Submitted to: Frontiers in Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/14/2014
Publication Date: 4/4/2014
Citation: Oh, M., Wu, X., Kim, S.Y., Clouse, S.D., Huber, S.C. 2014. The Carboxy-terminus of BAK1 regulates kinase activity and is required for normal growth of Arabidopsis. Frontiers in Plant Physiology. DOI: 10.3389/fpls.2014.00016. Interpretive Summary: Receptor kinases are proteins embedded in the cell outer (plasma) membrane that transmit signals from the outside of the cell to the interior, without the signal itself entering the cell. The signals are often molecules (ligands) that bind to extracellular portion of the receptor kinase. In plants, this binding induces the receptor kinase to interact with a specific co-receptor kinase, which then results in their mutual activation by a process involving protein phosphorylation. However, details of that activation process are not clear. In the present study, we determined that specific regions of the two proteins may bind to one another, thereby insuring proper physical proximity. In studies with the brassinosteroid hormone receptor kinase, known as BRI1, and its co-receptor kinase, known as BAK1, we determined that the carboxy-terminal polypeptide of BAK1 may be playing a particularly important role. Our working model is that the carboxy-terminal polypeptide of BAK1 inhibits its protein kinase activity, and that inhibition is relieved when the carboxy-terminal polypeptide binds to BRI1. As a result, the two proteins are properly positioned so that BAK1 can phosphorylate and activate BRI1. These results increase our understanding of how plant receptor kinases function, and may provide a general model for receptor:co-receptor interactions. Understanding these molecular interactions may also provide novel approaches to engineer signaling pathways in plants.
Technical Abstract: In brassinosteroid (BR) signaling, binding of brassinolide to the BRI1 receptor kinase promotes interaction with its co-receptor, BAK1. Juxtaposition of the kinase domains that occurs then allows reciprocal transphosphorylation and activation of both kinases, but details of that process are not entirely clear. In the present study we show that the carboxy (C) - terminal polypeptide of BAK1 plays an important role. First, we demonstrate that the C-terminal domain is an inhibitor of both the auto- and trans-phosphorylation activity of the recombinant BAK1 cytoplasmic domain protein. However, recombinant BAK1 lacking the C-terminal domain is unable to transactivate the peptide kinase activity of BRI1 in vitro. Accordingly, expression of a BAK1 truncation missing the C-terminal domain (designated BAK1-'CT-Flag) in transgenic plants lacking endogenous bak1 and its functional paralog, bkk1, produced viable plants that were substantially smaller than plants expressing full-length BAK1-Flag. The positive role played by the C-terminal domain may involve binding to BRI1, because a synthetic peptide based on the BAK1 C-terminal sequence interacted with recombinant BRI1 in vitro. Our current working model is that while phosphorylation of serine and threonine residues within the activation loop of BAK1 is a prerequisite for kinase activity, the ability of BAK1 to transphosphorylate BRI1 or downstream protein components requires relief of the autoinhibitory effect of the C-terminal domain and that likely happens by its binding to BRI1. These results increase the molecular understanding of how juxtaposed kinase domains activate one another, and may provide a general model for receptor:co-receptor interactions.