Burkholderia cepacia is an important opportunistic human pathogen that affects immunocompromised individuals, particularly cystic fibrosis (CF) patients. Colonization of the lungs of a CF patient by B. cepacia can lead not only to a decline in respiratory function but also to an acute systemic infection, such as bacteremia. We have previously demonstrated that a CF clinical isolate of B. cepacia, strain J2315, can invade and survive within cultured respiratory epithelial cells. In order to further characterize the mechanisms of invasion of B. cepacia, we screened a transposon-generated mutant library of strain J2315 for mutants defective in invasion of A549 respiratory epithelial cells. Here we describe isolation and characterization of a nonmotile mutant of B. cepacia with reduced invasiveness due to disruption of fliG, which encodes a component of the motor-switch complex of the flagellar basal body. We also found that a defined null mutation in fliI, a gene encoding a highly conserved ATPase required for protein translocation via the flagellar type III secretion system, also resulted in loss of motility and a significant reduction in invasion. Both mutants lacked detectable intracellular flagellin and failed to export detectable amounts of flagellin into culture supernatants, suggesting that disruption of fliG and fliI impaired flagellar biogenesis. The reduction in invasion did not appear to be due to defective adherence of the flagellar mutants to A549 cells, suggesting that functional flagella and motility are required for full invasiveness of B. cepacia. Our findings indicate that flagellum-mediated motility may facilitate penetration of host epithelial barriers by B. cepacia, contributing to establishment of infection and systemic spread of the organism.Over the last several decades Burkholderia cepacia has emerged as an important opportunistic human pathogen of the lower respiratory tract that affects immunocompromised individuals, particularly cystic fibrosis (CF) patients (18). Chronic colonization of the lungs of a CF patient by B. cepacia can lead not only to a decline in respiratory function, due to the onset of a necrotizing pneumonia, but also to an acute systemic infection, such as bacteremia or septicemia (11,46). In addition to being an invasive pathogen that is capable of entering deeper tissues and becoming blood borne, B. cepacia can survive in epithelial cells and macrophages (4,35,42,48), which may contribute to the persistence of the organism in the host. The rapid clinical decline due to B. cepacia colonization is known as cepacia syndrome, and this decline leads to mortality in approximately 20 to 35% of chronically colonized individuals (25, 47). Furthermore, colonization by B. cepacia reduces the life expectancy of a CF patient by 50%, from 30 to 15 years (24). The inherent resistance of B. cepacia to multiple antibiotics makes eradication of this pathogen from the lungs of infected individuals especially difficult. Despite the known association of B. cepacia with fatal pulmonary inf...

Although cellular processes depend on protein-protein interactions, our understanding of molecular recognition between proteins remains far from comprehensive. Protein-protein interfaces are structural and energetic mosaics in which a subset of interfacial residues, called hot spots, contributes disproportionately to the affinity of the complex. These hot-spot residues can be further clustered into hot regions. It has been proposed that binding energetics between residues within a hot region are cooperative, whereas those between hot regions are strictly additive. If this idea held true for all protein-protein interactions, then energetically significant long-range conformational effects would be unlikely to occur. In the present study, we show cooperative binding energetics between distinct hot regions that are separated by >20 Å. Using combinatorial mutagenesis and surface plasmon resonance binding analysis to dissect additivity and cooperativity in a complex formed between a variable domain of a T cell receptor and a bacterial superantigen, we find that combinations of mutations from each of two hot regions exhibited significant cooperative energetics. Their connecting sequence is composed primarily of a single ␤-strand of the T cell receptor variable Ig domain, which has been observed to undergo a strand-switching event and does not form an integral part of the stabilizing core of this Ig domain. We propose that these cooperative effects are propagated through a dynamic structural network. Cooperativity between hot regions has significant implications for the prediction and inhibition of protein-protein interactions.binding energy ͉ cooperativity ͉ protein-protein interaction ͉ surface plasmon resonance ͉ T cell activity

Type III secretion systems are utilized by a number of gram-negative bacterial pathogens to deliver virulence-associated proteins into host cells. Using a PCR-based approach, we identified homologs of type III secretion genes in the gram-negative bacterium Burkholderia cepacia, an important pulmonary pathogen in immunocompromised patients and patients with cystic fibrosis. One of the genes, designated bscN, encodes a member of a family of ATP-binding proteins believed to generate energy driving virulence protein secretion. Genetic dissection of the regions flanking the bscN gene revealed a locus consisting of at least 10 open reading frames, predicted to encode products with significant homology to known type III secretion proteins in other bacteria. A defined null mutation was generated in the bscN gene, and the null strain and wild-type parent strain were examined by use of a murine model of B. cepacia infection. Quantitative bacteriological analysis of the lungs and spleens of infected C57BL/6 mice revealed that the bscN null strain was attenuated in virulence compared to the parent strain, with significantly lower bacterial recovery from the lungs and spleens at 3 days postinfection. Moreover, histopathological changes, including an inflammatory cell infiltrate, were more pronounced in the lungs of mice infected with the wild-type parent strain than in those of mice infected with the isogenic bscN mutant. These results implicate type III secretion as an important determinant in the pathogenesis of B. cepacia.

Superantigens (SAGs) bind simultaneously to major histocompatibility complex (MHC) and T-cell receptor (TCR) molecules, resulting in the massive release of inflammatory cytokines that can lead to toxic shock syndrome (TSS) and death. A major causative agent of TSS is toxic shock syndrome toxin-1 (TSST-1), which is unique relative to other bacterial SAGs owing to its structural divergence and its stringent TCR specificity. Here, we report the crystal structure of TSST-1 in complex with an affinity-matured variant of its wild-type TCR ligand, human T-cell receptor b chain variable domain 2.1. From this structure and a model of the wild-type complex, we show that TSST-1 engages TCR ligands in a markedly different way than do other SAGs. We provide a structural basis for the high TCR specificity of TSST-1 and present a model of the TSST-1-dependent MHC-SAG-TCR T-cell signaling complex that is structurally and energetically unique relative to those formed by other SAGs. Our data also suggest that protein plasticity plays an exceptionally significant role in this affinity maturation process that results in more than a 3000-fold increase in affinity.