Primary structure of streptococcal Pep M5 protein: Absence of extensive sequence repeats

Streptococcal M protein: molecular design and biological behavior

M protein is a major virulence determinant for the group A streptococcus by virtue of its ability to allow the organism to resist phagocytosis. Common in eucaryotes, the fibrillar coiled-coil design for the M molecule may prove to be a common motif for surface proteins in gram-positive organisms. This type of structure offers the organism several distinct advantages, ranging from antigenic variation to multiple functional domains. The close resemblance of this molecular design to that of certain mammalian proteins could help explain on a molecular level the formation of epitopes responsible for serological cross-reactions between microbial and mammalian proteins. Many of the approaches described in the elucidation of the M-protein structure may be applied for characterizing similar molecules in other microbial systems.

Type 1 and 3 M-proteins of Streptococcus pyogenes: peptic extraction and fibrinogen binding properties

The pepsin extraction of group A type 1 streptococci for the isolation of M protein fragments was studied at different pH values and at different time intervals. The extracts were compared by SDS PAGE and fused rocket immunoelectrophoresis. Type 1 M protein fragments were prepared in preparative scale by pepsin extraction of type 1 streptococci at pH 5.5 for 60 min. The fragments were separated by affinity chromatography on immobilized fibrinogen and finally purified for sequence studies by gel chromatography. Pepsin extraction of group A type 3 streptococci was also studied at different pH values. In contrast to type 1, the SDS PAGE pattern changed drastically in dependence on the pH. Affinity chromatography on immobilized fibrinogen is also effective in the separation of the pH 5.5 type 3 streptococcal pepsin extract.

Difference in the structural features of streptococcal M proteins from nephritogenic and rheumatogenic serotypes

The association of only certain M protein serotypes of group A streptococci with acute glomerulonephritis is very well recognized. Structural information on the M protein, a dimeric alpha-helical coiled-coil molecule, has come so far from three rheumatogenic serotypes, 5, 6, and 24. However, M proteins from the nephritogenic serotypes have not been well characterized. In the present study, we have isolated a biologically active 20,000 Mr pepsin fragment of type 49 M protein (PepM49), a nephritogenic serotype, and purified it to homogeneity using DEAE-Sephadex and gel filtration. The amino acid composition of PepM49 is similar to those of the rheumatogenic M protein serotypes PepM5, PepM6, and PepM24. However, the sequence of the NH2-terminal 60 residues of PepM49 shows little homology to any of these M protein serotypes, although the latter have significant homology among themselves. Nevertheless, PepM49 exhibits a strong heptad periodicity in its nonpolar residues, suggesting its overall conformational similarity with the other M molecules. During the course of the present studies, Moravek et al. (17) reported the NH2-terminal sequence of another M protein serotype, PepM1, which also does not exhibit much homology with the PepM5, PepM6, and PepM24 proteins. Our analysis of this sequence revealed that the PepM1 protein also exhibits a heptad periodicity of the nonpolar amino acids. A closer examination has revealed that the pattern of heptad periodicity in PepM49 and PepM1 proteins is more regular and more similar to each other than has been previously seen for the PepM5, PepM6, and PepM24 proteins. PepM1 is also a nephritogenic serotype. Taken together, these findings indicate an underlying conservation of the tertiary structure of the various M protein serotypes, despite the complexity in their antigenic variation and suggest that the nephritogenic M protein serotypes M1 and M49 may be further apart evolutionarily from the rheumatogenic serotypes 5, 6, and 24. The distinct differences in the structural features of the PepM1 and PepM49 proteins relative to the PepM5, PepM6, and PepM24 proteins are also suggestive of a correlation with the earlier broader classification of the group A streptococci into rheumatogenic and nephritogenic serotypes.

