Two Conserved Amino Acids Characterized in the Island Domain Are Essential for the Biological Functions of Brassinolide Receptors

Abstract

Brassinosteroids (BRs) play important roles in plant growth and development, and BR perception is the pivotal process required to trigger BR signaling. In angiosperms, BR insensitive 1 (BRI1) is the essential BR receptor, because its mutants exhibit an extremely dwarf phenotype in Arabidopsis. Two other BR receptors, BRI1-like 1 (BRL1) and BRI1-like 3 (BRL3), are shown to be not indispensable. All BR receptors require an island domain (ID) responsible for BR perception. However, the biological functional significance of residues in the ID remains unknown. Based on the crystal structure and sequence alignments analysis of BR receptors, we identified two residues 597 and 599 of AtBRI1 that were highly conserved within a BR receptor but diversified among different BR receptors. Both of these residues are tyrosine in BRI1, while BRL1/BRL3 fixes two phenylalanines. The experimental findings revealed that, except BRI1Y597F and BRI1Y599F, substitutions of residues 597 and 599 with the remaining 18 amino acids differently impaired BR signaling and, surprisingly, BRI1Y599F showed a weaker phenotype than BRI1Y599 did, implying that these residues were the key sites to differentiate BR receptors from a non-BR receptor, and the essential BR receptor BRI1 from BRL1/3, which possibly results from positive selection via gain of function during evolution.

The brassinosteroid receptor geneBRI1safeguards cell-autonomous brassinosteroid signaling across tissues

“…Loss-of-function genetics has established BRI1 as the dominant Arabidopsis brassinosteroid receptor because it is essential for growth (Li and Chory, 1997, Li et al, 2022), whereas minor roles or specific functions were assigned to its homologs BRL1 and BRL3 (Cano-Delgado et al, 2004, Fabregas et al, 2018). In bri1 single mutants, BRL1 and BRL3 are presumably still active in stele tissues.…”

An update on evolutionary, structural, and functional studies of receptor-like kinases in plants

“…A recent study showed that two conserved amino acids Tyr597 and Tyr 599 are required by BRI1 for BR perception. Mutating these two amino acids in the ID abolished the BR signaling by knocking out or knocking down the phosphorylation activity of BES1 ( Li et al., 2022 ). Previous studies proved that the activation loop, αC helix, and αC-β4 loop are the most critical hotspots for kinase activation, and any abnormality in these regions can lead to the complete eradication of phosphorylation activity and cancer-related diseases in humans ( Taylor et al., 2012 ; Gogl et al., 2019 ; Yeung et al., 2020 ).…”