Structural and Functional Analysis of Cell Wall-anchored Polypeptide Adhesin BspA in Streptococcus agalactiae

Abstract

Streptococcus agalactiae (group B Streptococcus, GBS) is the predominant cause of early-onset infectious disease in neonates and is responsible for life-threatening infections in elderly and immunocompromised individuals. Clinical manifestations of GBS infection include sepsis, pneumonia, and meningitis. Here, we describe BspA, a deviant antigen I/II family polypeptide that confers adhesive properties linked to pathogenesis in GBS. Heterologous expression of BspA on the surface of the non-adherent bacterium Lactococcus lactis confers adherence to scavenger receptor gp340, human vaginal epithelium, and to the fungus Candida albicans. Complementary crystallographic and biophysical characterization of BspA reveal a novel ␤-sandwich adhesion domain and unique asparagine-dependent super-helical stalk. Collectively, these findings establish a new bacterial adhesin structure that has in effect been hijacked by a pathogenic Streptococcus species to provide competitive advantage in human mucosal infections. Streptococcus agalactiae (group B Streptococcus (GBS)4 ) is a commensal of the gastrointestinal tract and part of the normal microbiota of the female rectovaginal tract, where it is carried asymptomatically by ϳ10 -40% of women of childbearing age (1). However, as an opportunistic pathogen, GBS is the leading cause of neonatal meningitis and sepsis in the developed world. The predominant route by which GBS is transmitted to infants is from the mother, either during birth or following breach of the placental barrier in utero. Antenatal GBS screening strategies are therefore commonly used to identify colonized women, who then receive antimicrobial prophylaxis. Nevertheless, GBS remains a major cause of morbidity and mortality in infants worldwide (2).Because maternal GBS colonization is the primary risk factor for vertical transmission of neonatal infection (3), there has been considerable focus on the mechanisms underlying GBS colonization of the female genitourinary (GU) tract. A number of putative colonization determinants have been identified, including the following: ␣C protein, mediating entry of GBS into cervical epithelial cells (4); pili, shown to contribute to vaginal attachment (5); and Srr-1, which binds keratin-4 (5) and fibrinogen (6) on the surface of vaginal epithelium. In addition, GBS expresses numerous extracellular matrix-binding proteins, including C5a peptidase (ScpB) and FbsA, which may promote attachment to mucosal tissues of the GU tract (7).Antigen I/II (AgI/II) family polypeptide adhesins are found widely across the Streptococcus genus and have been best characterized for those streptococci indigenous to the oral cavity (8). In silico analyses have recently revealed the presence of genes encoding AgI/II family polypeptides in GBS, which we have designated here group B Streptococcus surface proteins (Bsp). These proteins conform to a conserved primary structure consisting of seven distinct regions (Fig. 1). The N-terminal region comprises a signal (leader) peptide, an N-terminal domain, and an ...

The Group B Streptococcal surface antigen I/II protein, BspC, interacts with host vimentin to promote adherence to brain endothelium and inflammation during the pathogenesis of meningitis

“…The proposed domain organization of streptococcal AgI/II polypeptides comprises a stalk consisting of the α-helical A (alanine-rich repeats) domain and the polyproline II (PPII) helical P domain, separating the V (variable) domain and the C-terminal domain, which contains the LP X TG motif required for cell wall anchorage (14). While the GBS BspC structure is not known, the structure of several regions of the GBS homolog, BspA, has been solved (15). We generated a hypothetical model of full length BspC using PyMOL (The PyMOL Molecular Graphics System, Version 2.1 Schrödinger, LLC) for the purpose of showing the overall domain structure (Fig.…”

“…These proteins mediate attachment of Streptococcus mutans and Streptococcus gordonii to tooth surfaces and can stimulate an immune response from the colonized host (14). Genes encoding AgI/II polypeptides are found in streptococcal species indigenous to the human mouth as well as other pathogenic streptococci such as GBS, S. pyogenes (Group A Streptococcus , GAS), and S. suis (14, 15). Intriguingly, the GAS AgI/II protein AspA (Group A Streptococcus s urface p rotein) is absent in many GAS M serotypes and is found predominantly among M serotypes implicated in puerperal sepsis and neonatal infections, including M2, M4, and M28.…”

Enterococcus adhesin PrgB facilitates type IV secretion by condensation of extracellular DNA

“…Combining results obtained from modelling experiments and crystal structure analysis, it is proposed that PrgB most likely shows an overall head-stalk architecture. This stalk separates the globular variable head (V-domain) from the C-terminal region and is proposed to project the V-domain away from the cell surface (Larson et al, 2011;Rego et al, 2016). By structural modelling, Schmitt et al could demonstrate that the C-proximal PrgB part resembles the corresponding regions of streptococcal proteins SpaP and SspB (AgI/II antigen family) (Brady et al, 2010;Schmitt et al, 2018).…”

“…Combining results obtained from modelling experiments and crystal structure analysis, it is proposed that PrgB most likely shows an overall head‐stalk architecture. This stalk separates the globular variable head (V‐domain) from the C‐terminal region and is proposed to project the V‐domain away from the cell surface (Larson et al, ; Rego et al, ). By structural modelling, Schmitt et al .…”

The Streptococcus agalactiae complement interfering protein combines multiple complement‐inhibitory mechanisms by interacting with both C4 and C3 ligands

“…In particular, phage insertions generate interstrain diversity and provide the pathogen with a number of virulence factors that facilitate its survival in the host (37). Of note, the gene coding for CIP is located in a hot spot phage insertion region of ∼20 kbp (15), which also contains the bspC locus implicated in biofilm formation (38, 39). Interestingly, group A Streptococcus isolates that belong to serotypes associated with maternal–fetal urogenital infections contain a similar phage insertion that also encodes a secreted protein 46% identical to CIP (15), suggesting interspecies lateral transfer.…”