Pneumococcal Pili Are Composed of Protofilaments Exposing Adhesive Clusters of Rrg A

Abstract

Pili have been identified on the cell surface of Streptococcus pneumoniae, a major cause of morbidity and mortality worldwide. In contrast to Gram-negative bacteria, little is known about the structure of native pili in Gram-positive species and their role in pathogenicity. Triple immunoelectron microscopy of the elongated structure showed that purified pili contained RrgB as the major compound, followed by clustered RrgA and individual RrgC molecules on the pilus surface. The arrangement of gold particles displayed a uniform distribution of anti-RrgB antibodies along the whole pilus, forming a backbone structure. Antibodies against RrgA were found along the filament as particulate aggregates of 2–3 units, often co-localised with single RrgC subunits. Structural analysis using cryo electron microscopy and data obtained from freeze drying/metal shadowing technique showed that pili are oligomeric appendages formed by at least two protofilaments arranged in a coiled-coil, compact superstructure of various diameters. Using extracellular matrix proteins in an enzyme-linked immunosorbent assay, ancillary RrgA was identified as the major adhesin of the pilus. Combining the structural and functional data, a model emerges where the pilus RrgB backbone serves as a carrier for surface located adhesive clusters of RrgA that facilitates the interaction with the host.

Toll-like Receptor (TLR) 2 Mediates Inflammatory Responses to Oligomerized RrgA Pneumococcal Pilus Type 1 Protein

“…Our data using whole bacteria as well as purified pilus components further implicate a specific protein, RrgA, as the major determinant of TLR2 activity. RrgA has been described as an adhesin and binds to fibronectin, collagen 1, and laminin (17,50). Pilus-or RrgA-negative mutants of pneumococci have been shown to be relatively less virulent than their wild type isogenic controls.…”

Stability and Assembly of Pilus Subunits of Streptococcus pneumoniae

“…Pilus formation requires the expression of seven genes encoded by the rlrA pathogenicity islet, including a RofA-like transcriptional regulator (RlrA), three sortases (SrtC-1, SrtC-2, and SrtC-3), and three structural proteins (RrgA, RrgB, and RrgC) (10). Electron microscopy observations of immunogold-labeled pneumococcus allowed investigations on the structural composition of the pilus and showed that RrgB is the major pilin subunit forming the elongated fiber shaft, but data concerning the precise localization of the minor RrgA and RrgC pilin subunits remained controversial (10,(25)(26)(27). Recently, an elegant structural analysis of the native pneumococcal pilus using a combination of electron microscopy techniques provided evidence that the RrgB-shaft is composed of monomeric covalently linked subunits oriented headto-tail (28).…”

Sortase D Forms the Covalent Bond That Links BcpB to the Tip of Bacillus cereus Pili

“…Another minor pilin subunit, RrgA of Streptococcus pneumonia, promotes bacterial adherence to respiratory epithelia (31). RrgA has been reported to bind to the extracellular matrix components fibronectin, collagen I, and laminin (32). Finally, Group A streptococcal pili engage the scavenger receptor gp340 on pharyngeal cells to promote adherence and aggregation (33).…”