Streptococcal diseases worldwide: present status and prospects

Infections caused by streptococci pathogenic for man are some of the most common bacterial diseases in temperate zones and occur very frequently in tropical and subtropical countries. The highest morbidity occurs from infections caused by group A streptococci; these infections can lead to rheumatic fever and acute glomerulonephritis. The incidence of rheumatic fever and the prevalence of rheumatic heart disease are several times higher in tropical countries than temperate countries.Recent developments in fundamental and applied research are throwing light on various aspects of the problem, e.g., the rapid (non-culture) identification of group A streptococcal infection. Analyses of the chemical structure of the M-protein molecule of group A streptococcus and of the biological properties of the epitopes of the M-protein have provided encouraging results. Furthermore, synthetic analogues of the protective immunodominant polypeptides of the M-protein have been prepared. The prospect of a streptococcal vaccine for preventing group A streptococcal diseases is thus more realistic.The control of infections caused by group B streptococci is important for the health of neonates. The identification of the chemical structure of the major group B streptococcal types may lead to development of a vaccine in the future. An alternative approach would entail the use of anti-group-B immunoglobulins, but a number of questions have to be answered before the new control measures can be introduced. The streptococci causing bacterial pneumonia, subacute bacterial endocarditis and possibly dental caries have been widely studied and promising advances have been made towards the introduction of better control of the diseases caused by these pathogens.

Antigenic domains of the streptococcal Pep M5 protein. Localization of epitopes crossreactive with type 6 M protein and identification of a hypervariable region of the M molecule

Pep M5, the pepsin-derived N-terminal half of the group A streptococcal type 5 M protein exhibits immunologic crossreaction with type 6 M protein, localizing some of the M6-crossreactive epitope(s) within this segment of the M5 protein. Based on the amino acid sequence of the Pep M5 protein, two structurally distinct domains have been recognized within its coiled-coil structure. We have now found that peptides derived from both the structurally distinct domains of the Pep M5 protein contain antigenic epitopes. Furthermore, only the peptides from the C-terminal domain of the Pep M5 protein crossreacted with rabbit anti-M6 sera, whereas those from the N-terminal domain did not. Consistent with this, sequence analyses of the arginyl peptides of the Pep M6 protein, the pepsin-derived N-terminal half of the M6 protein, revealed extensive homology of some of these peptides with regions within the C-terminal domain of the Pep M5 molecule. While an arginyl peptide of the Pep M6 protein exhibits 84% homology with region 150-186 of the Pep M5 protein, the C-terminal hexadecapeptide of the Pep M6 protein is virtually identical with the corresponding region of the Pep M5 protein. These results are suggestive of conformational similarities in the region around the pepsin-susceptible site within the M5 and M6 proteins. In addition, one or more epitopes of the M5 protein that are crossreactive with the M6 protein may be placed close to the pepsin-susceptible site of the M5 protein. Previous studies have suggested the N-terminal half of the M proteins to be the variable part of the molecule among the different M protein serotypes. The present results suggest that the N-terminal quarter of the M protein may represent the hypervariable domain of the M molecule.

Epitopes of streptococcal M proteins shared with cardiac myosin

We present evidence that M proteins from three different serotypes of group A streptococci share epitopes with cardiac myosin. Rabbit antisera evoked by a purified fragment of type 5 M protein crossreacted with myosin, but not alpha-tropomyosin, actin, or myosin light chains. In enzyme-linked immunosorbent assays, the myosin-crossreactive antibodies were totally inhibited by type 5 M protein and partially inhibited by types 6 and 19 M proteins. The affinity-purified myosin antibodies opsonized type 5 streptococci, indicating that they were directed against protective M protein epitopes on the surface of the organisms. Immunoblot analyses demonstrated the binding of the crossreactive antibodies to myosin heavy chains. Sera from patients with acute rheumatic fever showed significantly stronger reactions with myosin than did sera from their siblings, hospitalized controls, or patients with poststreptococcal glomerulonephritis.

