Structure Of Brassinolide's Receptor Solved

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Ringed Receptor The protein that detects that plant hormone brassinolide adopts an unexpected superhelix conformation.

The first X-ray crystal structures of a key plant hormone receptor protein have been solved by two independent research teams working in the U.S. and China (Nature, DOI: 10.1038/nature10178 and 10.1038/nature10153). The receptor’s structure—among the last of the major plant hormone receptor structures to be solved--senses brassinolide, a steroid that helps bulk up foliage, fight pathogens, and mediate fertilization.

The discovery sets the stage for plant researchers to improve the yields of lettuce and cotton, two crops that depend on brassinolide signaling for large harvests. The structure will also help plant scientists understand and bolster agricultural crops’ immunity to microbial pathogens.

Surprisingly, the brassinolide receptor “looks totally different than what was expected,” says Ping He, a plant biochemist at Texas A&M University. People so believed the receptor would adopt a horse-shoe shape, He explains, that the wrong structure was widely “put in textbooks.”

Instead, the brassinolide receptor, called BRI1, adopts a superhelix conformation that sequesters the hormone inside the ring, with the help of an “island” domain, so named  because it appears to float in the center of the receptor’s circular interior.

The two research teams--one led by structural biologist Jijie Chai at Tsinghua University, in Beijing, and the other by  Joanne Chory, a plant biologist at the Salk Institute in La Jolla, Calif.,--knew of each other’s work,  but did not share data prior to submitting the papers for publication. When Chai finally read Chory’s paper, “I could not find a difference between their structure and ours,” he says. The impressive similarity “certainly helped” convince reviewers who were expecting BRI1 to look like its mammalian analogue, the horse-shoe-shaped steroid receptor called TLR3, Chory says.

The researchers were also surprised to find that BRI1 does not seem to dimerize as TLR3 does, Chai says.

Chemists could use BRI1’s structure to develop small, receptor-activating molecules that aren’t as complicated—and therefore as expensive to produce or purify--as brassinolide. The structure might also help plant scientists engineer the receptor to increase crop sensitivity to brassinolide, thus speeding the launch of defensive strategies against invading microbial pests, He adds.

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