Structure of the choline-binding domain of Spr1274 in Streptococcus pneumoniae

Inter-hairpin linker sequences determine the structure of the ββ-solenoid fold: a "bottom-up" study of pneumococcal LytA choline-binding module

The ββ-solenoid structures are part of many proteins involved in the recognition of bacterial cell wall. They are elongated polypeptides consisting of repeated β-hairpins connected by linker sequences and disposed around a superhelical axis stabilised by short-range interactions. Among the most studied ββ-solenoids are those belonging to the family of choline-binding modules (CBMs) from the respiratory pathogen Streptococcus pneumoniae (pneumococcus) and its bacteriophages, and their properties have been employed to develop several biotechnological and biomedical tools. We have carried out a theoretical, spectroscopic and thermodynamic study of the ββ-solenoid structure of the CBM from the pneumococcal LytA autolysin using peptides of increasing length containing 1-3 repeats of this structure. Our results show that hints of native-like tertiary structure are only observed with a minimum of three β-hairpins, corresponding to one turn of the solenoid superhelix, and identify the linker sequences between hairpins as the major directors of the solenoid folding. This study paves the way for the rational structural engineering of ββ-solenoids aimed to find novel applications.

Choline Binding Proteins from Streptococcus pneumoniae: A Dual Role as Enzybiotics and Targets for the Design of New Antimicrobials

Streptococcus pneumoniae (pneumococcus) is an important pathogen responsible for acute invasive and non-invasive infections such as meningitis, sepsis and otitis media, being the major cause of community-acquired pneumonia. The fight against pneumococcus is currently hampered both by insufficient vaccine coverage and by rising antimicrobial resistances to traditional antibiotics, making necessary the research on novel targets. Choline binding proteins (CBPs) are a family of polypeptides found in pneumococcus and related species, as well as in some of their associated bacteriophages. They are characterized by a structural organization in two modules: a functional module (FM), and a choline-binding module (CBM) that anchors the protein to the choline residues present in the cell wall through non-covalent interactions. Pneumococcal CBPs include cell wall hydrolases, adhesins and other virulence factors, all playing relevant physiological roles for bacterial viability and virulence. Moreover, many pneumococcal phages also make use of hydrolytic CBPs to fulfill their infectivity cycle. Consequently, CBPs may play a dual role for the development of novel antipneumococcal drugs, both as targets for inhibitors of their binding to the cell wall and as active cell lytic agents (enzybiotics). In this article, we review the current state of knowledge about host- and phage-encoded pneumococcal CBPs, with a special focus on structural issues, together with their perspectives for effective anti-infectious treatments.

Pneumococcal surface proteins: when the whole is greater than the sum of its parts

Surface-exposed proteins of pathogenic bacteria are considered as potential virulence factors through their direct contribution to host-pathogen interactions. Four families of surface proteins decorate the cell surface of the human pathogen Streptococcus pneumoniae. Besides lipoproteins and LPXTG proteins, also present in other gram-positive bacteria, the pneumococcus presents the choline-binding protein (CBP) family and the non-classical surface proteins (NCSPs). The CBPs present specific structural features that allow their anchorage to the cell envelope through non-covalent interaction with choline residues of lipoteichoic acid and teichoic acid. NCSP is an umbrella term for less characterized proteins displaying moonlighting functions on the pneumococcal surface that lack a leader peptide and membrane-anchor motif. Considering the unceasing evolution of microbial species under the selective pressure of antibiotic use, detailed understanding of the interaction between pathogen and the host cells is required for the development of novel therapeutic strategies to combat pneumococcal infections. This article reviews recent progress in the investigation of the three-dimensional structures of surface-exposed pneumococcal proteins. The modular nature of some of them produces a great versatility and sophistication of the virulence functions that, in most cases, cannot be deduced by the structural analysis of the isolated modules.

New adhesin functions of surface-exposed pneumococcal proteins

Background

Streptococcus pneumoniae is a widely distributed commensal Gram-positive bacteria of the upper respiratory tract. Pneumococcal colonization can progress to invasive disease, and thus become lethal, reason why antibiotics and vaccines are designed to limit the dramatic effects of the bacteria in such cases. As a consequence, pneumococcus has developed efficient antibiotic resistance, and the use of vaccines covering a limited number of serotypes such as Pneumovax^® and Prevnar^® results in the expansion of non-covered serotypes. Pneumococcal surface proteins represent challenging candidates for the development of new therapeutic targets against the bacteria. Despite the number of described virulence factors, we believe that the majority of them remain to be characterized. This is the reason why pneumococcus invasion processes are still largely unknown.

Results

Availability of genome sequences facilitated the identification of pneumococcal surface proteins bearing characteristic motifs such as choline-binding proteins (Cbp) and peptidoglycan binding (LPXTG) proteins. We designed a medium throughput approach to systematically test for interactions between these pneumococcal surface proteins and host proteins (extracellular matrix proteins, circulating proteins or immunity related proteins). We cloned, expressed and purified 28 pneumococcal surface proteins. Interactions were tested in a solid phase assay, which led to the identification of 23 protein-protein interactions among which 20 are new.

Conclusions

We conclude that whether peptidoglycan binding proteins do not appear to be major adhesins, most of the choline-binding proteins interact with host proteins (elastin and C reactive proteins are the major Cbp partners). These newly identified interactions open the way to a better understanding of host-pneumococcal interactions.