The West Nile Virus (WNV) envelope protein, E, promotes membrane fusion during viral cell entry by undergoing a low-pH triggered conformational reorganization. We have examined the mechanism of WNV fusion and sought evidence for potential intermediates during the conformational transition by following hemifusion of WNV virus-like particles (VLPs) in a single particle format. We have introduced specific mutations into E, to relate their influence on fusion kinetics to structural features of the protein. At the level of individual E subunits, trimer formation and membrane engagement of the threefold clustered fusion loops are rate-limiting. Hemifusion requires at least two adjacent trimers. Simulation of the kinetics indicates that availability of competent monomers within the contact zone between virus and target membrane makes trimerization a bottleneck in hemifusion. We discuss the implications of the model we have derived for mechanisms of membrane fusion in other contexts.DOI: http://dx.doi.org/10.7554/eLife.04389.001

Entomopathogenic nematodes (EPNs) in the genera Heterorhabditis and Steinernema are lethal parasites of insects that are of interest as models for understanding parasite-host interactions and as biocontrol agents for insect pests. EPNs harbor a bacterial endosymbiont in their gut that assists in insect killing. EPNs are capable of infecting and killing a wide range of insects, yet how the nematodes and their bacterial endosymbionts interact with the insect immune system is poorly understood. Here, we develop a versatile model system for understanding the insect immune response to parasitic nematode infection that consists of seven species of EPNs as model parasites and five species of Drosophila fruit flies as model hosts. We show that the EPN Steinernema carpocapsae, which is widely used for insect control, is capable of infecting and killing D. melanogaster larvae. S. carpocapsae is associated with the bacterium Xenorhabdus nematophila, and we show that X. nematophila induces expression of a subset of antimicrobial peptide genes and suppresses the melanization response to the nematode. We further show that EPNs vary in their virulence toward D. melanogaster and that Drosophila species vary in their susceptibilities to EPN infection. Differences in virulence among different EPN-host combinations result from differences in both rates of infection and rates of postinfection survival. Our results establish a powerful model system for understanding mechanisms of host-parasite interactions and the insect immune response to parasitic nematode infection. E ntomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis are insect-parasitic nematodes that are phylogenetically distant but share a similar life cycle as a result of convergent evolution (1). EPNs offer numerous advantages as model parasitic nematodes, including small size, short generation time, and amenability to in vitro culturing (2). EPN infective larvae are associated with bacterial endosymbionts: Steinernema species are associated with bacteria in the genus Xenorhabdus, and Heterorhabditis species are associated with bacteria in the genus Photorhabdus (1). At least some EPNs are capable of infecting the fruit fly Drosophila melanogaster, providing a genetically tractable system for understanding the immune response to parasitic nematodes and their bacterial endosymbionts (3-6). However, the insect immune response to EPN infection is poorly understood.During a particular developmental stage called the infective juvenile (IJ), EPNs infect insects (Fig. 1A). IJs are developmentally arrested, third-stage larvae analogous to the dauer stage of freeliving nematodes (7). IJs actively seek out insect hosts using chemosensory cues (8-10) and infect either by entering through natural body openings or by penetrating the insect cuticle (11). IJs harbor their bacterial endosymbiont in their gut and deposit it into the insect upon infection, where it assists the nematode in killing the insect, digesting insect tissues, and inhibiting the growth of o...

In Pseudomonas aeruginosa the alp system encodes a programmed cell death pathway that is switched on in a subset of cells in response to DNA damage and is linked to the virulence of the organism. Here we show that the central regulator of this pathway, AlpA, exerts its effects by acting as an antiterminator rather than a transcription activator. In particular, we present evidence that AlpA positively regulates the alpBCDE cell lysis genes, as well as genes in a second newly identified target locus, by recognizing specific DNA sites within the promoter, then binding RNA polymerase directly and allowing it to bypass intrinsic terminators positioned downstream. AlpA thus functions in a mechanistically unusual manner to control the expression of virulence genes in this opportunistic pathogen.