The group A streptococcal M-type 3 protein gene exhibits a C terminus typical for class I M proteins

Characterization of group A streptococcal strains Sv and Su: determination of emm gene typing and presence of small vir regulon

The emm gene typing of GAS (group A streptococcus) strains Sv and Su and the molecular structure of the vir regulon were decided. An emm(-like) gene from the chromosomal DNA of GAS strain Sv was amplified with forward and reverse primers, which were selected from the best conserved portion in leader sequences of different strains and the C-terminal conserved portion, respectively, for determination of the M protein gene type. Strain Sv was defined as serotype M23, because deduced N-terminal amino acid positions of the products are identical to those of the M type 23 (emml) gene derived from GAS strain M23-MEMPHIS (M serotype 23, GenBank accession number U11953). When the vir regulon of strain Sv was examined by polymerase chain reaction mapping and compared with that of GAS strain Su, they had a similar size in length. In addition, when sequencing analysis of the DNA fragment of 4791 base pairs (bp) encoding three open reading frames (orf, mga, and emm) and the upstream region of scpA from genomic DNAs of both strains was performed, the sequence of the DNA from strain Sv was, except for 1 bp (T for C at position 4124), identical to that of the DNA from strain Su. These data show that both strains possess the genes in the order of mga (virR or mry) -emm -scpA designated as the small vir regulon. The effect of the formation of alternative pathway C3 convertase of complement on the GAS strains Sv and Su was also examined. When GAS strains Sv and Su were incubated in NHS containing radiolabeled C3 in the presence of Mg-EGTA, binding of C3 to Su bacteria was dose-dependent, whereas less binding of C3 to Sv bacteria was seen. Taken together, the data suggest that M protein could be expressed on the surface of the Sv bacteria, but not on the Su bacteria.

Human antibodies to the conserved region of the M protein: opsonization of heterologous strains of group A streptococci

A 20-mer peptide (p145) in the carboxyl-terminal region of the M protein of group A streptococci (GAS) has previously been defined as the target of bactericidal antibodies. Sequence analysis of seven field isolates from indigenous Australians living in an area highly endemic for GAS and five laboratory reference strains (encompassing nine unique serotypes plus three nontypeables) demonstrates that this region is highly conserved (sequence identity ranging from 65 to 95%) with six of the 12 sequences being identical to p145. Most of the sequence dissimilarity is contained within the last seven amino acids of p145. Competitive ELISA demonstrates that human antibodies specific for p145 cannot discriminate between p145 and synthetic peptides representing four from four of the variant sequences tested. Ig purified from endemic sera was able to opsonize each of the GAS isolates and free p145 as well as a peptide expressing a minimal conformational epitope within p145 (requiring amino acids between positions 2 and 13 of p145), but not an irrelevant peptide, were able to partially or completely inhibit opsonization of all isolates and reference strains. Thus adult endemic sera contain antibodies which are bactericidal for multiple GAS serotypes and which are specific for a sequence of 12 amino acids contained within the p145 region of the M protein.

Purification and characterization of M3 protein expressed on the surface of group A streptococcal type 3 strain C203

Monoclonal antibodies (mAbs) have been produced by immunizing BALB/C mice with whole M+ bacteria in incomplete Freund adjuvant and the resulting mAbs for M3 protein have been selected by an indirect immuno-fluorescent technique using formaldehyde-fixed M+ and M- bacteria. Four mAbs reacted with a 65 kDa protein in an extract obtained from the cell wall of M+ bacteria after treatment with N-acetyl muramidase and lysozyme. The purified 65 kDa protein neutralized the phagocytic activity of rabbit anti-M3 antibody. The N-terminal amino acid sequence of the 65 kDa protein was identical with that of protein generated by the M3 gene which has been previously cloned and sequenced. The evidence indicates that the 65 kDa protein is M3 protein. The M3 protein bound not only human fibrinogen but also human serum albumin (HSA). When the M3 protein was purified by gel-filtration and ion-exchange chromatography in the absence of phenylmethyl sulfonyl fluoride (PMSF), four fragments (35 kDa, 32 kDa, 30 kDa, and 25 kDa) in addition to the intact molecule appeared. N-terminal amino acid sequence analysis showed that 35 kDa and 25 kDa fragments were ANAAD and DARSV, respectively, being identical at positions 1-5 and 198-202 to the M3 gene derived protein. Therefore, the 35 kDa and 25 kDa fragments, which were presumed to be cleavage products, may be derived from the C-terminal part and N-terminal part of the intact molecule, respectively. When the effect of purified M3 protein in the bactericidal activity of normal human blood in the presence of M- bacteria was investigated, the M3 protein was responsible for the organism's resistance to attack by phagocytic cells.

Multiple binding of type 3 streptococcal M protein to human fibrinogen, albumin and fibronectin

M proteins are major virulence factors of group A streptococci which enable the bacteria to resist phagocytic attack. Their binding capacity for different plasma proteins seems to be one reason for the antiphagocytic activity of M protein. In the present study we demonstrate that M3 protein, isolated from the streptococcal culture supernatant of strain 4/55, and the recombinant form (rM3), purified from an E. coli lysate after cloning in phage lambda-EMBL3, show a multiple binding to fibrinogen, albumin and fibronectin in Western blot and dot binding assays. Binding of M3 protein to the multifunctional extracellular matrix and plasma protein fibronectin may not only influence phagocytosis but may also contribute to the adherence of these bacteria to endothelial and epithelial cells.