A Brassinosteroid-Hypersensitive Mutant of BAK1 Indicates That a Convergence of Photomorphogenic and Hormonal Signaling Modulates Phototropism

Whippo, Craig W.; Hangarter, Roger P.

doi:10.1104/pp.105.064444

Cited by 50 publications

(37 citation statements)

References 66 publications

(93 reference statements)

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“…Similarly, exogenous application of BR rescues the germination defects of severe GA biosynthesis/insensitive mutants in Arabidopsis (Steber and McCourt, 2001). Also, the Arabidopsis elongated mutant, which was discovered as a suppressor of a GA biosynthesis dwarf mutant (Halliday et al, 1996), was later found to carry a gain-of-function mutation in BAK1, a coreceptor for BRI1 (Whippo and Hangarter, 2005). Alternatively, the members of ATBS1/KIDARI/PRE1 might function redundantly in several different signaling pathways by regulating expression of a common For both (B) and (D), 10 mg of total RNAs of each sample were separated by agarose gel, stained with ethidium bromide for a loading control, transferred to nylon membrane, and hybridized with a 32 P-labeled AIF1 cDNA fragment.…”

Section: Discussionmentioning

confidence: 99%

Regulation ofArabidopsisBrassinosteroid Signaling by Atypical Basic Helix-Loop-Helix Proteins

et al. 2009

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Basic helix-loop-helix (bHLH) proteins are highly conserved transcription factors critical for cell proliferation and differentiation. Recent studies have implicated bHLH proteins in many plant signaling processes, including brassinosteroid (BR) signaling. Here, we report identification of two families of atypical bHLH proteins capable of modulating BR signaling. We found that activation-tagged bri1 suppressor 1-Dominant (atbs1-D), previously identified as a dominant suppressor of a weak BR receptor mutant bri1-301, was caused by overexpression of a 93-amino acid atypical bHLH protein lacking amino acids critical for DNA binding. Interestingly, atbs1-D only suppresses weak BR mutants, while overexpression of a truncated ATBS1 lacking the basic motif also rescues bri1-301, suggesting that ATBS1 likely stimulates BR signaling by sequestering negative BR signaling components. A yeast two-hybrid screen using ATBS1 as bait discovered four ATBS1-Interacting Factors (AIFs) that are members of another atypical bHLH protein subfamily. AIF1 exhibits an overlapping expression pattern with ATBS1 and its homologs and interacts with ATBS1 in vitro and in vivo. AIF1 overexpression nullifies the suppressive effect of atbs1-D on bri1-301 and results in dwarf transgenic plants resembling BR mutants. By contrast, silencing of AIF1 partially suppressed the bri1-301 phenotype. Our results suggested that plants use these atypical bHLH proteins to regulate BR signaling.

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Section: Discussionmentioning

confidence: 99%

Regulation ofArabidopsisBrassinosteroid Signaling by Atypical Basic Helix-Loop-Helix Proteins

et al. 2009

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“…A previously described mutation in BAK1, elg (elongated), was originally identified as a suppressor of the gibberellin biosynthesis mutant, ga4 (19). The elg mutation results in a substitution of an aspartic acid to an asparagine (D122N) in the third LRR of BAK1 (20) (Fig. 1A and Fig.…”

Section: Resultsmentioning

confidence: 99%

Extracellular leucine-rich repeats as a platform for receptor/coreceptor complex formation

