Lipid phase influences the binding of Bacillus thuringiensis Cyt2Aa2 toxin on model lipid membranes

Oligomer assembly of Bacillus thuringiensis Cyt2Aa2 on lipid membranes reveals a thread-like structure

Bacillus thuringiensis, a well-known insecticidal bacterium, produces several insecticidal proteins, including cytolytic (Cyt) proteins. Cyt proteins bind directly to the lipid membrane and form large protein complexes. In addition to the protein ladder bands, information on the oligomeric structure in lipid membranes is necessary to understand the mechanism of Cyt proteins on target cells. In this work, we have investigated the oligomeric Cyt2Aa2 complex with synthetic lipid and with erythrocyte membranes. When the activated Cyt2Aa2 protein was incubated with these lipid membranes, the protein ladder pattern relevant to hemolytic activity was detected in SDS-PAGE. Moreover, AFM topographic images revealed a fusilli-like structure and a ring-like structure for synthetic POPC and POPC/Chol, respectively. Furthermore, TEM micrographs provided an additional information on the oligomeric structure of Cyt2Aa2 in erythrocytes. Cyt2Aa2 appears to oligomerise/aggregate into mixed structures between the filamentous structure and small protein complexes in erythrocytes. In addition, a nanopore was found to be a substructure of the filamentous structure. These results strengthen the understanding of Cyt2Aa2 behavior in these two membrane systems, the fusilli and ring-like structures, depending on the type of lipid membrane. Furthermore, the structure of Cyt2Aa2 in insect target membranes remains to be investigated.

The Bacterium P. aeruginosa Disperses Ordered Membrane Domains by Targeting Phase Boundaries

Various pathogens use receptors on the host's plasma membrane for their cellular uptake. For the bacterium Pseudomonas aeruginosa, interactions between its lectin LecA and the host cell glycosphingolipid globotriaosylceramide (also known as Gb3) are crucial for its internalization via the so-called lipid zipper mechanism. In this study, we investigated the interactions of the P. aeruginosa strain PAO1 with phase-separated lipid bilayers containing Gb3. Surprisingly, bacteria are mostly bound to the interphase of liquid-ordered (Lo) and liquid-disordered (Ld) membrane domains. Simultaneously with the formation of bacterial aggregates and the accumulation of membrane lipids, the lipid bilayers were drastically reorganized and Lo domains were dissolved. Surprisingly, Gb3 was found to play a role in the localization of the bacterium at the interface, less so LecA. When microspheres were used as a minimal mimic of the bacterium, these beads also localized preferentially at the Lo-Ld phase boundaries, but in contrast to living bacteria, beads were unable to cause membrane reorganization and dissolution of the Lo domain, even when coated with LecA. Targeting phase boundaries as "weak points" in membranes and thereby reorganizing and destabilizing the host cell plasma membrane could be an attractive entry strategy for P. aeruginosa and many other bacteria and viruses.

Bacillus thuringiensis toxin Cyt2Aa forms filamentous oligomers when exposed to lipid membranes or detergents

The bacterium Bacillus thuringiensis (Bt) produces insecticidal proteins during the sporulation phase. These proteins are located in parasporal crystals consisting of two delta-endotoxin classes, crystal (Cry) and cytolytic (Cyt) toxins. In vitro, Cyt toxins show cytolytic activity against bacterial and a variety of insect and mammalian cells. They bind to cell membranes with unsaturated phospholipids and sphingomyelin. Although Bt and its parasporal crystals containing both Cry and Cyt toxins have been successfully used as bioinsecticides, the molecular mechanism of action of Cyt toxins is not yet fully understood. To address this, we exposed Cyt2Aa to lipid membranes and visualized membrane disruption process using cryo-electron microscopy. We observed two types of Cyt2Aa oligomers. First, Cyt2Aa forms smaller curved oligomers on the membrane surface that become linear over time, and detach when the membrane ruptures. Similar linear filamentous oligomers were also formed by Cyt2Aa in the presence of detergents without prior exposure to lipid membranes, which exhibited attenuated cytolytic activity. Furthermore, our data suggest that Cyt2Aa adopts different conformations between its monomeric and oligomeric forms. Overall, our results provide new evidence for a detergent-like mechanism of action of Cyt2Aa rather than the pore-forming model of target membrane disruption of this important class of insecticidal proteins.

Molecular cloning of a new cry2A-type gene from Bacillus thuringiensis strain Nn10 and its expression studies

In the present study, eight indigenous Bacillus thuringiensis isolates of Western Ghats of India with more than 90% toxicity against Helicoverpa armigera were characterized for cry2A gene sub families. Seven of the eight isolates harboured cry2Aa, cry2Ab and cry2Ac genes alone and or in combination. Further, the indigenous cry2Aa gene(s) from Bacillus thuringiensis isolate Nn10 which showed 100% mortality against Helicoverpa armigera was cloned and expressed into recombinant Bt strains for management of resistance development in insects. The ORF of cry2Aa (∼1.9 kb) gene(s) from Nn10 isolate was ligated with T/A vector (pTZ57 R/T) and expressed in E. coli, DH5α. Automated sequence analysis of newly cloned recombinant cry2Aa revealed 99% homology to 916 bases in the 3' region of minus strand and 100% homology with 720 bases in the 5' region of holotype cry2Aa1 gene. The partial Cry2Aa amino acid sequence of Bt strain, Nn10, deduced from the nucleotide sequence generated by M13F primer showed four amino acid variation in comparison to Cry2Aa1 holotype, at 338, 345, 346 and 489^th position of ORF and the sequence was submitted to the NCBI. Further the expression of ORF of cry2Aa of Nn10 into acrystalliferous Bt strain, 4Q7 using expression vector pHT3P2T under the transcriptional control of cry3Aa promoter and cry2Aa terminator. SDS PAGE analysis of recombinant protein exhibited a prominent band of about 65 kDa. Bioassay studies revealed that recombinant proteins, Cry2Aa of Nn10 was toxic to Helicoverpa armigera with LC50 value of 7.26 μg ml^-1.

Molecular Imprinted Based Quartz Crystal Microbalance Sensors for Bacteria and Spores

A molecular imprinting strategy was combined with mass-sensitive transducers to generate robust and reliable biomimetic sensor systems for the detection of bioparticles. The patterning of polymers with bioanalytes enabled us to detect Escherichia coli (E. coli) bacteria with quartz crystal microbalance (QCM). The QCM sensor results were compared with direct Atomic Force Microscopy (AFM) measurements—bacteria cells adhering to the sensor coatings were counted. The recognition sites generated by Bacillus subtilis (B. subtilis) spores could successfully and reversibly recognize the template analyte and ensured rapid sensing. Cross sensitive measurements clearly showed the advantage of the molecular imprinting strategy, by which spores of Bacillus species (subtilis and thuringiensis) could easily be differentiated and selectively detected. The growth of B. subtilis from its spores was observed at 42 °C in appropriate nutrient solution of glucose and ammonium sulfate over a period of 15 h. Moreover, the growth of B. subtilis bacteria from its respective spores was studied by increasing the glucose concentration until saturation effect of the sensor. The polymeric sensor coatings were patterned to fix the B. subtilis in order to investigate osmotic effects according to a frequency response of 400 Hz by altering the ionic strength of 0.1 M.