Reversible protein phosphorylation, catalysed by protein kinases, is the most widely studied post-translational modification (PTM), whereas the analysis of other modifications such as S-thiolation is in its relative infancy. In a yeast-two-hybrid (Y2H) screen, we identified a number of novel putative brassinosteroid insensitive 1 (BR1)-associated receptor-like kinase 1 (BAK1) interacting proteins including several proteins related to redox regulation. Glutaredoxin (GRX) C2 (AtGRXC2) was among candidate proteins identified in the Y2H screen and its interaction with recombinant Flag-BAK1 cytoplasmic domain was confirmed using an in vitro pull-down approach. We show that BAK1 peptide kinase activity is sensitive to the oxidizing agents H2O2 and diamide in vitro, suggesting that cysteine oxidation might contribute to control of BAK1 activity. Furthermore, BAK1 was glutathionylated and this reaction could occur via a thiolate-dependent reaction with GSSG or a H2O2-dependent reaction with GSH and inhibited kinase activity. Surprisingly, both reactions were catalysed by AtGRXC2 at lower concentrations of GSSG or GSH than reacted non-enzymatically. Using MALDI-TOF MS, we identified Cys353, Cys374 and Cys408 as potential sites of glutathionylation on the BAK1 cytoplasmic domain and directed mutagenesis suggests that Cys353 and Cys408 are major sites of GRXC2-mediated glutathionylation. Collectively, these results highlight the potential for redox control of BAK1 and demonstrate the ability of AtGRXC2 to catalyse protein glutathionylation, a function not previously described for any plant GRX. The present work presents a foundation for future studies of glutathionylation of plant receptor-like protein kinases (RLKs) as well as for the analysis of activities of plant GRXs.

Binding of brassinolide to the BRASSINOSTEROID-INSENSTIVE 1 (BRI1) receptor kinase promotes interaction with its co-receptor, BRI1-ASSOCIATED RECEPTOR KINASE 1 (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 may play a role. First, we demonstrate that the C-terminal domain is a strong inhibitor of the transphosphorylation 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. Thus, the C-terminal domain may play both a positive and negative role. Interestingly, a synthetic peptide corresponding to the full C-terminal domain (residues 576 to 615 of BAK1) interacted with recombinant BRI1 in vitro, and that interaction was enhanced by phosphorylation at the Tyr-610 site. Expression of a BAK1 C-terminal domain truncation (designated BAK1-ΔCT-Flag) in transgenic Arabidopsis plants lacking endogenous bak1 and its functional paralog, bkk1, produced plants that were wild type in appearance but much smaller than plants expressing full-length BAK1-Flag. The reduction in growth may be attributed to a partial inhibition of BR signaling in vivo as reflected in root growth assays but other factors are likely involved as well. Our working model is that in vivo, the inhibitory action of the C-terminal domain of BAK1 is relieved by binding to BRI1. However, that interaction is not essential for BR signaling, but other aspects of cellular signaling are impacted when the C-terminal domain is truncated and result in inhibition of growth. These results increase the molecular understanding of the C-terminal domain of BAK1 as a regulator of kinase activity that may serve as a model for other receptor kinases.