAM- and PAM-expressing streptococci acquired surface-bound plasmin. In contrast, plasminogen activation failed to occur on PAM- and Arp/PAM-expressing streptococci carrying an inactivated streptokinase gene: this block was overcome by exogenous streptokinase. Together, these results provide evidence for an unusual co-operation between a surface-bound protein, PAM, and a secreted protein, streptokinase, resulting in bacterial acquisition of a host protease that is likely to spur parasite invasion of host tissues.

Absorption of kininogen from human plasma by Streptococcus pyogenes is followed by the release of bradykinin

H-kininogen (high-molecular-mass kininogen, HK) is the precursor of the vasoactive peptide hormone bradykinin (BK). Previous work has demonstrated that HK binds to Streptococcus pyogenes through M-proteins, fibrous surface proteins and important virulence factors of these bacteria. Here we find that M-protein-expressing bacteria absorb HK from human plasma. The HK bound to the bacteria was found to be cleaved, and analysis of the degradation pattern suggested that the cleavage of HK at the bacterial surface is associated with the release of BK. Moreover, addition of activated plasma prekallikrein to bacteria preincubated with human plasma, resulted in BK release. This mechanism, by which a potent vasoactive and proinflammatory peptide is generated at the site of infection, should influence the host-parasite relationship during S. pyogenes infections.

NMR analysis of the interaction between protein L and Ig light chains

Protein L is a cell wall protein expressed by some strains of the anaerobic bacterial species Peptostreptococcus magnus. It binds to immunoglobulin (Ig) light chains predominantly of the kappa subtype from a wide range of animal species. This binding is mediated by five highly homologous repeats designated as B1-B5, each of which comprises 72 to 76 amino acid residues. The fold of the Ig light chain-binding B1 domain of protein L has previously been shown to comprise an alpha-helix packed against a four-stranded beta-sheet. The Ig-binding region of the protein L domain involves most of the residues in the second beta-strand, the C-terminal residues of the alpha-helix, and residues in the loop connecting the alpha-helix with the third beta-strand. In the present study, we have identified the protein L-binding site of an Ig light chain by use of stable isotope-assisted NMR spectroscopy. The light chain of a murine monoclonal anti-17alpha-hydroxy-progesterone Fab fragment (IgG2b, kappa) was selectively labeled with 13C at carbonyl groups of Ala, Arg, Cys, Ile, Lys, Met, Phe, Trp, or Tyr. The residues in which the carbonyl 13C chemical shift was significantly perturbed upon binding of the protein L B1 domain were preferentially found in the second beta-strand of the variable kappa domain and parts of its flanking beta-strands. None of these residues were affected by the addition of the antigen against which the monoclonal Fab fragment is directed. Therefore, we conclude that protein L binds to the outer surface of the framework region of the V(L) domain, primarily involving the V(L) second strand, and that this binding is independent of antigen-binding. The present NMR data, in combination with sequence comparisons between kappa light chains with and without protein L affinity, suggest that the amino acid substitutions at positions 9, 20, and/or 74 of the kappa light chains could crucially affect the interaction between protein L and the V(L) domain.

Characterization of the binding properties of protein LG, an immunoglobulin-binding hybrid protein

Protein LG is a 50-kDa hybrid molecule containing four Ig-light-chain-binding domains from protein L of Peptostreptococcus magnus and two IgG-Fe-binding repeats from streptococcal protein G. Here we analyse the binding of protein LG to Ig from several mammalian species. Protein LG was shown to bind human IgG of all subclasses and other Ig classes that carry kappa chains. The binding to human IgG was only marginally influenced by changes in temperature (4-37 degrees C) or salt concentration (0-1.6 M), and was stable over a wide pH range (pH 4-10). Protein LG bound to Ig from 11 of 12 mammalian species, including those of rabbit, mouse and rat. The affinity constants obtained for the interactions between protein LG and polyclonal IgG from rabbit (4.0 x 10(9) M-1), mouse (1.7 x 10(9) M-1) and rat (1.3 x 10(9) M-1) were similar to the value previously reported for the interaction between the hybrid protein and human polyclonal IgG (5.9 x 10(9) M-1). The interaction between protein LG and a mouse IgG mAb was not influenced by the presence of the specific protein antigen, nor was the binding of this antibody to its ligand affected by protein LG. Inhibition experiments demonstrated that the Ig-binding site of one of the fusion partners retained its ligand-binding capacity when the other component was occupied. Protein LG selectively absorbed 85-90% of the total Ig present in human and rabbit sera and 75-80% of the Ig in sera from mouse and rat. Human serum depleted of C1q, factor D and properdin and preabsorbed by protein LG could be used as a source for other complement factors. These data demonstrate that protein LG is a very versatile Ig-binding protein.

