Michael Hothorn
@structplantbio.bsky.social
Structural biologist interested in all things green. Professor at University of Geneva, Switzerland. https://web.structplantbio.org/
@fxrico.bsky.social deleted the single BSU1 gene in Marchantia, resulting in a mass of undifferentiated cells & strong patterning defects. Cell cycle markers are highly active in the thallus. Our data point to a function for Kelch phosphatases in the cell cycle not in BR signaling (6/6).
January 7, 2026 at 7:53 AM
@fxrico.bsky.social deleted the single BSU1 gene in Marchantia, resulting in a mass of undifferentiated cells & strong patterning defects. Cell cycle markers are highly active in the thallus. Our data point to a function for Kelch phosphatases in the cell cycle not in BR signaling (6/6).
In Andrea’s structure the C-terminal tail of BSU1 is phosphorylated and bound to the active site. Andrea and Pierre showed that this phosphorylation is caused by the CDK1-cyclin-CKS1 complex, very similar to PP1, where no structure is available. The phosphorylation inhibits BSU1 activity (5/6).
January 7, 2026 at 7:51 AM
In Andrea’s structure the C-terminal tail of BSU1 is phosphorylated and bound to the active site. Andrea and Pierre showed that this phosphorylation is caused by the CDK1-cyclin-CKS1 complex, very similar to PP1, where no structure is available. The phosphorylation inhibits BSU1 activity (5/6).
Oded and Felix Rico generated CRISPR/Cas9 knock-out mutants of all Kelch phosphatases in Arabidopsis. These mutants have a fertility defect, suggesting a function in embryo development, many more stomata than wild type, but wild type-like BR responses. (4/6)
January 7, 2026 at 7:50 AM
Oded and Felix Rico generated CRISPR/Cas9 knock-out mutants of all Kelch phosphatases in Arabidopsis. These mutants have a fertility defect, suggesting a function in embryo development, many more stomata than wild type, but wild type-like BR responses. (4/6)
BSU1 was originally identified in an activation tagging screen in the weak BR receptor mutant bri1-5. Oded Pri-Tal found, to our surprise, that also phosphatase-dead versions of the enzyme can rescue the bri1-5 mutant. A mutant in the RVxF substrate binding groove does not rescue. (3/6)
January 7, 2026 at 7:50 AM
BSU1 was originally identified in an activation tagging screen in the weak BR receptor mutant bri1-5. Oded Pri-Tal found, to our surprise, that also phosphatase-dead versions of the enzyme can rescue the bri1-5 mutant. A mutant in the RVxF substrate binding groove does not rescue. (3/6)
A crystal structure from Andrea Moretti of the Kelch phosphatase BSU1 reveals that it looks virtually identical to PP1, a Ser/Thr phosphatase involved in cell cycle regulation. Consistently, Pierre Raia could show that BSU1 is an efficient Ser/Thr, not a tyrosine phosphatase in vitro. (2/6)
January 7, 2026 at 7:49 AM
A crystal structure from Andrea Moretti of the Kelch phosphatase BSU1 reveals that it looks virtually identical to PP1, a Ser/Thr phosphatase involved in cell cycle regulation. Consistently, Pierre Raia could show that BSU1 is an efficient Ser/Thr, not a tyrosine phosphatase in vitro. (2/6)
My PhD supervisor Klaus Scheffzek (1958-2025) has passed away. tinyurl.com/3dcm73p6 Klaus was an expert structural biologist with degrees in physics and biology, an excellent scientist, a great teacher, and pianist. He made key contributions to G-protein signaling @embl.org @mpi-mr.bsky.social
December 12, 2025 at 12:16 PM
My PhD supervisor Klaus Scheffzek (1958-2025) has passed away. tinyurl.com/3dcm73p6 Klaus was an expert structural biologist with degrees in physics and biology, an excellent scientist, a great teacher, and pianist. He made key contributions to G-protein signaling @embl.org @mpi-mr.bsky.social
Nice work by the Mayer and @jessenlab.bsky.social labs out in @elife.bsky.social. Inositol pyrophosphates regulate the vacuolar inorganic polyphosphate and Pi pools, allowing Pi to be re-imported into the cytosol when cells experience phosphate starvation. elifesciences.org/articles/108...
