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Review
. 2020 Jun 18;21(12):4340.
doi: 10.3390/ijms21124340.

Regulation of Three Key Kinases of Brassinosteroid Signaling Pathway

Juan Mao  1   2 , Jianming Li  1   2   3
Affiliations
Review

Regulation of Three Key Kinases of Brassinosteroid Signaling Pathway

Juan Mao et al. Int J Mol Sci. .

Abstract

Brassinosteroids (BRs) are important plant growth hormones that regulate a wide range of plant growth and developmental processes. The BR signals are perceived by two cell surface-localized receptor kinases, Brassinosteroid-Insensitive1 (BRI1) and BRI1-Associated receptor Kinase (BAK1), and reach the nucleus through two master transcription factors, bri1-EMS suppressor1 (BES1) and Brassinazole-resistant1 (BZR1). The intracellular transmission of the BR signals from BRI1/BAK1 to BES1/BZR1 is inhibited by a constitutively active kinase Brassinosteroid-Insensitive2 (BIN2) that phosphorylates and negatively regulates BES1/BZR1. Since their initial discoveries, further studies have revealed a plethora of biochemical and cellular mechanisms that regulate their protein abundance, subcellular localizations, and signaling activities. In this review, we provide a critical analysis of the current literature concerning activation, inactivation, and other regulatory mechanisms of three key kinases of the BR signaling cascade, BRI1, BAK1, and BIN2, and discuss some unresolved controversies and outstanding questions that require further investigation.

Keywords: GSK3-like kinases; brassinosteroids; protein phosphatases; receptor-like kinases; somatic embryogenesis receptor-like kinases.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A current model of BR signaling. When BRs (brassinosteroids) are absent (left), BRI1 (Brassinosteroid-Insensitive1) is kept inactive by its autoinhibitory C-terminus and BKI1 (BRI1 Kinase Inhibitor1) association. BIN2 (Brassinosteroid-Insensitive2) is constitutively active and phosphorylates BES1 (bri1-EMS suppressor1)/BZR1 (Brassinazole-resistant1) transcription factors to promote their degradation and 14-3-3-mediated cytosolic retention, and to directly inhibit their DNA-binding activities. When BRs are present (right) and bind to the extracellular domains of BRI1 and its co-receptor BAK1 (BRI1 Associated receptor Kinase1) to activate the two receptor kinases, leading to dissociation of BKI1 from BRI1, phosphorylation and activation of BSKs (BR-signaling kinases)/CDGs (Constitutive Differential Growth) and BSU1 (bri1 suppressor1). The activated BSU1 dephosphorylates and inactivates BIN2 while KIB1 (Kink suppressed in bzr1-1D1) promotes BIN2 degradation, causing nuclear accumulation of PP2A (protein phosphatase 2A)-dephosphorylated BES1/BZR1 that bind BRRE (BR response element)/E-box-containing promoters to regulate expression of thousands of BR-responsive genes important for plant growth and development.
Figure 2
Figure 2
A structural model of the activated BRI1 kinase domain. Shown here are three (0°, 60°, and 180°) rotational views of a rainbow-colored ribbon model of the crystal structure of the BRI1 kinase domain (Protein Data Bank No. 4oh4). Individual α-helices (αB-αI) and β-strands (β1-β5) were labelled. The magenta-colored sticks indicate phosphorylated Ser/Thr residues with the magenta dots surrounding Ser1044 and Thr1049 of the activation segment, the green sticks denote the Lys911 and Glu927 residues that form the salt bridge between the β3-strand and αC-helix, the red sticks mark the three positively charged residues of the phosphate-binding pocket, and the orange sticks represent the phosphorylated Tyr residue. The red spheres show adenylyl-imidodiphosphate (a non-hydrolysable ATP analog), the orange spheres indicate the gatekeeper Tyr956 residue that is also phosphorylated in in vitro assays, and the light-blue spheres denote the five regulatory-spine (R-spine) residues (from lower to upper: Asp1068, His1077, Phe1028, Ile931, and Leu942). The rainbow bar indicates the order of amino acids (AAs) from the N-terminus (blue) to the C-terminus (red).
Figure 3
Figure 3
A current model of regulating the BRI1 abundance on the plasma membrane (PM). Newly synthesized BRI1 is translocated through the Sec61 translocon from the ER (endoplasmic reticulum)-associated ribosomes into the ER where it undergoes chaperone-assisted protein folding. Correctly folded BRI1 is exported into the Golgi to continue its secretory journey through the TGN (trans-Golgi network)/EE (early endosome) to be targeted on the PM (1), whereas the incorrectly/misfolded BRI1, such as its mutant variants bri1-5 and bri1-9, are retained in the ER by ERQC (ER quality control) for refolding and degradation via ERAD (ER-associated degradation) that involves retrotranslocation and cytosolic proteasome. The PM-localized BRI1 or BRI1/BAK1 heterodimer is presumably ubiquitinated by the PUB12/13 (plant U-box protein12/13) E3 ligases and undergoes constitutive CME (clathrin-mediated endocytosis)-mediated internalization (2) or ligand-induced CIE (clathrin-independent endocytosis)-mediated internalization (3). The endocytosed BRI1 at the TGN/EE could be packaged into ILVs (intraluminal vesicles) and delivered to the vacuole for degradation via ESCRT (endosomal sorting complex required for transport)-mediated biogenesis of MVBs (multivesicular bodies) and eventual MVB-vacuole fusion (4). Alternatively, the TGN/EE-localized BRI1 can be recycled back to the PM via retromer-mediated cargo selection, microtubule-assisted vesicle trafficking, and exocyst/SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor)-involved exocytosis (5). The circled numbers indicate different secretion/trafficking routes. SNX1 (sorting nexin1) is one of the core retromer subunits, CLASP (cytoplasmic linker-associated protein) is a microtubule-associated protein, and ALIX (the Arabidopsis homolog of apoptosis-linked gene 2-interacting protein X) is a cytosolic ESCRT-associated protein.
Figure 4
Figure 4
A crystal structure of the BAK1 kinase domain. Shown here are three rotation (0°, 60°, and 180°) views of a rainbow-colored ribbon model of the crystal structure of the BAK1 kinase domain (Protein Data Bank No: 3uim). Individual α-helices (αB-αI) and β-strands (β1-β5) were labelled. The purple sticks indicate phosphorylated Ser/Thr residues with the Thr450 and Thr455 residues surrounded with purple dots. The red sticks show the bound adenylyl-imidodiphosphate (a non-hydrolysable ATP analog), the light-orange sticks denote the two residues that form the conserved salt bridge between the β3-strand and the αC-helix, the blue sticks represent the three positively charged residues that make up the phosphate-binding pocket, and the dark-orange sticks indicate phosphorylated Tyr residues. The pink spheres mark the R-spine residues, the red spheres show the two Cys residues that were S-glutathionylated in vitro, the orange spheres denote the gatekeeper Tyr residue, and the green spheres designate the Asp287 residue important for Ca2+-dependent BAK1 cleavage. The rainbow bar indicates the order of AAs from the N-terminus (blue) to the C-terminus (red).
Figure 5
Figure 5
A structural model of BIN2. Shown here are two rotational (0° and 60°) views of a modeled BIN2 structure obtained at SWISS-MODEL (https://swissmodel.expasy.org/). Individual β-strands (β1-β5) and α-helices (αC-αH) were labeled. The two extra antiparallel-β-strands that cap the N-lobe are not labelled. The TREE motif and the Pro284 residue mutated in known bin2/dwarf12/ucu1 alleles are indicated with grey-colored sticks. The magenta sticks show the three positively charged residues [Arg80, Arg164, and Lys189 (also acetylated)] of the conserved phosphate-binding pocket, the red sticks with red dots denote the Tyr200 residue, the green sticks with green dots mark the Lys69-Glu81 salt bridge between the β3-strand and the αC-helix, and the blue sticks represent the Lys167 residue that corresponds to the acetylated Lys183 residue of the human GSK3β. The red spheres show the Cys residues that were shown to be S-nitrosylated or S-glutathionylated in vitro, the orange spheres indicate the gatekeeper Met115 residue, and the pink spheres mark the 5 R-spine residues (from lower to top: Asp223, His163, Phe185, Met85, and Leu96). Important residues are also labelled with single letter codes with positions. The rainbow bar indicates the order of AAs from the N-terminus (blue) to the C-terminus (red).
Figure 6
Figure 6
Comparison between a structure model of BSU1’s phosphatase domain with crystal structures of PSP (protein Ser/Thr phosphatase) and PTP (protein tyrosine phosphatase). Shown here are a predicted structure of the BSU1’s C-terminal phosphatase domain [obtained at SWISS-MODEL at https://swissmodel.expasy.org/ using the protein sequence of BSU1 (accession No: NP_171844)], a crystal structure of a human PSP (HsPP1; Protein Data Bank No. 4moy), and a crystal structure of a human PTP (HsPTPN5 for the human PTP non-receptor type 5; Protein Data Bank No. 2cjz). The conserved metal-coordinating amino acid residues are indicated by colored sticks: orange colored Asp(D)510, Asp(D)544, and Asn(N)576, and three magenta-colored His(H) residues. The three dotted white lines separate the β-sandwich of two β-sheets from the two flanking α-helix domains in the structure models of BSU1 and PP1. The dotted spheres and the red sticks in PP1 represent two metal ions and the phosphate, respectively. The two conserved residues (Cys and Arg of the HCX5R catalytic signature motif) are colored with magenta in the human PTPN5 structure and a phosphorylated tyrosine is shown with blue sticks. The rainbow bar indicates the order of AAs from the N-terminus (blue) to the C-terminus (red) of each peptide.
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References

