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Title: Calcium/calmodulin inhibition of the BRI1 receptor kinase provides a possible link between calcium- and brassinosteroid-signaling

item OH, MAN-HO - University Of Illinois
item KIM, HYOUNG SEOK - University Of Illinois
item WU, XIA - University Of Illinois
item CLOUSE, STEVEN - North Carolina State University
item ZIELINSKI, RAYMOND - University Of Illinois
item Huber, Steven

Submitted to: Biochemical Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2012
Publication Date: 4/15/2012
Citation: Oh, M., Kim, H., Wu, X., Clouse, S.D., Zielinski, R.E., Huber, S.C. 2012. Calcium/calmodulin inhibition of the BRI1 receptor kinase provides a possible link between calcium- and brassinosteroid-signaling. Biochemical Journal. 443:515-223.

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 regulate receptor kinase activity. In the present study we identify a new mechanism that inhibits the brassinosteroid receptor kinase, known as BRI1. The new mechanism involves binding of calmodulin, which is a highly conserved protein in eukaryotes that functions as a calcium sensor. When the concentration of free calcium in the cell is elevated (as occurs in response to stress), calmodulin becomes activated and binds to a variety of cellular target proteins. We show that the BRI1 receptor kinase is one of those targets, and identify a possible functional consequence as calmodulin binding inhibits the kinase activity of BRI1. We also establish a new assay system to monitor the kinase activity of BRI1 that should be useful in studies of other receptor kinases. The results establish a possible link between calcium signaling and brassinosteroid hormone action. This is important because controlling regulatory mechanisms may provide new approaches to alter receptor kinase function and improve plant productivity.

Technical Abstract: The receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1) is a key component in brassinosteroid (BR) perception and signaling transduction, which has broad impacts on plant growth and development. In the present study, we demonstrate that Arabidopsis calmodulin (CaM) binds to the recombinant cytoplasmic domain of BRI1 in a Ca2+-dependent manner in vitro. In silico analysis predicted binding to Helix E of the BRI1 kinase subdomain VIa and a synthetic peptide based on this sequence interacted with Ca2+/ CaM. Co-expression of CaM with the cytoplasmic domain of BRI1 in E. coli strongly reduced autophosphorylation of BRI1, in particular on Tyr residues, and also reduced the BRI1-mediated transphosphorylation of E. coli proteins on tyrosine and threonine (and presumably serine) residues. Several isoforms of CaM and CaM-like proteins (CML) were more effective (AtCaM6, AtCaM7 and AtCML8) than others (AtCaM2, AtCaM4 and AtCML11) when co-expressed with BRI1 in E. coli. These results establish a novel assay for recombinant BRI1 transphosphorylation activity and collectively uncover a possible new link between Ca2+- and BR-signaling.