December 5, 2025 at 2:31 PM
Nice work by the Mayer and @jessenlab.bsky.social labs out in @elife.bsky.social. Inositol pyrophosphates regulate the vacuolar inorganic polyphosphate and Pi pools, allowing Pi to be re-imported into the cytosol when cells experience phosphate starvation. elifesciences.org/articles/108...
This project has been a joint effort with the Fiedler and Panse labs, supported by an @snf-fns.ch Sinergia grant and the @erc.europa.eu. The work from my lab was done by PhD student Kristina Sturm, postdocs Oded Pri-Tal, @fxrico.bsky.social, @hmchen93.bsky.social and technician Larissa Broger (6/6).
November 29, 2025 at 7:27 PM
This project has been a joint effort with the Fiedler and Panse labs, supported by an @snf-fns.ch Sinergia grant and the @erc.europa.eu. The work from my lab was done by PhD student Kristina Sturm, postdocs Oded Pri-Tal, @fxrico.bsky.social, @hmchen93.bsky.social and technician Larissa Broger (6/6).
Interestingly, only the α-subunit, not the α2β2 CK2 holoenzyme, is regulated by PP-InsPs in vitro. Genetic analyses in Arabidopsis and Marchantia indicate that α-subunits can act independently of β-subunits, underscoring the physiological relevance of PP-InsP-mediated regulation of CK2. (5/6)
November 29, 2025 at 7:25 PM
Interestingly, only the α-subunit, not the α2β2 CK2 holoenzyme, is regulated by PP-InsPs in vitro. Genetic analyses in Arabidopsis and Marchantia indicate that α-subunits can act independently of β-subunits, underscoring the physiological relevance of PP-InsP-mediated regulation of CK2. (5/6)
Mutations in the CK2 PP-InsP binding site alter flowering time in Arabidopsis. In parallel, the Panse lab showed that this site is essential for CK2 function in yeast (4/6).
November 29, 2025 at 7:23 PM
Mutations in the CK2 PP-InsP binding site alter flowering time in Arabidopsis. In parallel, the Panse lab showed that this site is essential for CK2 function in yeast (4/6).
Notably, we found the α-subunit of casein kinase 2 (CK2) interact with PP-InsPs and with the PP-InsP kinase VIH2. CK2 binds InsP6/7/8 with micromolar affinity. PP-InsP binding reduces substrate phosphorylation by occupying the kinase’s basic substrate-binding surface (3/6).
November 29, 2025 at 7:19 PM
Notably, we found the α-subunit of casein kinase 2 (CK2) interact with PP-InsPs and with the PP-InsP kinase VIH2. CK2 binds InsP6/7/8 with micromolar affinity. PP-InsP binding reduces substrate phosphorylation by occupying the kinase’s basic substrate-binding surface (3/6).
The first major surprise was the prevalence of deeply conserved protein complexes among the identified interactors, including the spliceosome, the ribosome, the CFlm complex, and numerous signaling proteins such as protein kinases. (2/6).
November 29, 2025 at 7:16 PM
The first major surprise was the prevalence of deeply conserved protein complexes among the identified interactors, including the spliceosome, the ribosome, the CFlm complex, and numerous signaling proteins such as protein kinases. (2/6).
Very nice application of our constitutive active BIR3-RK chimera from @hohmannulrich.bsky.social (doi.org/10.1105/tpc....) to dissect the contribution of ERECTA to vascular differentiation. doi.org/10.1073/pnas.... Congrats to @lauraragni.bsky.social @bertderybel.bsky.social @bayerlab.bsky.social
November 21, 2025 at 6:16 PM
Very nice application of our constitutive active BIR3-RK chimera from @hohmannulrich.bsky.social (doi.org/10.1105/tpc....) to dissect the contribution of ERECTA to vascular differentiation. doi.org/10.1073/pnas.... Congrats to @lauraragni.bsky.social @bertderybel.bsky.social @bayerlab.bsky.social
By the @cellsensing.bsky.social and Zipfel labs: The plant receptor kinase HSL3 senses a cyclic, disulfide-bond stabilized peptide phytocytokine. www.biorxiv.org/content/10.1...
October 25, 2025 at 7:19 AM
By the @cellsensing.bsky.social and Zipfel labs: The plant receptor kinase HSL3 senses a cyclic, disulfide-bond stabilized peptide phytocytokine. www.biorxiv.org/content/10.1...