Assembly of human contact phase proteins and release of bradykinin at the surface of curli-expressing Escherichia coli

Previous work has demonstrated that most strains of the human pathogen Streptococcus pyogenes bind kininogens through M protein, a fibrous surface protein and virulence determinant. Here we find that strains of several other pathogenic bacterial species, both Gram-positive and Gram-negative, isolated from patients with sepsis, also bind kininogens, especially kininogen (HK). The most pronounced interaction was seen between HK and Escherichia coli. Among clinical isolates of E. coli, the majority of the enterohaemorrhagic, enterotoxigenic, and sepsis strains, but none of the enteroinvasive and enteropathogenic strains, bound HK. Binding of HK to E. coli correlated with the expression of curli, another fibrous bacterial surface protein, and the binding of HK to purified curli was specific, saturable, and of high affinity; Ka = 9 x 10(7) M-1. Other contact phase proteins such as factor XI, factor XII, and prekallikrein bound to curliated E. coli, but not to an isogenic curli-deficient mutant strain, suggesting that contact phase activation may occur at the surface of curliated bacteria. Kininogens are also precursor molecules of the vasoactive kinins. When incubated with human plasma, curli-expressing bacteria absorbed HK. Addition of purified plasma kallikrein to the HK-loaded bacteria resulted in a rapid and efficient release of bradykinin from surface-bound HK. The assembly of contact phase factors at the surface of pathogenic bacteria and the release of the potent proinflammatory and vasoactive peptide bradykinin, should have a major impact on the host-microbe relationship and may contribute to bacterial pathogenicity and virulence.

Diagnosis of tuberculous meningitis: a comparative analysis of 3 immunoassays, an immune complex assay and the polymerase chain reaction

OBJECTIVE

To compare 3 immunoassays, an immune complex assay, and an application of the polymerase chain reaction (PCR) for the diagnosis of tuberculous meningitis (TBM).

MATERIAL

Cerebrospinal fluid (CSF) from 33 patients with TBM and from 34 control patients with infectious and non-infectious CNS diseases was analysed.

RESULTS

The antibody immunoassays were either nonspecific or insensitive. However, detection of mycobacterial IgG immune complexes correlated strongly with infection, as they were detected in the CSF from 64% of the patients with TBM compared to only 3 (9%) of the control samples. PCR analysis, using Mycobacterium tuberculosis-specific oligonucleotide primers, also strongly correlated with infection, as DNA was amplified from 54% of the samples from patients with TBM, but from only 2 (6%) of the control samples. Both 'false positive' samples were also positive in the immune complex assay and came from 2 patients with otogenic brain abscesses. It is conceivable that these patients suffered from otogenic tuberculosis with secondary non-mycobacterial meningitis. When combining the immune complex assay with DNA-detection by PCR, 100% of the culture positive and 74% of culture negative samples were found to be positive, while maintaining a high specificity.

CONCLUSION

Parallel analysis to detect mycobacterial immune complexes and M. tuberculosis-specific DNA by PCR from the CSF of patients may offer a sensitive and specific tool for the diagnosis of TBM.

Mapping of the immunoglobulin light chain-binding site of protein L

Protein L is a cell surface protein expressed by some strains of the anaerobic bacterial species Peptostreptococcus magnus. The molecule binds specifically and with high affinity to immunoglobulins (Ig) of a wide range of animal species. The Ig-binding activity is mediated through five highly homologous domains, each 72 to 76 amino acid residues long, which interact with framework regions in the variable domain of Ig light chains. The interaction does not interfere with the antigen binding capacity of the antibody. The fold of the Ig light chain-binding domains of Protein L is comprised of an alpha-helix packed against a four stranded beta-sheet and is similar to the fold of the IgG heavy chain-binding domains of streptococcal protein G, despite the fact that the two proteins show no significant sequence homology. In the present work, heteronuclear NMR spectroscopy has been utilized to define the interaction between the N-terminal Ig-binding domain of Protein L and the variable domain of a human Ig kappa light chain. The Ig-binding region of the Protein L domain involves most of the residues in the second beta-strand, the C-terminal residues of the alpha-helix and the loop connecting the alpha-helix with the third beta-strand. The Ig light chain-binding surface of Protein L thus resembles the surface of Protein G which binds to the C gamma 1 domain of IgG, but is different from the portion of Protein G involved in the contact with the C gamma 2-C gamma 3 interface region. The data suggest that the global fold shared by the Ig-binding domains of Proteins L and G provide bacteria with a flexible template for the evolution of surface structures capable of interacting with different conserved parts of Ig molecules of the infected host.

