Abstract
Plants acquire the ability to adapt to the environment using transmembrane receptor-like kinases (RLKs) to sense the challenges from their surroundings and respond appropriately. RLKs perceive a variety of ligands through their variable extracellular domains (ECDs) that activate the highly conserved intracellular kinase domains (KDs) to control distinct biological functions through a well-developed downstream signaling cascade. A new study has emerged that brassinosteroid-insensitive 1 (BRI1) family and excess microsporocytes 1 (EMS1) but not GASSHO1 (GSO1) and other RLKs control distinct biological functions through the same signaling pathway, raising a question how the signaling pathway represented by BRI1 is specified. Here, we confirm that BRI1-KD is not functionally replaceable by GSO1-KD since the chimeric BRI1-GSO1 cannot rescue bri1 mutants. We then identify two subdomains S1 and S2. BRI1 with its S1 and S2 substituted by that of GSO1 cannot rescue bri1 mutants. Conversely, chimeric BRI1-GSO1 with its S1 and S2 substituted by that of BRI1 can rescue bri1 mutants, suggesting that S1 and S2 are the sufficient requirements to specify the signaling function of BRI1. Consequently, all the other subdomains in the KD of BRI1 are functionally replaceable by that of GSO1 although the in vitro kinase activities vary after replacements, suggesting their functional robustness and mutational plasticity with diverse kinase activity. Interestingly, S1 contains αC-β4 loop as an allosteric hotspot and S2 includes kinase activation loop, proposedly regulating kinase activities. Further analysis reveals that this specific function requires β4 and β5 in addition to αC-β4 loop in S1. We, therefore, suggest that BRI1 specifies its kinase function through an allosteric regulation of these two subdomains to control its distinct biological functions, providing a new insight into the kinase evolution.
Highlights
All living organisms sense and transduce signals through cell surface receptors to respond to the various challenges from the environment
A typical Receptor-like protein kinases (RLKs) consists of three distinct functional domains: N-terminal extracellular domain (ECD) that binds a ligand, a transmembrane domain (TM) that anchors the protein within the membrane, and C-terminal intracellular kinase domain (KD) that transduces the signal downstream with serine–threonine–tyrosine specificity (Shiu and Bleecker, 2001a,b)
Using the domain-swap approach, here, we report that the chimeric receptor of brassinosteroid-insensitive 1 (BRI1) ECD fused GSO1-KD (BRI1-GSO1) cannot activate the intracellular signaling cascade shared with BRI1 since it was not able to rescue bri1-301 mutant phenotypes
Summary
All living organisms sense and transduce signals through cell surface receptors to respond to the various challenges from the environment. Receptor-like protein kinases (RLKs) are one of the most important and largest groups of transmembrane cell surface receptors in plants, which have more than 600 members in Arabidopsis alone, playing a fundamental role in intracellular and extracellular communications (Walker and Zhang, 1990; Walker, 1993; Shiu and Bleecker, 2001a). A typical RLK consists of three distinct functional domains: N-terminal extracellular domain (ECD) that binds a ligand, a transmembrane domain (TM) that anchors the protein within the membrane, and C-terminal intracellular kinase domain (KD) that transduces the signal downstream with serine–threonine–tyrosine specificity (Shiu and Bleecker, 2001a,b). Based on their sequence similarities, expression profiles, biological functions, and interactions with other protein molecules, around 89 LRR-RLKs have been designated so far, and around 60 of them are functionally characterized (Wu et al, 2016)
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