Jaillais

Belkhadir

Balsemão-Pires

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

142

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Receptor kinases with leucine-rich repeat (LRR) extracellular domains form the largest family of receptors in plants. In the few cases for which there is mechanistic information, ligand binding in the extracellular domain often triggers the recruitment of a LRRcoreceptor kinase. The current model proposes that this recruitment is mediated by their respective kinase domains. Here, we show that the extracellular LRR domain of BRI1-ASSOCIATED KINASE1 (BAK1), a coreceptor involved in the disparate processes of cell surface steroid signaling and immunity in plants, is critical for its association with specific ligand-binding LRR-containing receptors. The LRRs of BAK1 thus serve as a platform for the molecular assembly of signalcompetent receptors. We propose that this mechanism represents a paradigm for LRR receptor activation in plants. -5). Ligand perception at the cell surface by either BRI1 or FLS2 induces the subsequent recruitment of BAK1 to a ligand-bound receptor complex (6-10). This process triggers transphosphorylation at multiple serines and threonines of the respective kinase domains inside the cell (11-13). Perhaps because BRI1 is a long-lived protein that apparently cycles between the plasma membrane and endosomes (14), there are multiple mechanisms to maintain the kinase domain in a basal state. BRI1 kinase is auto-inhibited by its C-terminal tail (15), by auto-phosphorylation on threonine 872 (11), and by a protein, BRI1 KINASE INHIBITOR 1 (BKI1), which associates with BRI1's kinase domain (10, 16). BKI1 inhibits BR signaling by binding to the BRI1's kinase domain, thereby inhibiting the interaction between BRI1-and BAK1-kinase (10, 16). Upon ligand binding, BRI1 phosphorylates BKI1 on a tyrosine within its membrane-targeting region, which dissociates BKI1 from the cell membrane and targets it to the cytoplasm, where it is inactive (10). Dissociation of BKI1 from BRI1 allows formation of a stable BRI1-BAK1 complex that is competent to induce downstream signaling (17).The interplay between BRI1 and BAK1 kinase domains is further regulated by BAK1 autophosphorylation on tyrosine 610 (tyr-610), which is required to stimulate BRI1 kinase activity in vitro and for proper BR signaling in vivo (18). Of note, BAK1 tyr-610 phosphorylation is not required for flagellin response and it is possible that tyr-610 phosphorylation might be involved in the proper interaction with its cognate receptors. However, tyr-610 mutations affect only BRI1 kinase activation but not its interaction with BRI1 intracellular domain (18). Therefore, a critical unanswered question is how ligand-bound LRR-RKs selectively recruit BAK1. Here, we report that the LRR domain of BAK1 is required for its recruitment to a ligand-bound LRR-RK and allows the kinase domains to be in physical contact for subsequent reciprocal transphosphorylation. Furthermore, our data indicate that the extracellular domain (ECD) of BAK1 is critical for the high affinity formation of the correct receptor/coreceptor pair.

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“…Previously identified elg mutant, which harbors a mutation in the extracellular region of BAK1, confers BR hypersensitivity [47]. The interaction between MSBP1 and the mutant BAK1 protein was weaker than WT protein ( Figure 1D), and the expression of BR biosynthesis genes was more severely suppressed by BL in the elg mutant ( Figure 3A).…”

Section: Overexpression Of Msbp1 Partially Suppresses Br Hypersensitimentioning

confidence: 94%

“…It was shown that the homeostasis of BR concentration was maintained through feedback regulation of the expression of multiple BR-biosynthetic genes [47][48][49]. Signaling-defective mutants, bri1 and bak1, showed reduced inhibition of transcription of BR-biosynthesis genes by BL [6,11].…”

Section: Enhanced Bak1 Expression Rescues the Suppressed Br Signalingmentioning

confidence: 99%

Membrane steroid-binding protein 1 (MSBP1) negatively regulates brassinosteroid signaling by enhancing the endocytosis of BAK1

et al. 2009

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Brassinosteroids (BRs) are perceived by transmembrane receptors and play vital roles in plant growth and development, as well as cell in responses to environmental stimuli. The transmembrane receptor BRI1 can directly bind to brassinolide (BL), and BAK1 interacts with BRI1 to enhance the BRI1-mediated BR signaling. Our previous studies indicated that a membrane steroid-binding protein 1 (MSBP1) could bind to BL in vitro and is negatively involved in BR signaling. To further elucidate the underlying mechanism, we here show that MSBP1 specifically interacts with the extracellular domain of BAK1 in vivo in a BL-independent manner. Suppressed cell expansion and BR responses by increased expression of MSBP1 can be recovered by overexpressing BAK1 or its intracellular kinase domain, suggesting that MSBP1 may suppress BR signaling through interacting with BAK1. Subcellular localization studies revealed that both MSBP1 and BAK1 are localized to plasma membrane and endocytic vesicles and MSBP1 accelerates BAK1 endocytosis, which results in suppressed BR signaling by shifting the equilibrium of BAK1 toward endosomes. Indeed, enhanced MSBP1 expression reduces the interaction between BRI1 and BAK1 in vivo, demonstrating that MSBP1 acts as a negative factor at an early step of the BR signaling pathway.

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A Brassinosteroid-Hypersensitive Mutant of BAK1 Indicates That a Convergence of Photomorphogenic and Hormonal Signaling Modulates Phototropism

Cited by 50 publications

References 66 publications

Regulation ofArabidopsisBrassinosteroid Signaling by Atypical Basic Helix-Loop-Helix Proteins

Regulation ofArabidopsisBrassinosteroid Signaling by Atypical Basic Helix-Loop-Helix Proteins

Extracellular leucine-rich repeats as a platform for receptor/coreceptor complex formation

Membrane steroid-binding protein 1 (MSBP1) negatively regulates brassinosteroid signaling by enhancing the endocytosis of BAK1

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