Three-dimensional solution structure of an immunoglobulin light chain-binding domain of protein L. Comparison with the IgG-binding domains of protein G

Protein L is a multidomain protein expressed at the surface of some strains of the anaerobic bacterial species Peptostreptococcus magnus. It has affinity for immunoglobulin (Ig) through interaction with framework structures in the variable Ig light chain domain. The Ig-binding activity is located to five homologous repeats called B1-B5 in the N-terminal part of the protein. We have determined the three-dimensional solution structure of the 76 amino acid residue long B1 domain using NMR spectroscopy and distance geometry-restrained simulated annealing. The domain is composed of a 15 amino acid residue long disordered N-terminus followed by a folded portion comprising an alpha-helix packed against a four-stranded beta-sheet. These secondary structural elements are well determined with a backbone atomic root mean square deviation from their mean of 0.54 A. The B domains of protein L show very limited sequence homology to the domains of streptococcal protein G interacting with the heavy chains of IgG. However, despite this fact, and their different binding properties, the fold of the B1 domain was found to be similar to the fold of the IgG-binding protein G domains [Wikström, M., Sjöbring, U., Kastern, W., Björck, L., Drakenberg, T., & Forsén, S. (1993) Biochemistry 32, 3381-3386]. In the present study, the solution structure of the B1 domain enabled a more detailed comparison which can explain the different Ig-binding specificities of these two bacterial surface proteins. Among the differences observed, the alpha-helix orientation is the most striking.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasminogen, absorbed by Escherichia coli expressing curli or by Salmonella enteritidis expressing thin aggregative fimbriae, can be activated by simultaneously captured tissue-type plasminogen activator (t-PA)

Curli are fimbrial structures expressed by Escherichia coli that specifically interact with matrix proteins such as fibronectin and laminin. Similar structures are also expressed by Salmonella enteritidis and have been denoted thin aggregative fimbriae. Bacteria expressing curli and thin aggregative fimbriae were found to bind radiolabelled plasminogen as well as the tissue-type plasminogen activator (t-PA). By contrast, E. coli carrying a gene locus with an insertionally inactivated chromosomal curlin subunit were unable to bind the two human proteins. The purified subunit polypeptides of curli and thin aggregative fimbriae bound plasminogen and t-PA with high affinity (1 x 10(8) to 2 x 10(8) M-1). The binding of plasminogen and t-PA to curli-expressing E. coli was only partially inhibited by fibronectin and laminin. Plasminogen absorbed from human plasma by curli-expressing E. coli was readily converted to plasmin by t-PA; both plasmin and t-PA were functionally active when bound to the bacteria. A simultaneous binding of fibrinolytic proteins and matrix proteins to fimbriae of E. coli and S. enteritidis could provide these pathogens with both adhesive and invasive properties.

Streptokinase activates plasminogen bound to human group C and G streptococci through M-like proteins

An ability to interact with plasminogen or plasmin could provide micro-organisms with a mechanism for invasion. Thus, group A, C and G streptococci secrete streptokinase which binds and activates plasminogen. Some streptococci also express surface structures which bind plasminogen without causing its activation. Plasminogen-binding surface proteins were extracted from one group C and one group G streptococcal isolate. Both proteins were found to bind plasmin, fibrinogen and serum albumin in addition to plasminogen. Gene fragments encoding the streptococcal proteins were amplified by PCR and were subsequently cloned and expressed in Escherichia coli. DNA sequence determination revealed for both genes open reading frames encoding proteins which contained repetitive domains and a carboxyl-terminal unrepeated region that were typical of M and M-like proteins. Though the amino-terminal regions of the group C and G streptococcal proteins demonstrated a rather high overall similarity between themselves, they were not similar to the variable regions of other M-like proteins with one exception: there was a 46% identity between the first 22 amino acids of the group G streptococcal protein and the corresponding sequence of PAM, the plasminogen-binding M-like protein of type M53 group A streptococci. Like the proteins extracted from the streptococci, the recombinant proteins bound plasminogen, fibrinogen and albumin. The three plasma proteins bound to separate sites on the streptococcal M-like proteins. Plasminogen bound by the group C and G streptococcal proteins was readily activated by streptokinase, providing evidence for a functional link between the secreted plasminogen-activator and proteins exposed on the bacterial surface.