Tropomyosin lysine reactivities and relationship to coiled-coil structure

We have carried out a detailed analysis of tropomyosin structure using lysines as specific probes for the protein surface in regions of the molecule that have not been investigated by other methods. We have measured the relative reactivities of lysines in rabbit skeletal muscle alpha, alpha-tropomyosin with acetic anhydride using a competitive labeling procedure. We have identified 37 of 39 lysines and find that they range 20-fold in reactivity. The observed reactivities are related to the coiled-coil model of the tropomyosin molecule [Crick, F.H.C. (1953) Acta Crystallogr. 6, 689-697; McLachlan, A.D., Stewart, M., & Smillie, L.B. (1975) J. Mol. Biol. 98, 281-291] and other available chemical and physical information about the structure. In most cases, the observed lysine reactivities can be explained by allowable interactions with neighboring amino acid side chains on the same or facing alpha-helix. However, we found no correlation between reactivity and helical position of a given lysine. For example, lysines in the outer helical positions included lysines of low as well as high reactivity, indicating that they vary widely in their accessibility to solvent and that the coiled coil is heterogeneous along its length. Furthermore, the middle of the molecule (residues 126-182) that is susceptible to proteolysis and known to be the least stable region of the protein also contains some of the least and most reactive lysines. We have discussed the implications of our results on our understanding the structures of tropomyosin and other coiled-coil proteins as well as globular proteins containing helical regions.

Presence of two distinct regions in the coiled-coil structure of the streptococcal Pep M5 protein: relationship to mammalian coiled-coil proteins and implications to its biological properties

The complete amino acid sequence of Pep M5, a biologically active 197-residue fragment comprising nearly half of the group A streptococcal M5 protein, has structural features characteristic of an alpha-helical coiled-coil protein. Fourier analyses of the nonpolar residues show strong periodicities based on repeats of 7 residues (7/2 and 7/3). Except for the nonhelical NH2-terminal 12-residue segment, the 7-residue periodicity in the distribution of nonpolar residues extends through the remainder of the Pep M5 molecule, with some discontinuities and irregularities. The molecule contains two distinct regions that differ in the pattern of distribution of the nonpolar and charged residues. The 7-residue pattern "a, b, c, d, e, f, g" in region 13-121 is atypical in that position "a" is predominantly occupied by asparagine, rather than nonpolar residues. On the other hand, the periodicity in region 122-196 is more typical of that found in other coiled-coil proteins, such as the myosin rod region, keratin, desmin, and vimentin, rather than tropomyosin. Although the periodicity in nonpolar residues is not highly regular, the predominance of basic and acidic residues in the inner "e" and "g" positions, respectively, suggests that ionic interactions between chains may contribute significantly to the stability of the coiled-coil. The distribution of charged residues in the outer positions within the two regions of the molecule is also distinct. The NH2-terminal region carries a significantly higher net negative charge than the COOH-terminal region, suggesting that the former region may play an important role in some of the biological functions of the Pep M5 molecule.

Multiple, heart-cross-reactive epitopes of streptococcal M proteins

We present evidence that a highly purified pepsin extract of type 5 streptococcal M protein (pep M5) contains at least three epitopes that are cross-reactive with sarcolemmal membrane proteins of human myocardium. The tissue-cross-reactive determinants of pep M5 are also partially shared with pep M6 and pep M19. Three rabbits immunized with a single 300 micrograms dose of pep M5 developed significant levels of heart-cross-reactive antibodies, as determined by indirect immunofluorescence tests. All three sera also contained antibodies that cross-reacted with pep M6 and pep M19. The heart tissue--specific antibodies that were eluted from sarcolemmal membranes opsonized types 5, 6, and 19 streptococci, indicating that they were directed against protective M protein epitopes on the surface of virulent organisms. Immunofluorescence inhibition tests, using purified M proteins as soluble inhibitors of heart-cross-reactive antibodies, revealed the number and M protein serotype distribution of the tissue-cross-reactive epitopes. Immunoblot analyses demonstrated the sarcolemmal membrane proteins containing the various cross-reactive antigenic determinants.

