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	Figure 1. AKT2-MRH1 interaction in yeast.(a) Yeast 2-Hybrid interaction test between the cytosolic C-terminal fragment of AKT2 fused to the DNA-binding domain as bait (BD:CtAKT2) and the cytosolic C-terminal fragment of the MRH1 kinase, derived from the cDNA library screening, fused to the activation domain as prey (AD:CtMRH1). Yeast growth on a highly selective medium lacking adenine, histidine, tryptophan, and leucine, supplemented with an antibiotic (AbA), indicate expression of several reporter genes. No growth and thus no reporter gene expression is observed if as bait only the DNA-binding domain (BD) is co-expressed with the AD:CtMRH1 prey. Images were taken after 5 days of growth. 5 μl of mated cultures were plated after dilution and incubated at 30 °C. (b) Split Ubiquitin interaction assay between the full-length AKT2 channel fused to the synthetic PLV transcription factor and the C-terminal domain of ubiquitin (PLV:Cub:AKT2 in a bait vector pMetYC-gate) and the full-length MRH1 kinase fused to the modified N-terminal domain of ubiquitin (NubG:MRH1 in prey vector pNX32-GW and MRH1:NubG in prey vector pXN32-GW). Reporter-gene expression can be observed only in one configuration. No auto-activity of the bait was observed. Liquid cultures of the L40 strain harboring both vectors were grown until the mid-logarithmic phase and standardized to OD600 = 1.0. Following a 1:10 dilution, 5 μl were plated on a medium containing 5-Brom-4-chlor-3-indoxyl-β-D-galactopyranosid but lacking leucine, tryptophan, adenine and uracil. Images were taken after 7 days of incubation at 30 °C. Results shown in (a) were repeated once and in (b) are representative for 2 independent repeats.
	Figure 2. (a,b) Typical localization of AKT2-eGFP (green) and MRH1-mRFP (red) expressed under the control of the UBI10 promoter and (c) co-localization of both in Arabidopsis protoplasts derived from rosette leaves of 3–4 weeks old plants. Blue color corresponds to the auto-fluorescence signal from chlorophyll. Dashed squares indicate zoomed region presented in images below. Images represent single focal planes after 16h–20h post transformation. Shown experiments are representatives of at least 3 independent repeats.
	Figure 3. (a) FRET measurements in Arabidopsis protoplasts between AKT2:eGFP and MRH1:mRFP or mRFP, only (mean ± SD, n = 10). EF: mean transfer efficiency in a bleached region; CF: mean transfer efficiency in a non-bleached region. For the negative control (AKT2:eGFP and mRFP) no difference between EF and CF values was observed. In contrast, for the AKT2:eGFP + MRH1:mRFP pair the means were significantly different (p > 0.001, Welch two-sample t-test). (b–e) Co-expression of AKT2:eGFP and MRH1:mRFP. (b,c) Example of an AKT2:eGFP signal before and after bleaching of MRH1:mRFP. (d,e) Corresponding regions with signals coming only from MRH1:mRFP before and after bleaching. All experiments were performed on Col0 plants.
	Figure 4. (a) Representative image showing akt2-2, mrh1-1 and WT-Col0 plants grown under short day conditions (8 h light/16 h dark) in a glasshouse 10 weeks after sowing. (b) Stem length measurements of all three genotypes grown in a glasshouse in long day (16 h light/8 h dark) and short day conditions (8 h light/16 h dark). Measurements started with the first bolting plant and continued with a 5-day interval until the end of main stalk growth. Data are displayed as mean ± SD (n > 20).
	Figure 5. (a,b,c) Typical BiFC signals (yellow) recorded for MRH1:Nt-Venus + AKT2:Ct-Venus (a), AKT2:Nt-Venus + AKT2:Ct-Venus (b), and MRH1:Nt-Venus + MRH1:Ct-Venus (c) expressed in Xenopus leavis oocytes. Plasma membrane was stained with FM 4-64FX (red) prior imaging. Plots on the right represent signal distribution along the arrows for reconstituted Venus protein (BiFC, black) and the membrane marker (FM 4-64, red). (d) Quantitative analysis of the overlap of the BiFC signal with the membrane marker. In comparison to the AKT2:Nt-Venus + AKT2:Ct-Venus pair, the signal overlap of the MRH1:Nt-Venus + AKT2:Ct-Venus pair is significantly larger (P < 0.012, Student´s t-test) indicating an increased integration of the AKT2-protein into the plasma membrane in the presence of MRH1. The signal overlap of the MRH1:Nt-Venus + AKT2:Ct-Venus pair and the MRH1:Nt-Venus + MRH1:Ct-Venus pair is not significantly different. Data shown are representative or are mean ± SE for n ≥ 15.
	Figure 6. MRH1 has structural properties of a pseudokinase.(a,b) MRH1 is different from WNK-type kinases that established a strategy for recovery of catalytic activity. (a) Sequence comparison of motif I (G-loop) and motif II of MRH1, a typical (BRI1) and a WNK-type kinase (WNK3) from A. thaliana. The catalytic lysine in motif II is mutated in MRH1 and WNK3. In WNK-type kinases the third glycine of the G-loop (motif I) is replaced by a lysine. MRH1 does not contain a lysine in neither of the two motifs. (b) Recovery strategy of WNK-type kinases. Schematic representation of the catalytic lysine in typical and WNK-type kinases. Motif I is shown in yellow and motif II in blue. In WNK-type kinases the missing catalytic lysine is replaced by a lysine in motif I. (c,d) Homology model of MRH1 illustrates the absence of coordinating lysine residues in the catalytic center. Three motifs essential for catalytic kinase activity are highlighted in yellow (motif I, G-loop), blue (motif II, β3 strand) and green (motif VII, DFG motif). (c) In a typical kinase (BRI1, PDB entry 4OH4) a lysine residue (K911 in BRI1, blue) in motif II coordinates the ATP molecule in the catalytic center (highlighted in licorice representation). (d) In MRH1 there is no lysine residue in the vicinity of the potential catalytic center (enlarged view). All 16 lysine residues in the kinase domain of MRH1 are highlighted in purple licorice representation.

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