Genetics, Structure, and Function of Group A Streptococcal Pili

Abstract

Streptococcus pyogenes (Group A Streptococcus; GAS) is an exclusively human pathogen. This bacterial species is responsible for a large variety of infections, ranging from purulent but mostly self-limiting oropharynx/skin diseases to streptococcal sequelae, including glomerulonephritis and rheumatic fever, as well as life-threatening streptococcal toxic-shock syndrome. GAS displays a wide array of surface proteins, with antigenicity of the M protein and pili utilized for M- and T-serotyping, respectively. Since the discovery of GAS pili in 2005, their genetic features, including regulation of expression, and structural features, including assembly mechanisms and protein conformation, as well as their functional role in GAS pathogenesis have been intensively examined. Moreover, their potential as vaccine antigens has been studied in detail. Pilus biogenesis-related genes are located in a discrete section of the GAS genome encoding fibronectin and collagen binding proteins and trypsin-resistant antigens (FCT region). Based on the heterogeneity of genetic composition and DNA sequences, this region is currently classified into nine distinguishable forms. Pili and fibronectin-binding proteins encoded in the FCT region are known to be correlated with infection sites, such as the skin and throat, possibly contributing to tissue tropism. As also found for pili of other Gram-positive bacterial pathogens, GAS pilin proteins polymerize via isopeptide bonds, while intramolecular isopeptide bonds present in the pilin provide increased resistance to degradation by proteases. As supported by findings showing that the main subunit is primarily responsible for T-serotyping antigenicity, pilus functions and gene expression modes are divergent. GAS pili serve as adhesins for tonsillar tissues and keratinocyte cell lines. Of note, a minor subunit is considered to have a harpoon function by which covalent thioester bonds with host ligands are formed. Additionally, GAS pili participate in biofilm formation and evasion of the immune system in a serotype/strain-specific manner. These multiple functions highlight crucial roles of pili during the onset of GAS infection. This review summarizes the current state of the art regarding GAS pili, including a new mode of host-GAS interaction mediated by pili, along with insights into pilus expression in terms of tissue tropism.

Keywords: FCT region; Streptococcus pyogenes; T serotyping; pili; thermoregulation.

FIGURE 1

Heterogeneic organization of FCT region. Gene content heterogeneity for the seven FCT forms is shown on the basis of genome sequences and previously reported data (Kratovac et al., 2007; Falugi et al., 2008). Representative M types for each FCT form are also shown. Pilus major and minor subunit (ancillary proteins 1 and 2; AP1 and AP2) genes are shown in pink and blue, respectively. SipA/LepA and sortase genes are colored orange and purple, respectively. Fibronectin-binding protein genes, including prtF2 family genes (pfbp and fbaB) and prtF1, are shown in green, while transcriptional regulator genes, including rofA, nra, and msmR, are shown in light green. Other genes are light gray in color. Deposited DNA sequences of strains (M type, accession number) used for each FCT form are as follows: form 1, MGAS10394 (M6, NC_006086); form 2, SF370 (M1, NC_002737); form 3, SSI-1 (M3, NC_004606); form 4, A735 (M12, AF447492); form 5, MGAS10750 (M4, NC_008024); form 6, MGAS10270 (M2, NC_008022); and form 9, STAB14018 (M75, CCP014542.1). Genome sequences for FCT forms 7 and 8 are not available.

FIGURE 2

Pilus components and related factors of pathogenic streptococci. Pilus major (backbone pilin) and minor (tip and base pilin) subunits of Streptococcus pyogenes (GAS), Streptococcus agalactiae (GBS), and Streptococcus pneumoniae are depicted. GBS pilus genes are located in three pilus island (PI) types, while S. pneumoniae pilus genes are located in two types of pilus islets (PIs). LPXTG or an LPXTG-like motif is shown under each pilin. Related pilus-specific sortases and their class, i.e., sortase class B or C, are also shown. Note that GAS SrtB and SrtC1/2 belongs to class C and B, respectively. A requirement of SipA/LepA for pilus assembly is also shown by “+.” Pilin subunits of FCT forms 5 and 6 are presented as gene tag numbers of MGANC_008024S10750 (serotype M4, genome accession number NC_008024) and MGAS10270 (serotype M2, genome accession number NC_008022), respectively. Subunits of GBS pili are also shown as tag numbers in 2603V/R (serotype V, genome accession number NC_004116.1) and COH1 (serotype III, genome accession number HG939456).

FIGURE 3

Proposed model for thermoregulation of pilus production from FCT form 3 strains. FCT form 3 nra-positive Streptococcus pyogenes produces pili in a temperature-dependent manner. The underlying mechanism includes post-transcriptional control of nra mRNA translation via a putative stem loop structure in the protein coding region of nra mRNA. The putative stem loop structure most likely functions as a thermometer to modulate the translational efficiency of nra mRNA by potential interactions with the translation initiation complex. Thermosensitive modulation of pilus production highlights the importance of pili in an initial infection phase and involvement of pili in bacterial fitness in the host.