An IgG-like Domain in the Minor Pilin GBS52 of Streptococcus agalactiae Mediates Lung Epithelial Cell Adhesion

Abstract

Streptococcus agalactiae is the leading cause of neonatal pneumonia, sepsis, and meningitis. The pathogen assembles heterotrimeric pilus structures on its surface; however, their function in pathogenesis is poorly understood. We report here the crystal structure of the pilin GBS52, which reveals two IgG-like fold domains, N1 and N2. Each domain is comprised of seven antiparallel beta strands, an arrangement similar to the fold observed in the Staphylococcus aureus adhesin Cna. Consistent with its role as an adhesin, deletion of gbs52 gene significantly reduces bacterial adherence to pulmonary epithelial cells. Moreover, latex beads linked to the GBS52 protein adhere to pulmonary but not to many other epithelial cells; binding to the former is specifically inhibited by antibodies against GBS52. Nonetheless, substantial binding is only observed with N2 domain-conjugated beads. This study presents the structure of a Gram-positive pilin that utilizes a distinct IgG fold variant to mediate pathogen adherence to a specific tissue.

The Corynebacterium diphtheriae shaft pilin SpaA is built of tandem Ig-like modules with stabilizing isopeptide and disulfide bonds

“…The closest structural homologues of the N-and C-domains of SpaA are the 2 CnaB-type domains of the Streptococcus agalactiae minor pilin GBS52 (15). In particular, the SpaA C-domain has significant sequence identity (25%) and structural similarity (rmsd 1.7 Å over 91 equivalent C␣ atoms) with the N2 domain of GBS52.…”

“…GBS52, a minor pilin from S. agalactiae that seems to correspond functionally to SpaB, comprises 2 CnaB-type domains (15), and its C-terminal N2 domain closely resembles the SpaA C-domain, including the internal isopeptide bond. Sequence comparisons show that the S. pyogenes minor pilin Cpa also has a C-terminal domain homologous with the C-domain of the S. pyogenes major pilin Spy0128, again including an internal isopeptide bond (5).…”

“…Defective sortase recognition or failure to present the Lys residue of the pilin motif appropriately would explain the loss of pilus assembly when intramolecular isopeptide bonds in SpaA or BcpA are deleted by mutation of the catalytic Asp/Glu. The isopeptide bonds in CnaA and GBS52 may similarly influence the binding of partner molecules, because their isopeptide bond-containing domains are important for specific binding of collagen (CnaA) and capable of binding to human pulmonary epithelial cells (GBS52) (15,29).…”

Intramolecular Isopeptide Bonds Give Thermodynamic and Proteolytic Stability to the Major Pilin Protein of Streptococcus pyogenes

“…Further investigations have shown that similar isopeptide bonds can be found as internal cross-links in other proteins. Examination of previously determined structures of a minor pilin GBS52 from Streptococcus agalactiae (13) and the CnaA and CnaB domains of a collagen-binding adhesin from Staphylococcus aureus (14 -16) revealed constellations of Lys-AsnGlu/Asp residues similar to those that form the internal crosslinks in Spy0128, and examination of the electron density confirmed their probable presence in those cases where the x-ray data were available (3). Sequence comparisons showed that similar domains, with corresponding Lys-Asn-Glu/Asp residues, are present in many cell surface proteins of Grampositive bacteria.…”

Structural vaccinology to thwart antigenic variation in microbial pathogens

“…By solving the crystallographic structure of BP2a (from GBS strain 515), Nuccitelli et al (3) discern four Ig-like domains (D1-D2-D3-D4), which could be anticipated to function as independent folding units (17,18). Individual domains are tested as subunit vaccines; however, only D3 elicits protection similar to full-length BP2a (3).…”