Phosphorylation of Tyr-610 in the Receptor Kinase BAK1 Plays a Role in Brassinosteroid Signaling and Basal Defense Gene Expression

Authors: Oh, Man Ho; WANG, XIAOFENG - North Carolina State University; WU, XIA - University Of Illinois; ZHAO, YOUFU - University Of Illinois; 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: 9/1/2010
Publication Date: 9/27/2010
Citation: Oh, M., Wang, X., Wu, X., Zhao, Y., Clouse, S.D., Huber, S.C. 2010. Phosphorylation of Tyr-610 in the receptor kinase BAK1 plays a role in Brassinosteroid signaling and basal defense gene expression. Proceedings of the National Academy of Sciences. 107(4):17827-17832.

Interpretive Summary: Protein phosphorylation is a common modification of proteins in both plants and animals that involves covalent binding of a phosphate molecule to the protein. Phosphorylation is catalyzed by enzymes known as protein kinases. Some protein kinases are embedded in the plasma membrane, with the kinase domain protruding into the cytoplasm and with an extracellular domain that can bind various ligand molecules, such as hormones or fragments of proteins known as peptides. In response to binding of the specific ligand to the extracellular domain, the cytoplasmic protein kinase domain becomes activated thereby transmitting the signal from the outside of the cell to the inside of the cell. This process initiates a signal transduction cascade that often result in changes in gene expression in the nucleus. Animal receptor kinases usually phosphorylate proteins on tyrosine residues, and hence are referred to as tyrosine kinases, while plant receptor kinases phosphorylate proteins on serine and threonine residues and are therefore referred to as serine/threonine kinases. Plants contain many more receptor kinases compared to animals, and although functions have only been elucidated in a few cases, it is nonetheless clear that receptor kinases regulate a diverse range of processes including control of plant growth and development and response to environmental stress and pathogens. It was recently reported that tyrosine phosphorylation plays an unexpected role in plant receptor kinase signaling and thus, at least some plant receptor kinases are dual-specificity kinases able to phosphorylate proteins on tyrosine as well as serine and threonine. We now document the occurrence and significance of tyrosine phosphorylation of a second plant receptor kinase, referred to as BAK1. BAK1 is of special interest because it is known to function in at least three distinct signaling pathways and we show that tyrosine phosphorylation is required for some but not all of these functions in vivo. Impact: The demonstration that site-specific tyrosine autophosphorylation is essential for BAK1-mediated signaling adds significantly to the emerging notion that tyrosine phosphorylation plays a broad role in plant receptor kinase signaling. Without recognition that many receptor kinases are likely to be dual-specificity kinases, the role of phosphotyrosine residues would not even be considered and is likely to be essential in many cases. A further and unexpected finding is that BAK1 apparently plays a role in the basal expression of many defense and stress genes (in the absence of stress or elicitors), and this function requires tyrosine phosphorylation. These results may provide new approaches to enhance plant tolerance to pathogens.

Technical Abstract: BAK1 is a leucine-rich repeat receptor-like kinase (LRR-RLK) that functions as a co-receptor with the brassinosteroid (BR) receptor BRI1 and the flagellin receptor FLS2, and also functions as a negative regulator of programmed cell death. BAK1 has been shown to autophosphorylate on numerous serine/threonine sites in vitro as well as to transphosphorylate associated receptor kinases both in vitro and in planta. It was recently reported that BAK1 can also autophosphorylate on tyrosine residues and in the present study we identify Tyr-610 in the carboxy-terminal domain of BAK1 as a major site of autophosphorylation. Expression of BAK1(Y610F)-Flag in transgenic plants lacking the endogenous bak1 and its functional paralog, bkk1, produced plants that were viable but extremely small and generally resembled BR signaling mutants. An acidic substitution for Tyr-610 to mimic phosphorylation restored normal growth. Transcriptome analysis in plants expressing BAK1(Y610F)-Flag compared to wild type BAK1-Flag confirmed that many BR-regulated genes were not properly expressed. In addition, the basal expression of many defense genes was dramatically reduced in the Y610F directed mutant even though the FLS2-mediated inhibition of seedling growth by the flg22 elicitor occurred normally. These results indicate that phosphorylation of Tyr-610 is essential for BR signaling but not for the suppression of cell death. In addition, tyrosine phosphorylation enhances the BAK1-mediated expression of many genes involved in plant innate immunity. The demonstration that site-specific tyrosine phosphorylation is essential for BAK1-mediated signaling adds significantly to the emerging notion that tyrosine phosphorylation plays a broad role in plant receptor kinase signaling.