Protein H--a surface protein of Streptococcus pyogenes with separate binding sites for IgG and albumin

Protein H, a molecule expressed at the surface of some strains of Streptococcus pyogenes, has affinity for the constant (IgGFc) region of immunoglobulin (Ig) G. In absorption experiments with human plasma, protein H-sepharose could absorb not only IgG but also albumin from plasma. The affinity constant for the reaction between albumin and protein H was 7.8 x 10(9) M-1, which is higher than the affinity between IgG and protein H (Ka = 1.6 x 10(9) M-1). Fragments of protein H were generated with deletion plasmids and polymerase chain reaction (PCR) technology. Using these fragments in various protein-protein interaction assays, the binding of albumin was mapped to three repeats (C1-C3) in the C-terminal half of protein H. On the albumin molecule, the binding site for protein H was found to overlap the site for protein G, another albumin- and IgGFc-binding bacterial surface protein. Also IgGFc-binding could be mapped with the protein H fragments and the region was found N-terminally of the C repeats. A synthetic peptide (25 amino acid residues long) based on a sequence in this region was shown to inhibit the binding of protein H to immobilized IgG or IgGFc. This sequence was not found in previously described IgGFc-binding proteins. However, two other cell surface proteins of S. pyogenes exhibited highly homologous regions. The results identify IgGFc- and albumin-binding regions of protein H and further define and emphasize the convergent evolution among bacterial surface proteins interacting with human plasma proteins.

PAM, a novel plasminogen-binding protein from Streptococcus pyogenes

The ability of group A streptococci to bind human plasminogen and plasmin has attracted interest, because it could provide the bacteria with a mechanism for invasion. M or M-like proteins account for the binding of several plasma proteins to group A streptococci. To investigate whether M or M-like proteins were responsible for the binding of plasminogen to group A streptococci, acid-extracted material from a type M53 streptococcal isolate was tested for its ability to bind plasminogen. Indeed, a 42-kDa plasminogen-binding protein was solubilized. Two oligonucleotides homologous with conserved sequences in known M protein genes were used as primers in the polymerase chain reaction, with chromosomal DNA from the M53 isolate. When cloned and expressed in Escherichia coli, a resulting fragment encoded a 43-kDa plasminogen-binding protein. Nucleotide sequence determination of the gene fragment revealed an open reading frame encoding a polypeptide of 43,580 Da, which matched the amino-terminal amino acid sequence of the plasminogen-binding protein extracted from M53 streptococci. The DNA sequence data also proved the relationship of the encoded protein, named PAM, to the M proteins. The plasminogen-binding domain was mapped to the amino-terminal third of PAM. Plasminogen absorbed by M53 streptococci or by immobilized PAM could be activated by streptokinase. The results provide further evidence of the diversity of the M protein family and suggest a new mechanism whereby these proteins contribute to the virulence of group A streptococci.

Proton nuclear magnetic resonance sequential assignments and secondary structure of an immunoglobulin light chain-binding domain of protein L

The 1H NMR assignments have been made for the immunoglobulin (Ig) light chain-binding B1 domain of protein L from Peptostreptococcus magnus. The secondary structure elements and the global folding pattern were determined from nuclear Overhauser effects, backbone coupling constants, and slowly exchanging amide protons. The B1 domain was found to be folded into a globular unit of 61 amino acid residues, preceded by a 15 amino acid long disordered N-terminus. The folded portion of the molecule contains a four-stranded beta-sheet spanned by a central alpha-helix. The fold is similar to the IgG-binding domains of streptococcal protein G, despite the fact that the binding sites on immunoglobulins for the two proteins are different; protein G binds IgG through the constant (Fc) part of the heavy chain, whereas protein L has affinity for the variable domain of Ig light chains.

Protein LG: a hybrid molecule with unique immunoglobulin binding properties

Immunoglobulin (Ig)-binding bacterial proteins have attracted theoretical interest for their role in molecular host-parasite interactions, and they are widely used as tools in immunology, biochemistry, medicine, and biotechnology. Protein L of the anaerobic bacterial species Peptostreptococcus magnus binds Ig light chains, whereas streptococcal protein G has affinity for the constant (Fc) region of IgG. In this report, Ig binding parts of protein L and protein G were combined to form a hybrid molecule, protein LG, which was found to bind a large majority of intact human Igs as well as Fc and Fab fragments, and Ig light chains. Binding to Ig was specific, and the affinity constants of the reactions between protein LG and human IgG, IgGFc fragments, and kappa light chains, determined by Scatchard plots, were 5.9 x 10(9), 2.2 x 10(9), and 2.0 x 10(9) M-1, respectively. The binding properties of protein LG were more complete as compared with previously described Ig-binding proteins when also tested against mouse and rat Igs. This hybrid protein thus represents a powerful tool for the binding, detection, and purification of antibodies and antibody fragments.

