The brassinolide receptor BRI1 is kept in a relatively inactive state by its intracellular tail and a small inhibitory protein known as BKI1. Based on earlier studies in Chory's lab, the Salk researchers knew that autophosphorylation of the receptor was necessary, but what triggered the release of the inhibitory protein remained unclear.
In an effort to understand the activation mechanism, the Salk researchers discovered that BKI1 acts through two evolutionarily conserved motifs: a 20-amino- acid sequence that binds the receptor kinase domain and a lysine-arginine-rich motif that anchors the inhibitory peptide to the plasma membrane. Phosphorylation of a key tyrosine within the membrane-targeting motif releases BKI1 from the membrane, relieving kinase inhibition and allowing the formation of an active signaling complex.
The phosphorylation of BKI1 is not only the first documented example of tyrosine transphosphorylation in plants, the underlying principle also closely resembles the mechanism used by bona fide receptor tyrosine kinases to regulate their activity. "Plant and animal receptor kinases evolved independently, yet their activation relies on similar mechanisms," says Chory.
By defining common features in plant and animal receptor signaling pathways, the Salk researchers hope to learn more about what the requirements for a robust signaling system are. Althou
|Contact: Gina Kirchweger|