Kinase Function of Brassinosteroid Receptor Specified by Two Allosterically Regulated Subdomains

Area of Science:

• Plant molecular biology
• Cell signaling
• Biochemistry

Background:

• Transmembrane receptor-like kinases (RLKs) enable plants to adapt to environmental cues.
• RLKs possess variable extracellular domains for ligand perception and conserved intracellular kinase domains for signal transduction.
• The BRI1 family and EMS1 RLKs utilize the same signaling pathway, prompting investigation into BRI1 pathway specificity.

Purpose of the Study:

• To investigate how the brassinosteroid-insensitive 1 (BRI1) receptor-like kinase specifies its unique signaling pathway.
• To identify the specific domains within the BRI1 kinase domain responsible for its signaling function.
• To explore the evolutionary implications of kinase domain plasticity.

Main Methods:

• Construction and functional analysis of chimeric receptor-like kinases (RLKs) involving BRI1 and GASSHO1 (GSO1).
• Complementation assays in *bri1* mutants to assess the function of chimeric RLKs.
• Site-directed mutagenesis and *in vitro* kinase activity assays.

Main Results:

• The kinase domain of BRI1 (BRI1-KD) is not functionally interchangeable with GSO1-KD, as demonstrated by failed rescue of *bri1* mutants with BRI1-GSO1 chimeras.
• Two specific subdomains, S1 and S2, within the BRI1 kinase domain were identified as sufficient to confer BRI1 signaling specificity.
• Other subdomains of BRI1-KD are functionally robust and replaceable by GSO1 counterparts, exhibiting variable *in vitro* kinase activities.

Conclusions:

• Subdomains S1 and S2 are critical determinants of BRI1 signaling specificity.
• BRI1 specifies its kinase function through allosteric regulation involving the αC-β4 loop in S1 and the kinase activation loop in S2.
• This finding provides novel insights into kinase evolution and the mechanisms underlying RLK signaling specificity in plants.