Evaluation of polymerase chain reaction, tuberculostearic acid analysis, and direct microscopy for the detection of Mycobacterium tuberculosis in sputum

Tuberculosis remains a major global cause of morbidity and mortality. There is an urgent need for improved bacteriologic diagnosis of Mycobacterium tuberculosis infection. Three methods for rapid identification of M. tuberculosis in sputum samples (direct microscopy, gas chromatography-mass spectrometry [GC-MS], and polymerase chain reaction [PCR]), were compared with culture on Lowenstein-Jensen medium. Growth of M. tuberculosis was observed in 38 of 145 sputum samples. Detection of acid-fast bacilli by direct microscopy gave a sensitivity of 66% and a specificity of 100%. Detection of tuberculostearic acid by GC-MS gave a sensitivity of 55% and a specificity of 87%. Amplification by PCR of a fragment of the insertion sequence IS6110 gave a sensitivity of 95% and a specificity of 93% compared with culture and a corrected specificity of 99% compared with both culture and clinical data. This study indicates that PCR can be adapted for clinical use and is the method of choice for rapid diagnosis of pulmonary tuberculosis.

Convergent evolution among immunoglobulin G-binding bacterial proteins

Protein G, a bacterial cell-wall protein with high affinity for the constant region of IgG (IgGFc) antibodies, contains homologous repeats responsible for the interaction with IgGFc. A synthetic peptide corresponding to an 11-amino acid-long sequence in the COOH-terminal region of the repeats was found to bind to IgGFc and block the interaction with protein G. Moreover, two other IgGFc-binding bacterial proteins (proteins A and H), which do not contain any sequences homologous to the peptide, were also inhibited in their interactions with IgGFc by the peptide. Finally, a decapeptide based on a sequence in IgGFc blocked the binding of all three proteins to IgGFc. This unusually clear example of convergent evolution emphasizes the complexity of protein-protein interactions and suggests that bacterial surface-protein interaction with host protein adds selective advantages to the microorganism.

Isolation and molecular characterization of a novel albumin-binding protein from group G streptococci

Many streptococcal strains are known to bind the two most abundant plasma proteins, namely, immunoglobulin G and albumin. Protein G isolated from group C and G streptococci has been demonstrated to have separate binding regions for each of these proteins. However, some group G streptococcal strains bind only serum albumin. This report describes the isolation of a 48-kDa albumin-binding protein from such a strain (DG12). The affinity constant of this protein for human serum albumin was determined to be 5 x 10(9) M-1, and the protein interacted strongly also with serum albumin from several other mammalian species. The gene encoding the albumin-binding protein was cloned and expressed in Escherichia coli. DNA sequence analysis of this gene revealed a unique NH2-terminal sequence and three types of repeats in the encoded protein. One of these repeated sequences has significant homology with the albumin-binding domains of protein G, and it was demonstrated that the albumin binding of the DG12 protein was localized within these domains. Another type of repeat is localized in the putative wall-spanning region of the molecule. This repeat sequence, which has the length of only 4 amino acids (LysProGluVal), is repeated 14 times. The relationship of the albumin-binding protein to other cell-wall-associated proteins of pathogenic streptococci is discussed.

Structure of peptostreptococcal protein L and identification of a repeated immunoglobulin light chain-binding domain

The gene for protein L, an immunoglobulin (Ig) light chain-binding protein expressed by some strains of the anaerobic bacterial species Peptostreptococcus magnus, was cloned and sequenced. The gene translates into a protein of 719 amino acid residues. Following a signal sequence of 18 amino acids and a NH2-terminal region ("A") of 79 residues, the molecule contains five homologous "B" repeats of 72-76 amino acids each. Further, toward the COOH terminus, two additional repeats ("C") were found. These are not related to the "B" repeats, but are highly homologous to each other. After the C repeats (52 amino acids each), a hydrophilic, proline-rich putative cell wall-spanning region ("W") was found, followed at the COOH-terminal end by a hydrophobic membrane anchor ("M"). Fragments of the gene were expressed, and the corresponding peptides were analyzed for Ig-binding activity. The B repeats were found to be responsible for the interaction with Ig light chains. An Escherichia coli high level expression system was adapted for the production of large amounts of two Ig-binding protein L fragments comprising one and four B repeats, respectively.