Unique and common protective epitopes among different serotypes of group A streptococcal M proteins defined with hybridoma antibodies

A set of four monoclonal antibodies was produced against a highly purified pepsin extract of type 5 streptococcal M protein. Three of the four antibodies cross-reacted with purified M proteins from heterologous serotypes and opsonized the respective heterologous organisms. Our studies suggest that monoclonal antibodies may be useful in identifying subpeptides of various M proteins containing common, protective epitopes that are capable of evoking antibodies that would protect against several different potentially "rheumatogenic" serotypes.

Application of reductive dihydroxypropylation of amino groups of proteins in primary structural studies: identification of phenylthiohydantoin derivative of epsilon-dihydroxypropyl-lysine residues by high-performance liquid chromatography

The general utility of reductive alkylation of amino groups of proteins with glyceraldehyde (2,3-dihydroxypropionaldehyde) in the presence of sodium cyanoborohydride, i.e. dihydroxypropylation, as an aid in generating arginine peptides of proteins by tryptic digestion has been investigated. The dihydroxypropylation of the amino groups of ribonuclease A and the streptococcal Pep M5 protein proceeds predominantly to the stage of monoalkylation. The derivatized lysine namely, epsilon-dihydroxypropyl-lysine is stable to acid hydrolysis, and is eluted slightly ahead of histidine in the amino acid analyzer. The peptide bonds of epsilon-dihydroxypropyl-lysine residues are resistant to tryptic digestion. The arginine peptides of dihydroxypropylated ribonuclease A, and dihydroxypropylated streptococcal Pep M5 protein have been isolated by reversed-phase high-performance liquid chromatography (HPLC) of the tryptic digest of the derivatized proteins. The phenylthiohydantoin (PTH) derivative of epsilon-dihydroxypropyl-lysine has been prepared. It is eluted at a position intermediate to that of the PTH derivatives of proline and tryptophan in reversed-phase HPLC on DuPont Zorbax ODS columns. Thus the PTH-epsilon-dihydroxypropyl-lysine could be identified during the sequence studies of the dihydroxypropylated peptides. The presence of dihydroxypropyl groups on the epsilon-amino groups of lysine residues in the dihydroxypropylated peptides does not interfere with the Edman degradation studies. The ease of the dihydroxypropylation reaction, the resistance of the peptide bonds of epsilon-dihydroxypropyl-lysine residues to trypsin, and the identification of the PTH derivative of epsilon-dihydroxypropyl-lysine residues by reversed-phase HPLC makes the dihydroxypropylation procedure a valuable addition to the arsenal of procedures for limiting the tryptic digestion to the arginine residues of proteins and peptides.

Streptococcal M6 protein expressed in Escherichia coli. Localization, purification, and comparison with streptococcal-derived M protein

Type 6 streptococcal M protein produced by E. coli bearing plasmid pJRS42.13 (ColiM6) accumulates in the periplasmic space of this new host. No immunoreactive M protein was found either on the surface of the organism or in the culture medium. The ColiM6 protein was purified from the periplasm and the final preparation consisted of three protein bands of apparent molecular weight 55,000, 57,000, and 59,000. These three bands were identical in migration in SDS PAGE to that of the M protein present in freshly prepared crude periplasm. The amino acid composition of the ColiM6 protein was nearly identical to that of M protein isolated from streptococci with phage lysin (LysM6). Furthermore, except for the amino terminal residue of the LysM6 molecule, the amino terminal sequence of the ColiM6 molecule was identical to those of both LysM6 and M protein released from the streptococcus by limited peptic digestion (PepM6). These results reveal that the molecule produced in the E. coli and transported into the periplasm may be the complete M protein as it exists on the streptococcus. The results also indicate that the systems that process M protein for transport through the cytoplasmic membrane are similar in the streptococcus and E. coli. The purified ColiM6 protein was able to remove opsonic antibodies from both human and rabbit serum, as well as to stimulate the production of opsonic antibodies in rabbits, indicating that the immunodeterminants on this molecule are the same as those found on streptococcal-derived M molecules.