    1. Clouse S.D., Sasse J.M. Brassinosteroids: Essential regulators of plant growth and development. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1998;49:427–451. doi: 10.1146/annurev.arplant.49.1.427. - DOI - PubMed
    1. Wang Z.Y., Bai M.Y., Oh E., Zhu J.Y. Brassinosteroid signaling network and regulation of photomorphogenesis. Annu. Rev. Genet. 2012;46:701–724. doi: 10.1146/annurev-genet-102209-163450. - DOI - PubMed
    1. Guo H., Li L., Aluru M., Aluru S., Yin Y. Mechanisms and networks for brassinosteroid regulated gene expression. Curr. Opin. Plant Biol. 2013;16:545–553. doi: 10.1016/j.pbi.2013.08.002. - DOI - PubMed
    1. Yang C.J., Zhang C., Lu Y.N., Jin J.Q., Wang X.L. The mechanisms of brassinosteroids’ action: From signal transduction to plant development. Mol. Plant. 2011;4:588–600. doi: 10.1093/mp/ssr020. - DOI - PubMed
    1. Nawaz F., Naeem M., Zulfiqar B., Akram A., Ashraf M.Y., Raheel M., Shabbir R.N., Hussain R.A., Anwar I., Aurangzaib M. Understanding brassinosteroid-regulated mechanisms to improve stress tolerance in plants: A critical review. Environ. Sci. Pollut. Res. Int. 2017;24:15959–15975. doi: 10.1007/s11356-017-9163-6. - DOI - PubMed

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