Streptococcal protein G. Gene structure and protein binding properties

Protein G was solubilized from 31 human group C and G streptococcal strains with the muralytic enzyme mutanolysin. As judged by the mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the binding patterns of the solubilized protein G molecules in Western blot experiments, the strains could be divided into three groups, represented by the group G streptococcal strains G148 and G43 and the group C streptococcal strain C40. The 65-kDa G148 protein G and the 58-kDa C40 protein G showed affinity for both immunoglobulin G (IgG) and human serum albumin (HSA), whereas the 40-kDa G43 protein G bound only IgG. Despite the different molecular patterns, the three protein G species had identical NH2-terminal amino acid sequences. Apart from the 65-kDa peptide, digestion of G148 streptococci with mutanolysin also produced a 52-kDa IgG- and HSA-binding peptide and a 14-kDa HSA-binding peptide. It was demonstrated that these peptides resulted from cleavage of 65-kDa protein G by proteolytic components in the mutanolysin preparation. The protein G genes of the C40 and G43 strains were cloned and sequenced, and their structure was compared to the previously published sequence of the G148 protein G gene. As compared to G148, both the C40 and G43 genes lacked a 210-base pair fragment in the IgG-binding region, accounting for the 10-fold lower affinity of these proteins for IgG. The G43 gene also lacked a 450-base pair fragment in the 5'-end of the gene, explaining why the G43 protein G did not bind HSA. The differences in protein G structure did not correlate with the clinical origin of the strains used in this study. The IgG-binding region of protein G was further mapped. Thus, a peptide corresponding to a single IgG-binding unit was obtained by the cloning and expression of a 303-base pair polymerase chain reaction-generated DNA fragment. The affinity of this 11.5-kDa peptide for human IgG was 8.0 x 10(7) M-1, as determined by Scatchard plots. Finally, a 55-amino acid-long synthetic peptide, corresponding to one of the three repeated domains in the COOH-terminal half of strain G148 protein G, effectively blocked binding of protein G to IgG.

Polymerase chain reaction for detection of Mycobacterium tuberculosis

A polymerase chain reaction for the specific detection of mycobacteria belonging to the Mycobacterium tuberculosis complex was developed. Using a single primer pair derived from the nucleotide sequence of protein antigen b of M. tuberculosis, we achieved specific amplification of a 419-base-pair DNA fragment in M. tuberculosis and M. bovis. After DNA was extracted from mycobacteria by using a simple, safe lysis procedure, we detected the 419-base-pair sequence in samples containing few mycobacteria. Preliminary data suggested that this technique could be applied to clinical specimens for early and specific diagnosis of tuberculosis.

Ig-binding bacterial proteins also bind proteinase inhibitors

Protein G is a streptococcal cell wall protein with separate binding sites for IgG and human serum albumin (HSA). In the present work it was demonstrated that alpha 2-macroglobulin (alpha 2M) and kininogen, two proteinase inhibitors of human plasma, bound to protein G, whereas 23 other human proteins showed no affinity. alpha 2M was found to interact with the IgG-binding domains of protein G, and in excess alpha 2M inhibited IgG binding and vice versa. A synthetic peptide, corresponding to one of the homologous IgG-binding domains of protein G, blocked binding of protein G to alpha 2M. Protein G showed affinity for both native and proteinase complexed alpha 2M but did not bind to the reduced form of alpha 2M, or to the C-terminal domain of the protein known to interact with alpha 2M receptors on macrophages. Binding of protein G to alpha 2M and kininogen did not interfere with their inhibitory activity on proteinases, and the interaction between protein G and the two proteinase inhibitors was not due to proteolytic activity of protein G. The finding that protein G has affinity for proteinase inhibitors was generalized to comprise also other Ig binding bacterial proteins. Thus, alpha 2M and kininogen, were shown to bind both protein A of Staphylococcus aureus and protein L of Peptococcus magnus. The results described above suggest that Ig-binding proteins are involved in proteolytic events, which adds a new and perhaps functional aspect to these molecules.

Ig-binding bacterial proteins also bind proteinase inhibitors

Protein G is a streptococcal cell wall protein with separate binding sites for IgG and human serum albumin (HSA). In the present work it was demonstrated that alpha 2-macroglobulin (alpha 2M) and kininogen, two proteinase inhibitors of human plasma, bound to protein G, whereas 23 other human proteins showed no affinity. alpha 2M was found to interact with the IgG-binding domains of protein G, and in excess alpha 2M inhibited IgG binding and vice versa. A synthetic peptide, corresponding to one of the homologous IgG-binding domains of protein G, blocked binding of protein G to alpha 2M. Protein G showed affinity for both native and proteinase complexed alpha 2M but did not bind to the reduced form of alpha 2M, or to the C-terminal domain of the protein known to interact with alpha 2M receptors on macrophages. Binding of protein G to alpha 2M and kininogen did not interfere with their inhibitory activity on proteinases, and the interaction between protein G and the two proteinase inhibitors was not due to proteolytic activity of protein G. The finding that protein G has affinity for proteinase inhibitors was generalized to comprise also other Ig binding bacterial proteins. Thus, alpha 2M and kininogen, were shown to bind both protein A of Staphylococcus aureus and protein L of Peptococcus magnus. The results described above suggest that Ig-binding proteins are involved in proteolytic events, which adds a new and perhaps functional aspect to these molecules.