A lot of pressure on the different grant schemes, and pretty much independent of career stage.
October 17, 2025 at 10:20 AM
A lot of pressure on the different grant schemes, and pretty much independent of career stage.
Amazing work by the Santiago lab @unil.bsky.social. The malectin-LRR receptor kinase IGP1 senses cello-oligomers to alert the plant immune system & enhance disease resistance. Very nice discovery & mechanism.
www.biorxiv.org/content/10.1...
www.biorxiv.org/content/10.1...
September 30, 2025 at 2:18 PM
Amazing work by the Santiago lab @unil.bsky.social. The malectin-LRR receptor kinase IGP1 senses cello-oligomers to alert the plant immune system & enhance disease resistance. Very nice discovery & mechanism.
www.biorxiv.org/content/10.1...
www.biorxiv.org/content/10.1...
Today’s Department seminar by @luciastrader.bsky.social @salkinstitute.bsky.social on auxin transcription factors.
September 3, 2025 at 9:21 AM
Today’s Department seminar by @luciastrader.bsky.social @salkinstitute.bsky.social on auxin transcription factors.
Complete transport cycle of human XPR1, regulated by InsP8 and KIDINS220 @cp-molcell.bsky.social doi.org/10.1016/j.mo...
August 26, 2025 at 11:36 AM
Complete transport cycle of human XPR1, regulated by InsP8 and KIDINS220 @cp-molcell.bsky.social doi.org/10.1016/j.mo...
Unexpected hit in the list of the most highly cited papers of the 21st century.
August 25, 2025 at 12:19 PM
Unexpected hit in the list of the most highly cited papers of the 21st century.
Cool stuff, discovery of protein oliogophosphorylation by the Fiedler lab @fmp-berlin.de @natchem.nature.com. www.nature.com/articles/s41... Now what if NME1 is indeed the missing eukaryotic inorganic polyphosphate kinase?
August 25, 2025 at 9:46 AM
Cool stuff, discovery of protein oliogophosphorylation by the Fiedler lab @fmp-berlin.de @natchem.nature.com. www.nature.com/articles/s41... Now what if NME1 is indeed the missing eukaryotic inorganic polyphosphate kinase?
Very nice cryo-EM structure of rice PHT1 phosphate transporter by Zhu Liu's lab in Wuhan. www.biorxiv.org/content/10.1...
August 11, 2025 at 11:59 AM
Very nice cryo-EM structure of rice PHT1 phosphate transporter by Zhu Liu's lab in Wuhan. www.biorxiv.org/content/10.1...
But perhaps the nicest finding from
Alberto was that the exact positioning of the ligand in the receptor is absolutely critical for co-receptor recognition. Based on this we can now accurately predict how a bioactive BR needs to look like, and how receptor antagonists can be designed.
Alberto was that the exact positioning of the ligand in the receptor is absolutely critical for co-receptor recognition. Based on this we can now accurately predict how a bioactive BR needs to look like, and how receptor antagonists can be designed.
August 9, 2025 at 9:13 PM
But perhaps the nicest finding from
Alberto was that the exact positioning of the ligand in the receptor is absolutely critical for co-receptor recognition. Based on this we can now accurately predict how a bioactive BR needs to look like, and how receptor antagonists can be designed.
Alberto was that the exact positioning of the ligand in the receptor is absolutely critical for co-receptor recognition. Based on this we can now accurately predict how a bioactive BR needs to look like, and how receptor antagonists can be designed.
We also found that BRL2 actually is a bona fide BR receptor with a distinct ligand spectrum.
August 9, 2025 at 9:10 PM
We also found that BRL2 actually is a bona fide BR receptor with a distinct ligand spectrum.
Next, we mutated the entire hormone binding pocket in BRI1 and assayed the mutants for BR binding and their ability to rescue a bri1 null mutant. It turns out the binding site has amazing structural plasticity with only few contacts being critical.
August 9, 2025 at 9:09 PM
Next, we mutated the entire hormone binding pocket in BRI1 and assayed the mutants for BR binding and their ability to rescue a bri1 null mutant. It turns out the binding site has amazing structural plasticity with only few contacts being critical.