Streptococcus pneumoniae (pneumococcus) remains a significant health threat worldwide, especially to the young and old. While some of the biomolecules involved in pneumococcal pathogenesis are known and understood in mechanistic terms, little is known about the molecular details of bacterium/host interactions. We report here the solution structure of the ‘repeated’ adhesion domains (domains R1 and R2) of the principal pneumococcal adhesin, choline binding protein A (CbpA). Further, we provide insights into the mechanism by which CbpA binds its human receptor, polymeric immunoglobulin receptor (pIgR). The R domains, comprised of 12 imperfect copies of the leucine zipper heptad motif, adopt a unique 3‐α‐helix, raft‐like structure. Each pair of α‐helices is antiparallel and conserved residues in the loop between Helices 1 and 2 exhibit a novel ‘tyrosine fork’ structure that is involved in binding pIgR. This and other structural features that we show are conserved in most pneumococcal strains appear to generally play an important role in bacterial adhesion to pIgR. Interestingly, pneumococcus is the only bacterium known to adhere to and invade human cells by binding to pIgR.
The polymeric immunoglobulin receptor (pIgR) is a type I transmembrane protein that delivers dimeric IgA (dIgA) and pentameric IgM to mucosal secretions. Here, we report the 1.9 A resolution X-ray crystal structure of the N-terminal domain of human pIgR, which binds dIgA in the absence of other pIgR domains with an equilibrium dissociation constant of 300 nM. The structure of pIgR domain 1 reveals a folding topology similar to immunoglobulin variable domains, but with differences in the counterparts of the complementarity determining regions (CDRs), including a helical turn in CDR1 and a CDR3 loop that points away from the other CDRs. The unusual CDR3 loop position prevents dimerization analogous to the pairing of antibody variable heavy and variable light domains. The pIgR domain 1 structure allows interpretation of previous mutagenesis results and structure-based comparisons between pIgR and other IgA receptors.
The production of the alphavirus virion is a multistep event requiring the assembly of the nucleocapsid core in the cytoplasm and the maturation of the glycoproteins in the endoplasmic reticulum and the Golgi apparatus. These components associate during the budding process to produce the mature virion. The nucleocapsid proteins of Sindbis virus and Ross River virus have been produced in a T7-basedEscherichia coli expression system and purified. In the presence of single-stranded but not double-stranded nucleic acid, the proteins oligomerize in vitro into core-like particles which resemble the native viral nucleocapsid cores. Despite their similarities, Sindbis virus and Ross River virus capsid proteins do not form mixed core-like particles. Truncated forms of the Sindbis capsid protein were used to establish amino acid requirements for assembly. A capsid protein starting at residue 19 [CP(19–264)] was fully competent for in vitro assembly, whereas proteins with further N-terminal truncations could not support assembly. However, a capsid protein starting at residue 32 or 81 was able to incorporate into particles in the presence of CP(19–264) or could inhibit assembly if its molar ratio relative to CP(19–264) was greater than 1:1. This system provides a basis for the molecular dissection of alphavirus core assembly.
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