The Fc binding site for streptococcal protein G is in the C gamma 2-C gamma 3 interface region of IgG and is related to the sites that bind staphylococcal protein A and human rheumatoid factors

The isolated 35 kDa fragment of protein G obtained by papain digestion of group G streptococci was found to bind solid phase intact IgG, Fc (2C gamma 2 + 2C gamma 3 domains), F(ab')2 and F(acb)2 (F(ab')2 + 2C gamma 2 domains) fragments but not pFc' (2C gamma 3 domains) fragments. The level of binding to rabbit F(acb)2 and rabbit F(ab')2 fragments was similar. Protein G binding to solid phase Fc fragments was inhibited by IgG, Fc, staphylococcal protein A and its monovalent fragment D, but was enhanced by F(ab')2 fragments. Chemical modification of tyrosine but not histidine residues of IgG abrogated its ability to inhibit the binding of protein G to solid phase Fc fragments. Protein G was found to strongly inhibit the binding of a monoclonal and a polyclonal human rheumatoid factor to IgG. These findings indicate that protein G binds with separate sites to the Fc and F(ab')2 fragments of IgG, that the interaction with the Fc fragment occurs at the C gamma 2-C gamma 3 domain interface region and that tyrosine but not histidine residues in this area are likely involved. The relationship of the Fc fragment-binding site specificity of protein G to that of other microbial IgG binding proteins and human rheumatoid factors is discussed.

The Fc binding site for streptococcal protein G is in the C gamma 2-C gamma 3 interface region of IgG and is related to the sites that bind staphylococcal protein A and human rheumatoid factors

The isolated 35 kDa fragment of protein G obtained by papain digestion of group G streptococci was found to bind solid phase intact IgG, Fc (2C gamma 2 + 2C gamma 3 domains), F(ab')2 and F(acb)2 (F(ab')2 + 2C gamma 2 domains) fragments but not pFc' (2C gamma 3 domains) fragments. The level of binding to rabbit F(acb)2 and rabbit F(ab')2 fragments was similar. Protein G binding to solid phase Fc fragments was inhibited by IgG, Fc, staphylococcal protein A and its monovalent fragment D, but was enhanced by F(ab')2 fragments. Chemical modification of tyrosine but not histidine residues of IgG abrogated its ability to inhibit the binding of protein G to solid phase Fc fragments. Protein G was found to strongly inhibit the binding of a monoclonal and a polyclonal human rheumatoid factor to IgG. These findings indicate that protein G binds with separate sites to the Fc and F(ab')2 fragments of IgG, that the interaction with the Fc fragment occurs at the C gamma 2-C gamma 3 domain interface region and that tyrosine but not histidine residues in this area are likely involved. The relationship of the Fc fragment-binding site specificity of protein G to that of other microbial IgG binding proteins and human rheumatoid factors is discussed.

Protein G genes: structure and distribution of IgG-binding and albumin-binding domains

Protein G (also designated Fc receptor type III) is the IgG-binding protein of group C and G streptococci. Protein G has also been shown to bind human serum albumin but at a site that is structurally separated from the IgG-binding region. From the known gene sequence of protein G, two synthetic oligonucleotides were constructed for use as probes in DNA-hybridization experiments to study the structure and distribution of the albumin- and IgG-binding regions in bacterial strains belonging to different species. Thus, one of the probes corresponded to repeats within the IgG-binding region (I) and the other corresponded to repeats in the albumin-binding encoding region (II). Probe I showed strong hybridization to DNA isolated from 31 human group C and G strains, whereas hybridization to probe II was variable. With the three restriction endonucleases used, three restriction patterns were found in Southern blot experiments. No fundamental difference could be detected in hybridization experiments, either between strains of group C and G streptococci, or between isolates of different clinical origin. No hybridization to DNA from other bacterial species was found.

