Identification of two functional forms of immunoglobulin G3-binding protein expressed by group A streptococci

Two-domain motif for IgG-binding activity by group A streptococcal emm gene products

A biological role for the non-immune binding of human IgG by group A streptococci is evidenced by its strong association with a subpopulation of strains giving rise to tissue-specific infection. IgG-binding activity lies within many of the M and M-like surface proteins (encoded by emm genes), and several structurally distinct IgG-binding sites are known to exist. In this report, two adjacent IgG-binding domains, differing in their specificity for human IgG subclasses, are localized within the M-like protein, protein H. The putative coding regions for the two IgG-binding domains were mapped for 82 epidemiologically unrelated strains. Both coding regions are associated with phylogenetically distant emm genes, supporting a role for horizontal transfer and intergenomic recombination in the evolution of emm genes. In most instances, the two coding regions are tightly linked, suggesting that there exist strong selective pressures to maintain a two-domain binding motif. Both coding regions are found among all strains bearing emm gene markers associated with impetigo lesions as the principal tissue reservoir, but are absent from most strains that exhibit markers for a predominant nasopharyngeal reservoir. The data support the hypothesis that the pathogenic potential of an isolate is dictated, at least in part, by its unique array of multifunctional emm gene products.

Identification of an amino acid signature sequence predictive of protein G-inhibitable IgG3-binding activity in group-A streptococcal IgG-binding proteins

Sequence comparison of six known group-A streptococcal IgG-binding proteins, sharing the common property of protein G-inhibitable IgG3-binding-activity, identified a highly conserved 35-amino-acid (aa) sequence (74-100% similarity) within an EQ-rich central conserved core region of each protein. A search of aa sequence databases identified four additional proteins with > 50% similarity to this consensus sequence. All of these proteins demonstrated protein G-inhibitable IgG3-binding activity. Taken together, these results identify a signature sequence that predicts the presence of a protein G-inhibitable IgG3-binding domain(s) in group-A streptococcal IgG-binding proteins.

Characterization of a type II'o group A streptococcal immunoglobulin-binding protein

The opacity factor positive M type 2 group A streptococcal isolate, A207, expresses a unique functional type II'o IgG-binding protein which reacts with all four human IgG subclasses and rabbit IgG. In order to determine the gene product or products responsible for this activity, three genes of the vir regulon from this isolate were cloned, expressed and analysed. The fcr A2 gene coded for a protein binding hyman IgG1, IgG2 and IgG4 but not IgG3. The enn2 gene coded for a protein reacting exclusively with human IgA, while the emmL2 gene product bound IgG1, IgG2, IgG3 and IgG4 as well as rabbit but not horse or pig IgG. The IgG3-binding activity of the EmmL2 protein was functionally indistinguishable from the Form 1 IgG3-binding activity present in heat extracts of group A isolate A207.

Distinct profiles of immunoglobulin G-binding-protein expression by invasive serotype M1 isolates of Streptococcus pyogenes

Analysis of immunoglobulin G (IgG)-binding-protein expression by invasive group A streptococcal isolates of the M1 serotype collected as part of a Centers for Disease Control and Prevention surveillance study revealed two distinct phenotypes. One group of type M1 isolates expressed a surface protein reactive with all four human IgG subclasses (type IIo), while a second group expressed a surface protein demonstrating significant reactivity only with human IgG3 (type IIb). The functional forms of IgG-binding protein were antigenically related, and both were recognized by a rabbit polyclonal antiserum to serotype M1 but not by normal rabbit serum. While the quantities of antigenic M1 protein present in the extracts of representative isolates displaying each phenotype differed, the functional differences were found to be qualitative and not solely quantitative. The IgG-binding properties of these antigenically related M1 proteins could be readily distinguished from those of another IgG-binding protein, protein H. Type M1 isolates of the IIb phenotype differed from those of the IIo phenotype by secreting larger amounts of a casein-hydrolyzing protease into culture supernatants.