If you want to better understand the growth mechanism of plants, you must analyze biochemistry from the molecular details. Michael Hothorn and his research team from the Friedrich-Miescher laboratory of the Max Planck Society are doing this. Their latest research revealed that a plant membrane receptor requires an accessory protein to sense a growth-promoting hormone, which transmits this signal to the entire cell membrane.

Each cell is surrounded by lipid cell membranes. Signals from other cells and the environment must be sensed on the cell surface, transmitted to the entire cell membrane, and then converted into a specific reaction within the cell. All organisms have evolved some membrane receptor proteins to accomplish these complex tasks, but plant membrane receptors look very different compared to well-researched membrane receptors in animals and bacteria. The leucine-rich repeat sequence (LRR) receptor protein kinase family is responsible for most membrane signaling events in plants. The plant hormone receptor BRI1 belongs to the LRR receptor protein kinase family, which can sense a small steroid hormone to promote plant growth. It was previously confirmed that BRI1 can directly bind this small steroid hormone on the cell surface with its LRR domain.

Julia Santiago, a postdoctoral researcher in the Hothorn laboratory, has now confirmed that BRI1 requires an accessory protein to properly sense this hormone and transmit the signal to the entire cell membrane. This auxiliary SERK1 protein is known to play a role in the brassinosteroid signaling pathway, but it was surprising to see that it was needed early. By analyzing the BRI1-steroid hormone-SERK1 ternary complex protein crystals with strong X-rays, Santiago saw that SERK1 directly contributed to the formation of hormone binding pockets, and the two proteins interacted with the hormone. Thus, this steroid also acts as a molecular glue to promote the binding of BRI1 on the cell surface to the LRR domain of SERK1. This subsequently caused interactions within the cell's cytoplasmic kinase domain, which in turn activated a defined signaling pathway that triggered a growth response.

The interesting feature of SERK1 is that it can help activate several seemingly unrelated plant receptor kinases that bind extremely different ligands and trigger different responses. This new structure allows researchers to see the operating mechanism of SERK1 for the first time. It didn't shake hands with BRI1, it just used a few 'fingertips' to contact the receptor. In addition, its extremely conserved surface can still interact with other plant receptor kinases and potential ligands. "There must be some advantages to combine all these different functions into one accessory protein," Hothorn speculates. It is worth noting that the use of a common accessory protein can achieve communication between different signaling pathways.

This atomic model also provides some other new insights: "Looking at our model, we can now well predict which mutations in receptors or accessory proteins should affect downstream signaling pathways. We also know this Which part of the hormone is critical for its binding to receptors or accessory proteins. "Such detailed knowledge may drive the rational design of plant anabolic steroid hormones and receptor antagonists that can be used in basic research.

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