A "dock, lock, and latch" structural model for a staphylococcal adhesin binding to fibrinogen

Gram-positive pathogens such as staphylococci contain multiple cell wall-anchored proteins that serve as an interface between the microbe and its environment. Some of these proteins act as adhesins and mediate bacterial attachment to host tissues. SdrG is a cell wall-anchored adhesin from Staphylococcus epidermidis that binds to the Bbeta chain of human fibrinogen (Fg) and is necessary and sufficient for bacterial attachment to Fg-coated biomaterials. Here, we present the crystal structures of the ligand binding region of SdrG as an apoprotein and in complex with a synthetic peptide analogous to its binding site in Fg. Analysis of the crystal structures, along with mutational studies of both the protein and of the peptide, reveals that SdrG binds to its ligand with a dynamic "dock, lock, and latch" mechanism. We propose that this mechanism represents a general mode of ligand binding for structurally related cell wall-anchored proteins of gram-positive bacteria.     

Characterization of the RpoS status of clinical isolates of Salmonella enterica

The stationary-phase-inducible sigma factor, sigma(S) (RpoS), is the master regulator of the general stress response in Salmonella and is required for virulence in mice. rpoS mutants can frequently be isolated from highly passaged laboratory strains of Salmonella: We examined the rpoS status of 116 human clinical isolates of Salmonella, including 41 Salmonella enterica serotype Typhi strains isolated from blood, 38 S. enterica serotype Typhimurium strains isolated from blood, and 37 Salmonella serotype Typhimurium strains isolated from feces. We examined the abilities of these strains to produce the sigma(S) protein, to express RpoS-dependent catalase activity, and to resist to oxidative stress in the stationary phase of growth. We also carried out complementation experiments with a cloned wild-type rpoS gene. Our results showed that 15 of the 41 Salmonella serotype Typhi isolates were defective in RpoS. We sequenced the rpoS allele of 12 strains. This led to identification of small insertions, deletions, and point mutations resulting in premature stop codons or affecting regions 1 and 2 of sigma(S), showing that the rpoS mutations are not clonal. Thus, mutant rpoS alleles can be found in freshly isolated clinical strains of Salmonella serotype Typhi, and they may affect virulence properties. Interestingly however, no rpoS mutants were found among the 75 Salmonella serotype Typhimurium isolates. Strains that differed in catalase activity and resistance to hydrogen peroxide were found, but the differences were not linked to the rpoS status. This suggests that Salmonella serotype Typhimurium rpoS mutants are counterselected because rpoS plays a role in the pathogenesis of Salmonella serotype Typhimurium in humans or in the transmission cycle of the disease.     

Influence of the culture medium constituents and inoculum size on the accumulation of blue pigment and cell growth of Lavandula spica

The effects of the concentration of PO₄ ^3-, NO₃ ^–, Fe ²⁺, sucrose and inoculum size on the accumulation of blue pigment and growth of suspension cultures of Lavandula spica D.C. ( L. latifolia Vill.), were studied. The combination of 2.5 mM PO₄ ^3-, 14.1 mM NO₃ ^–, 1 mM Fe ²⁺, 30 g l^–1 sucrose and 10 g l^–1fresh weight of inoculum, promoted a 7-fold enhancement on the productivity of blue pigment in comparison to the control medium. Among all the culture medium constituents tested, phosphate exerted the highest beneficial effect and Fe²⁺ showed to be essential for the accumulation of the pigment. High levels of sucrose (60 or 90 g l^–1) did not stimulate the accumulation of blue pigment. In these conditions, cell growth and cell viability were drastically affected.     

Conidial anastomosis tubes in Colletotrichum

We describe the occurrence of special kinds of hyphae that create anastomoses directly between conidia. They can be found both in the laboratory and on infected plants. They first appear within asexual fruiting bodies approximately 15 days after conidiation has begun leading to the appearance of chains of connected conidia. Coincident with this we demonstrate in Colletotrichum lindemuthianum nuclear dynamics, including fragmentation, with cytoplasmic flow and passage of nuclei and organelles between conidia through the anastomosis tubes. We propose that conidial anastomosis tubes play an important role in the life cycle of these fungi.     

New insights into the roles of agrin

The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.     

Synthesis of novel fluorescent probes for the molecular chaperone Hsp90

Heat shock protein 90 (Hsp90) is a molecular chaperone necessary for maintaining oncogenic transformation. There is substantial interest in developing novel agents that bind to the N-terminal of the chaperone. Here we report the synthesis and characterization of two fluorescent Hsp90 inhibitors and probe their use in an Hsp90 fluorescent polarization assay.     

Special Issue Dedicated to Dr. Héctor S. Barra

Dedication

Special Issue Dedicated to Dr. Héctor S. Barra     

The Myxococcus xanthus lipopolysaccharide O-antigen is required for social motility and multicellular development

The gliding bacterium Myxococcus xanthus aggregates to form spore-filled fruiting bodies when nutrients are limiting. Defective fruiting-body formation and sporulation result from mutations in the sasA locus, which encodes the wzm wzt wbgA (formerly rfbABC ) lipopolysaccharide (LPS) O-antigen biosynthesis genes. Mutants carrying these same sasA mutations are defective in social motility and form small glossy colonies. We report here that the developmental and motility phenotypes of four mutants each containing different Tn 5 insertions in LPS O-antigen biosynthesis genes are similar to those of the original sasA locus mutants. All of the LPS O-antigen mutants tested exhibited defective developmental aggregation and sporulated at only 0.02–15% of the wild-type level. In addition, all of the LPS O-antigen mutants were determined by genetic analyses to be wild type for adventurous motility and defective in social motility, indicating that the LPS O-antigen is necessary for normal development and social motility. The two previously identified cell-surface components required for social motility, type IV pili and the protein-associated polysaccharide material termed fibrils, were detected on the surfaces of all of the LPS O-antigen mutants. This indicates that LPS O-antigen is a third cell-surface component required for social motility.     

Use of Potassium and Sucrose by Onion Guard Cells during a Daily Cycle of Osmoregulation

Solute content of stomata from intact onion cotyledons grownunder either greenhouse or growth chamber conditions was followedover the course of a daily light cycle to determine patternsof osmoregulation. Initial opening of stomata was well correlatedwith guard cell potassium accumulation under both growth conditions.Subsequently, however, there was a consistent decrease in guardcell potassium content despite constant or increasing aperture.Although a secondary increase in potassium was sometimes observedduring the second half of the light cycle, guard cell potassiumcontent was poorly correlated with aperture. Sucrose levelsin guard cells increased 60% during the period of decliningpotassium content, suggesting its use as an alternate osmoticum.Guard cells are postulated to use multiple pathways for theproduction of osmotica over the course of a complete daily cycleof stomatal movements. (Received December 5, 1995; Accepted April 9, 1996)