Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2011
Publication Date: 1/3/2012
Citation: Oh, M., Wang, X., Clouse, S.D., Huber, S.C. 2012. Deactivation of the Arabidopsis BRI1 receptor kinase by autophosphorylation within the glycine-rich loop involved in ATP binding. Proceedings of the National Academy of Sciences. 109:327-332. Interpretive Summary: Plants have a large family of receptor kinases that control a number of important processes ranging from growth and development to responses to the environment and pathogens. Receptor kinases work by sensing signals outside of the cell and transmitting that information to the nucleus where gene expression is altered in an appropriate way to the signal. The process starts with activation of the receptor kinase when the signal is present and persists until the receptor kinase is deactivated. However, little is known about the mechanisms that deactivate receptor kinases. In the present study we identify a new mechanism for the deactivation of the brassinosteroid receptor kinase, known as BRI1. The new mechanism involves binding of a phosphate molecule to a specific serine residue on BRI1 in a process known as autophosphorylation. The critical serine residue is located in the active site of the enzyme, and when phosphorylated inhibits BRI1 activity. This is a novel mechanism for the deactivation of the BRI1 receptor kinase, and potentially has relevance to a number of other plant receptor kinases. Controlling deactivation mechanisms may provide a new approach to alter receptor kinase function and improve plant productivity.
Technical Abstract: The activity of the dual-specificity brassinosteroid receptor kinase, BRI1, reflects the balance between phosphorylation-dependent activation and several potential mechanisms for deactivation of the receptor. In the present report, we identify regions of the juxtamembrane domain that are essential for tyrosine autophosphorylation, indicating that kinase specificity can be affected by residues outside of the kinase domain. We also elucidate a new mechanism for deactivation that involves autophosphorylation of serine-891 in the ATP-binding domain. Serine-891 was identified previously as a potential site of autophosphorylation by mass spectrometry, and sequence-specific antibodies and mutagenesis studies now unambiguously establish modification of this residue. In vivo, phosphorylation of Ser891 increased slowly with time following application of brassinolide to Arabidopsis seedlings, while phosphorylation of threonine residues increased rapidly and then remained constant. Transgenic plants expressing the BRI1(S891A)-Flag directed mutant have increased hypocotyl and petiole lengths relative to wild type BRI1-Flag (both in the bri1-5 background), whereas plants expressing the phosphomimetic S891D directed mutant are dwarfed and have extremely short hypocotyls and petioles. Collectively, these results suggest that autophosphorylation of serine-891 is one of the deactivation mechanisms that inhibits BRI1 activity and BR signaling in vivo. Many RD-type leucine-rich repeat receptor-like kinases have a phosphorylatable residue within the ATP-binding domain suggesting that this mechanism may play a broad role in receptor kinase deactivation. The phosphoserine-891 inhibitory site is not as rapidly dephosphorylated in vivo as phosphothreonine sites, which may have implications for subsequent cycles of activation/deactivation of this relatively long-lived receptor kinase.