Princeton researchers uncover vital role played by a protein elevated in many cancers
This powerful agnostic approach was made possible through "proximity labeling," a technique pioneered by Ting's group that involves genetically engineering proteins to mark their closest neighbors with a unique molecular tag.
"It's like spray painting," Ting said. "When spray painting, the paint distribution is going to be densest in the regions closest to your paint bottle." Biotin
For the new study, Luo's lab employed proximity labeling to mark all of the cell surface proteins on a particular group of neurons in the brains of developing and adult fruit flies.
The scientists used a special strain of flies bred to produce a catalytic protein, or enzyme, borrowed from horseradish plants. This enzyme, called peroxidase, functions like an unopened can of spray paint and only resides on the surface of a particular kind of neuron in the fly brains that the scientists wanted to study.
At different points in the flies' development, the scientists removed the brains and did the molecular equivalent of triggering the spray cans to paint all the nearby proteins. This involved immersing the cells in chemical baths that caused the horseradish proteins to mark their cell surface neighbors with a molecule called biotin.
Next, the researchers broke apart the fly brains and their constituent neurons, and pulled out all the proteins that got sprayed - or, rather, that contained the biotin tag. "The beauty here is that you're only enriching the cell surface proteins and ignoring all of the proteins inside the cell," said Luo, who is also an investigator at the Howard Hughes Medical Institute.
This study was the first time the researchers used the technique in a living organism rather than in a petri dish. "To get this to actually work in vivo"--in intact brains of live organisms--"was quite a technical leap," Ting said. Beyond the lamppost
In relatively few steps, the scientists were able to conduct a complete census of the more than 700 different kinds of protein that dot the surface of olfactory projection neurons in fruit flies. The team's haul included many cell surface proteins that had been painstakingly identified previously using other techniques.
Even more intriguing, their survey also turned up 20 new cell surface proteins in the brains of the developing flies, which subsequent experiments by the team revealed to be important for brain wiring.
"This shows that this approach is really unbiased," Ting said. "It allows you to look away from under the lamppost to find proteins that you may miss otherwise." Source: Journal reference:
Li, J., et al. (2020) Cell-Surface Proteomic Profiling in the Fly Brain Uncovers Wiring Regulators. Cell . doi.org/10.1016/j.cell.2019.12.029 .
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