Isolation and characterization of a 14-kDa albumin-binding fragment of streptococcal protein G

Protein G, a streptococcal cell wall protein, has separate binding sites for human albumin and IgG. Streptococci expressing protein G were treated with the bacteriolytic agent mutanolysin. Several IgG- and human serum albumin (HSA)-binding peptides were identified in the material thus solubilized and one of these, a 14-kDa peptide, was found to bind HSA but not IgG in Western blot experiments. This molecule was purified by affinity chromatography on Sepharose coupled with HSA followed by gel filtration on Sepharose 6B and a final affinity chromatography on IgG-Sepharose, by which low Mr W(15 to 20 kDa)IgG-binding peptides were removed. In different binding experiments the purified 14-kDa peptide bound exclusively HSA and the equilibrium constant between the peptide and HSA was determined to be 3.4 X 10(8) M-1. The relation between the 14-kDa molecule and protein G was studied by analyzing the N-terminal amino acid sequence of the peptide and comparing it with the previously determined protein G sequence. The 40 N-terminal amino acids were found to be identical with an amino acid sequence starting at position 62 in the protein G molecule. These and previous data enabled us to locate the albumin binding to the repetitively arranged domains in the N-terminal half of the protein G molecule.

Non-immune Fab- and Fc- mediated interactions of avian Ig with S. aureus and group C and G streptococci

Serum samples from 19 avian species representing 8 orders were tested for their capacity to inhibit the Fab- and Fc-mediated immunoglobulin binding to protein A-carrying S. aureus and protein G-carrying group C and G streptococci. Four species (mallard, dunlin, starling and blackbird) belonging to three different orders showed a high degree of Fc-mediated protein A- and protein G-reactivity. Five species demonstrated a high level and nine species exhibited a low level of Fab-mediated protein A-reactivity. The four species identified as Fc-reactive were capable of Fab-mediated immunoglobulin binding with streptococcal surface proteins but incapable of Fab-mediated protein A binding. SDS-PAGE analysis confirmed that the protein A-Sepharose affinity purified material contained proteins corresponding to immunoglobulin chains. Inhibition results by avian sera were confirmed by direct binding of protein A-reactive proteins to bacteria, by precipitation in gel and by Western blot analysis of binding to protein A and protein G, respectively.

Isolation and characterization of a 14-kDa albumin-binding fragment of streptococcal protein G

Protein G, a streptococcal cell wall protein, has separate binding sites for human albumin and IgG. Streptococci expressing protein G were treated with the bacteriolytic agent mutanolysin. Several IgG- and human serum albumin (HSA)-binding peptides were identified in the material thus solubilized and one of these, a 14-kDa peptide, was found to bind HSA but not IgG in Western blot experiments. This molecule was purified by affinity chromatography on Sepharose coupled with HSA followed by gel filtration on Sepharose 6B and a final affinity chromatography on IgG-Sepharose, by which low Mr W(15 to 20 kDa)IgG-binding peptides were removed. In different binding experiments the purified 14-kDa peptide bound exclusively HSA and the equilibrium constant between the peptide and HSA was determined to be 3.4 X 10(8) M-1. The relation between the 14-kDa molecule and protein G was studied by analyzing the N-terminal amino acid sequence of the peptide and comparing it with the previously determined protein G sequence. The 40 N-terminal amino acids were found to be identical with an amino acid sequence starting at position 62 in the protein G molecule. These and previous data enabled us to locate the albumin binding to the repetitively arranged domains in the N-terminal half of the protein G molecule.

Streptococcal protein G has affinity for both Fab- and Fc-fragments of human IgG

We have analyzed the binding of IgG and fragments of IgG [Fc, F(ab')2, and Fab] to group C and G streptococci and to protein G, the IgG binding cell wall protein of these bacteria. A direct correlation (r = 0.87, P less than 0.0001) was observed when the binding of radiolabelled, polyclonal IgG F(ab')2- and Fc-fragments to 23 group C and G streptococcal strains was compared. One strain (G-148) was treated with increasing amounts of pepsin, trypsin or papain and the Fab-binding structure was found to be much more sensitive to the enzymes as compared to the Fc-binding. A 35 K fragment of protein G was coupled to Sepharose, and both radiolabelled IgG F(ab')2- and Fc-fragments bound to the Sepharose beads. Binding of IgG fragments was inhibited by intact IgG or by the homologous IgG fragment, whereas Fc-fragments did not inhibit Fab binding or vice versa. Two radiolabelled protein G-fragments (28 and 35 K) showed different binding to polyclonal IgG, IgG F(ab')2-, IgG Fab- and IgG Fc-fragments. Thus, in a dot binding assay the 35 K fragment bound all IgG fragments tested, whereas the 28 K protein G fragment bound only intact IgG and IgG Fc-fragments. These results indicate two independent and separate binding sites for Fab- and Fc-fragments on protein G. Different binding sites on protein G were also indicated by Western blot analysis of four different protein G-fragments (28, 35, 42 and 65 K). In these experiments the 28 K fragment showed affinity only for Fc-fragments, while the higher mol. wt protein G preparations bound both IgG Fab- and Fc-fragments.