By Ariel Liu
In a new breakthrough in the ongoing fight against Zika virus, researchers at the SLAC National Accelerator Laboratory has determined how the structure of protein crystals produced by bacteria might be modified to combat the mosquitoes that carry Zika, according to SLAC’s press release.
The toxin BinAB is a larvicide commonly used in fighting mosquito-borne diseases such as malaria, West Nile virus and viral encephalitis. However, the larvicide is ineffective against the Aedes mosquitoes that transmit Zika and dengue fever.
The research conducted through SLAC’s X-ray free-electron laser called the Linac Coherent Light Source (LCLS) helps structural biologists better understand BinAB and provides information on how to design a composite toxin that can possibly attack more species of mosquitoes, including Aedes.
“A more detailed look at the protein’s structure provides information fundamental to understanding how the crystals kill mosquito larvae,” Jacques-Philippe Colletier, a scientist at the Institut de Biologie Structurale in Grenoble, France and the lead author for the paper, said in the press release. “This is a prerequisite for modifying the toxin to adapt it to our needs.”
The BinAB crystals, which are produced by Lysinibacillus sphaericus bacteria at the end of their life cycle, are consumed by mosquito larvae. When the crystals are exposed to the alkaline conditions in a mosquito larva’s gut, a key protein is activated and recognized by receptors, killing the mosquitoes.
Because Aedes larvae do not have the correct receptors, they can evade intoxication and become resistant to BinAB. Many other organisms, including humans, lack these receptors, as well as alkaline digestive systems. Therefore, the toxin is relatively safe to use.
“Part of the appeal is that the larvicide is safe because it’s so specific, but that’s also part of its limitation,” Michael Sawaya, a scientist at the University of California, Los Angeles-DOE Molecular Biology Institute and co-author on the paper, said in the SLAC press release.
From previous experiments, scientists knew that the BinAB larvicide is made up of the pair of proteins BinA and BinB. BinA is the lethal part of the complex, while BinB is responsible for binding the toxin to the mosquito’s intestine. The LCLS research team was able to uncover the molecular basis for how the two proteins bind together.
Only minimal information was known about the three-dimensional structure and biological behavior of BinAB prior to the LCLS experiment, according to SLAC’s press release.
“We chose to look at the BinAB larvicide because it is so widely used, yet the structural details were a mystery,” Brian Federici, professor of entomology at UC Riverside, said in the press release.
Conventional X-ray cannot study the crystals due to their small size, so the research team used genetic engineering techniques to increase the size of the crystals and a crystallography technique called de novo phasing. This technique involves using heavy metal markers to tag crystals and collecting tens of thousands of X-ray diffraction patterns to create a three-dimensional map of the electron density of the protein.
“The most immediate need is to now expand the spectrum of action of the BinAB toxin to counter the progression of Zika,” Colletier said in the press release. “BinAB is already effective against Culex [carrier of West Nile encephalitis] and Anopheles [carrier of malaria] mosquitos. With the results of the study, we now feel more confident that we can design the protein to target Aedes mosquitos.”
Additional contributors to the research include scientists from the Howard Hughes Medical Institutes at UCLA, Lawrence Berkeley National Laboratory and Stanford University.
Contact Ariel Liu at aliu15 ‘at’ stanford.edu.