The biologist's usual go-to method to study such molecules — growing a large quantity using molecular cloning — failed to produce enough correctly structured α7 to study.
"You can't study it directly in its natural form, so you have to engineer it," Chen said.
In the case of α7, Chen's collaborator, Dr. Steve Sine from Mayo Clinic, engineered a chimera, a Frankenstein molecule sharing about 70 percent of its structure in common with the α7 that reacted to stimuli in the same way that natural α7 does.
The next step was to form crystals with these proteins for high-resolution study. This turns out to be particularly difficult for neuronal receptors because they are intrinsically flexible — they need to bind to a neurotransmitter, a small molecule that acts as a messenger in the nervous system, and transmit the signal across the protein body. Moreover, these receptors are decorated with sugar molecules that add further flexibility to the system.
The crystallization of α7 was a painstaking process carried out by Shu-xing Li, the first author of the study and a postdoctoral fellow in Chen's lab. For every hundred crystals obtained, only one or two were good enough for structural analysis. Li had to sort through hundreds of crystals to collect enough data for structural analysis.
"In a sense, these crystals are probably among the world's most expensive crystals, certainly more expensive than diamond," Chen said. "But considering the rich information we can get from these crystals about human neuronal receptors, and the potential impact on drug development that can benefit human health, they are worth the effort."