Submitted to: Plant Signaling and Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2010
Publication Date: 3/1/2011
Citation: Oh, M., Wu, X., Clouse, S.D., Huber, S.C. 2011. Functional importance of EAK1 tyrosine phosphorylation in vivo. Plant Signaling and Behavior. 6:400-405. Interpretive Summary: Plant receptor kinases function in a diverse range of processes ranging from control of plant growth and development to response to environmental stress and pathogens. They transmit signals from the outside of the cell to the nucleus, where gene expression is altered in a manner that is appropriate to the original stimulus. However, the functions of the vast majority of the receptor kinases are unknown, as are many of the details involved in the signal transduction process itself. It is well known, however, that autophosphorylation of the receptor kinases often plays an important role in signaling, and one of the emerging concepts is that phosphorylation of tyrosine residue(s) occurs in addition to the well-recognized role for phosphorylation of serine and threonine residues. Tyrosine autophosphorylation was documented recently with the receptor kinase referred to as BAK1, which provides the second example of a plant receptor kinase regulated in this manner. BAK1 is of special interest because it participates as co-receptor in multiple signaling pathways and tyrosine phosphorylation may distinguish those functions. We discuss the two known examples of plant receptor kinases where tyrosine phosphorylation has been shown to occur and compare the tyrosine residues within their cytoplasmic domains that are conserved or have been shown to be essential for kinase activity. In addition, we show for the first time that binding of BAK1 to another receptor kinase, known as BRI1, is strictly dependent on divalent cations in vitro, which uncovers another factor that may regulate receptor kinase function in vivo. Impact: Tyrosine phosphorylation of plant receptor kinases is an emerging concept in the area of plant signal transduction. 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. These results may ultimately provide new approaches to enhance plant productivity. A further and unexpected finding is that the initial binding of a receptor kinase with its coreceptor may be dependent on divalent cations via allosteric effects within the kinase domains. This forms the foundation for future studies to identify potential receptor kinase signaling partners and identifies yet another factor that may regulate signaling in vivo.
Technical Abstract: The plant receptor kinase BRASSINOSTEROID ASSOCIATED KINASE 1 (BAK1) is known as a partner of several ligand-binding leucine-rich repeat receptor kinases, including BRASSINOSTEROID INSENSITIVE 1 (BRI1) and the flagellin receptor FLS2. Autophosphorylation of receptor kinases is recognized to be an important process in receptor kinase signaling, and at least with the recombinant protein, BAK1 was shown to autophosphorylate on Tyr residues in addition to numerous Ser/Thr residues documented previously. We recently identified Tyr-610 in the carboxy-terminal domain of BAK1 as a major site of autophosphorylation and showed that phosphorylation of this residue is essential for at least some functions of BAK1 in vivo. In particular, the function of BAK1 as co-receptor with BRI1 in brassinosteroid (BR) signaling is impaired in transgenic plants expressing the BAK1(Y610F)-Flag directed mutant. Recombinant cytoplasmic domains of BRI1 and BAK1 interact and transphosphorylate each other in vitro in a manner that mimics their interaction in vivo; while BAK1(Y610F) binds normally to BRI1 its ability to transphosphorylate and activate the kinase domain of BRI1 is severely compromised. To further elaborate on this earlier model, we present additional results showing that the interaction between BAK1 and BRI1 in vitro is Mg2+ dependent, suggesting that cytosolic [Mg2+] may play some role in receptor kinase signaling in vivo. We also compare the primary structures of BRI1 and BAK1 in terms of the occurrence of Tyr residues in the cytoplasmic domain, and identify differences in which residues are essential for kinase activity. Finally, transgenic plants expressing the BAK1(Y610F) directed mutant have alterations in the transcriptome that extend beyond the genes that are BR regulated in nontransgenic plants. In particular, the basal expression of many defense genes is significantly reduced in Y610F plants, which uncovers an expanded function for BAK1 in regulation of pathogen responses. Collectively, the results establish a site-specific role for Tyr phosphorylation of BAK1 in BR signaling and regulation of plant defense mechanisms, which may have implications for enhancing agricultural productivity.