Tyrosine Phosphorylation of the BRI1 Receptor Kinase Emerges as a Component of Brassinosteriod Signaling in Arabidopsis

Authors: Oh, Man Ho; WANG, XIAOFENG - North Carolina State University; KOTA, UMA - North Carolina State University; GOSHE, MICHAEL - North Carolina State University; CLOUSE, STEVEN - North Carolina State University; Huber, Steven

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2008
Publication Date: 1/13/2009
Citation: Oh, M., Wang, X., Kota, U., Goshe, M.B., Clouse, S.D., Huber, S.C. 2009. Tyrosine Phosphorylation of the BRI1 Receptor Kinase Emerges as a Component of Brassinosteriod Signaling in Arabidopsis. Proceedings of the National Academy of Sciences USA. 106:658-663.

Interpretive Summary: Brassinosteroids are essential plant hormones that control growth and tolerance to many stress conditions. Plant cells sense brassinosteroids when they bind to a receptor protein known as BRASSINOSTEROID INSENSITIVE 1 (BRI1). When the hormone binds to the extracellular portion of BRI1, the intracellular protein kinase domain becomes activated and undergoes autophosphorylation on multiple serine and threonine residues. We demonstrate for the first time that BRI1 also autophosphorylates on tyrosine residues and thus is a dual specificity kinase. We identified tyrosine-831 in BRI1 as one of the major sites of autophosphorylation and demonstrated that phosphorylation of this residue plays an important role in brassinosteroid signaling in vivo. Impact: Our results demonstrate that, in contrast to current dogma, tyrosine phosphorylation may be a prominent feature of many plant receptor kinases, including BRI1 involved in brassinosteroid signaling. The occurrence of tyrosine phosphorylation opens a new level of potential regulation of plant receptor kinases. Understanding growth-promoting signal transduction mechanisms may ultimately lead to strategies to control brassinosteroid signaling with beneficial impact on agriculture.

Technical Abstract: Brassinosteroids (BRs) are essential growth-promoting hormones that regulate many aspects of plant growth and development. Two leucine-rich repeat receptor-like kinases (LRR-RLKs) are involved in BR perception and signal transduction: BRASSINOSTEROID INSENSITIVE 1 (BRI1), which is the BR receptor, and its co-receptor BRI1-ASSOCIATED KINASE 1 (BAK1). Both proteins are classified as serine/threonine protein kinases, but here we report that recombinant cytoplasmic domains of BRI1 and BAK1 also autophosphorylate on tyrosine residues and thus are dual-specificity kinases. With BRI1, Tyr 831 and Tyr 956 are identified as autophosphorylation sites in vitro and in vivo. Interestingly, Tyr 956 in kinase subdomain V is essential for activity, as the Y956F mutant is catalytically inactive and thus this site cannot be simply manipulated by mutagenesis. In contrast, Tyr 831 in the juxtamembrane domain is not essential for kinase activity but plays an important role in BR signaling in vivo, as expression of BRI1(Y831F)-Flag in transgenic bri1-5 plants results in plants with larger leaves (but altered leaf shape) and early flowering relative to plants expressing wild type BRI1-Flag. Acidic substitutions of Tyr 831 restored normal leaf size (but not shape) and normal flowering time. This is the first example where a specific tyrosine residue has been shown to play an important role in vivo in plant receptor kinase function. Interestingly, six additional LRR-RLKs (of the twenty three tested) were also found to autophosphorylate on tyrosine in addition to serine and threonine, suggesting that tyrosine signaling should be considered with other plant receptor kinases as well.