Structural and functional characterization of the Streptococcus pneumoniae RrgB pilus backbone D1 domain

Streptococcus pneumoniae expresses on its surface adhesive pili, involved in bacterial attachment to epithelial cells and virulence. The pneumococcal pilus is composed of three proteins, RrgA, RrgB, and RrgC, each stabilized by intramolecular isopeptide bonds and covalently polymerized by means of intermolecular isopeptide bonds to form an extended fiber. RrgB is the pilus scaffold subunit and is protective in vivo in mouse models of sepsis and pneumonia, thus representing a potential vaccine candidate. The crystal structure of a major RrgB C-terminal portion featured an organization into three independently folded protein domains (D2-D4), whereas the N-terminal D1 domain (D1) remained unsolved. We have tested the four single recombinant RrgB domains in active and passive immunization studies and show that D1 is the most effective, providing a level of protection comparable with that of the full-length protein. To elucidate the structural features of D1, we solved the solution structure of the recombinant domain by NMR spectroscopy. The spectra analysis revealed that D1 has many flexible regions, does not contain any intramolecular isopeptide bond, and shares with the other domains an Ig-like fold. In addition, we demonstrated, by site-directed mutagenesis and complementation in S. pneumoniae, that the D1 domain contains the Lys residue (Lys-183) involved in the formation of the intermolecular isopeptide bonds and pilus polymerization. Finally, we present a model of the RrgB protein architecture along with the mapping of two surface-exposed linear epitopes recognized by protective antisera.     

A novel strategy to over-express and purify homologous proteins from Streptococcus pneumoniae

Functional studies of Streptococcus pneumoniae virulence factors are facilitated by the development of complementation/mutagenesis systems. These methods usually result in poor expression yields; therefore, biochemical and structural/functional characterizations are mostly performed with proteins expressed and purified from heterologous systems (e.g. Escherichia coli). However, heterologous expression does not guarantee correct protein structure and function. In this work, we developed a method to over-express and purify homologous proteins from S. pneumoniae. The system relies on the combined use of the shuttle plasmid pMU1328 and a natural constitutive pneumococcal promoter, P₉₆. Efficient over-expression of secreted, membrane or surface anchored proteins, either wild type or mutant, was achieved. As proof of principle the S. pneumoniae pilus-1 backbone RrgB was successfully purified as a His-tag secreted protein (RrgB-His_SP) from pneumococcal culture supernatants. N-terminal sequencing and mass spectrometry analysis of RrgB-His_SP allowed the determination of the leader sequence cleavage site in pneumococcus, while proteolysis studies confirmed the stability of RrgB-His_SP to trypsin digestion. The data presented here support the use of this novel homologous expression method for all S. pneumoniae proteins for which extensive characterization studies are planned. Moreover, given the promiscuity of the pMU1328 replicon, this system could be used in diverse bacterial species.