Aspergillus fumigatus is a fungal pathogen causing severe infections in immunocompromised patients. For clearance of inhaled conidia, an efficient response of the innate immune system is required. Macrophages represent the first line of defence and ingest and kill conidia. C-type lectins represent a family of receptors, which recognize pathogen-specific carbohydrates. One of them is β1–3 glucan, a major component of the fungal cell wall. Here we provide evidence that β1–3 glucan plays an important role for the elimination of A. fumigatus conidia. Laminarin, a soluble β1–3 glucan and antibodies to dectin-1, a well known β1–3 glucan receptor, significantly inhibited conidial phagocytosis. On resting conidia low amounts of surface accessible β1–3 glucan were detected, whereas high amounts were found on small spores that appear early during germination and infection as well as on resting conidia of a pksP mutant strain. Swollen conidia also display larger quantities of β1–3 glucan, although in an irregular spotted pattern. Resting pksP mutant conidia and swollen wild-type conidia are phagocytosed with high efficiency thereby confirming the relevance of β1–3 glucans for conidial phagocytosis. Additionally we found that TLR2 and the adaptor protein MyD88 are required for efficient conidial phagocytosis, suggesting a link between the TLR2-mediated recognition of A. fumigatus and the phagocytic response.

The ActA protein of the intracellular pathogen Listeria monocytogenes induces a dramatic reorganization of the actin‐based cytoskeleton. Two profilin binding proteins, VASP and Mena, are the only cellular proteins known so far to bind directly to ActA. This interaction is mediated by a conserved module, the EVH1 domain. We identify E/DFPPPPXD/E, a motif repeated 4‐fold within the primary sequence of ActA, as the core of the consensus ligand for EVH1 domains. This motif is also present and functional in at least two cellular proteins, zyxin and vinculin, which are in this respect major eukaryotic analogs of ActA. The functional importance of the novel protein–protein interaction was examined in the Listeria system. Removal of EVH1 binding sites on ActA reduces bacterial motility and strongly attenuates Listeria virulence. Taken together we demonstrate that ActA–EVH1 binding is a paradigm for a novel class of eukaryotic protein–protein interactions involving a proline‐rich ligand that is clearly different from those described for SH3 and WW/WWP domains. This class of interactions appears to be of general importance for processes dependent on rapid actin remodeling.

Summary: Listeria monocytogenes phage endolysins Ply118 and Ply500 share a unique enzymatic activity and specifically hydrolyse Listeria cells at the completion of virus multiplication in order to release progeny phage. With the aim of determining the molecular basis for the lytic specificity of these enzymes, we have elucidated their domain structure and examined the function of their unrelated and unique C‐terminal cell wall binding domains (CBDs). Analysis of deletion mutants showed that both domains are needed for lytic activity. Fusions of CBDs with green fluorescent protein (GFP) demonstrated that the C‐terminal 140 amino acids of Ply500 and the C‐terminal 182 residues of Ply118 are necessary and sufficient to direct the murein hydrolases to the bacterial cell wall. CBD500 was able to target GFP to the surface of Listeria cells belonging to serovar groups 4, 5 and 6, resulting in an even staining of the entire cell surface. In contrast, the CBD118 hybrid bound to a ligand predominantly present at septal regions and cell poles, but only on cells of serovars 1/2, 3 and 7. Non‐covalent binding to surface carbohydrate ligands occurred in a rapid, saturation‐dependent manner. We measured 4 × 10⁴ and 8 × 10⁴ binding sites for CBD118 and CBD500 respectively. Surface plasmon resonance analysis revealed unexpected high molecular affinity constants for the CBD–ligand interactions, corresponding to nanomolar affinities. In conclusion, we show that the CBDs are responsible for targeting the phage endolysins to their substrates and function to confer recognition specificity on the proteins. As the CBD sequences contain no repeats and lack all known sequence motifs for anchoring of proteins to the bacterial cell, we conclude that they use unique structural motifs for specific association with the surface of Gram‐positive bacteria.

Type III secretion systems (TTSSs or secretons), essential virulence determinants of many Gram‐negative bacteria, serve to translocate proteins directly from the bacteria into the host cytoplasm. Electron microscopy (EM) indicates that the TTSSs of Shigella flexneri are composed of: (1) an external needle; (2) a transmembrane domain; and (3) a cytoplasmic bulb. EM analysis of purified and negatively stained parts 1, 2 and a portion of 3 of the TTSS, together termed the ‘needle complex’ (NC), produced an average image at 17 Å resolution in which a base, an outer ring and a needle, inserted through the ring into the base, could be discerned. This analysis and cryoEM images of NCs indicated that the needle and base contain a central 2–3 nm canal. Five major NC components, MxiD, MxiG, MxiJ, MxiH and MxiI, were identified by N‐terminal sequencing. MxiG and MxiJ are predicted to be inner membrane proteins and presumably form the base. MxiD is predicted to be an outer membrane protein and to form the outer ring. MxiH and MxiI are small hydrophilic proteins. Mutants lacking either of these proteins formed needleless secretons and were unable to secrete Ipa proteins. As MxiH was present in NCs in large molar excess, we propose that it is the major needle component. MxiI may cap at the external needle tip.

Aspergillus fumigatius is a ubiquitous saprophytic fungus that has become the most prevalent airborne fungal pathogen for immunocompromised patients during the last two decades. In this r...