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Effective and sustained inhibition of non-enzymatic oncogenic driver proteins is a major pharmacological challenge. The clinical success of thalidomide analogues demonstrates the therapeutic efficacy of drug-induced degradation of transcription factors and other cancer targets 1 – 3 , but a substantial subset of proteins are resistant to targeted degradation using existing approaches 4 , 5 . Here we report an alternative mechanism of targeted protein degradation, in which a small molecule induces the highly specific, reversible polymerization of a target protein, followed by its sequestration into cellular foci and subsequent degradation. BI-3802 is a small molecule that binds to the Broad-complex, Tramtrack and Bric-à-brac (BTB) domain of the oncogenic transcription factor B cell lymphoma 6 (BCL6) and leads to the proteasomal degradation of BCL6 6 . We use cryo-electron microscopy to reveal how the solvent-exposed moiety of a BCL6-binding molecule contributes to a composite ligand–protein surface that engages BCL6 homodimers to form a supramolecular structure. Drug-induced formation of BCL6 filaments facilitates ubiquitination by the SIAH1 E3 ubiquitin ligase. Our findings demonstrate that a small molecule such as BI-3802 can induce polymerization coupled to highly specific protein degradation, which in the case of BCL6 leads to increased pharmacological activity compared to the effects induced by other BCL6 inhibitors. These findings open new avenues for the development of therapeutic agents and synthetic biology. Binding of the small molecule BI-3802 to the oncogenic transcription factor B cell lymphoma 6 (BCL6) induces polymerization of BCL6, leading to its ubiquitination by SIAH1 and proteasomal degradation.

Acetyl-CoA carboxylase catalyses the ATP-dependent carboxylation of acetyl-CoA, a rate-limiting step in fatty acid biosynthesis 1 , 2 . Eukaryotic acetyl-CoA carboxylases are large, homodimeric multienzymes. Human acetyl-CoA carboxylase occurs in two isoforms: the metabolic, cytosolic ACC1, and ACC2, which is anchored to the outer mitochondrial membrane and controls fatty acid β-oxidation 1 , 3 . ACC1 is regulated by a complex interplay of phosphorylation, binding of allosteric regulators and protein–protein interactions, which is further linked to filament formation 1 , 4 – 8 . These filaments were discovered in vitro and in vivo 50 years ago 7 , 9 , 10 , but the structural basis of ACC1 polymerization and regulation remains unknown. Here, we identify distinct activated and inhibited ACC1 filament forms. We obtained cryo-electron microscopy structures of an activated filament that is allosterically induced by citrate (ACC–citrate), and an inactivated filament form that results from binding of the BRCT domains of the breast cancer type 1 susceptibility protein (BRCA1). While non-polymeric ACC1 is highly dynamic, filament formation locks ACC1 into different catalytically competent or incompetent conformational states. This unique mechanism of enzyme regulation via large-scale conformational changes observed in ACC1 has potential uses in engineering of switchable biosynthetic systems. Dissecting the regulation of acetyl-CoA carboxylase opens new paths towards counteracting upregulation of fatty acid biosynthesis in disease. Cryo-electron microscopy studies of distinct, catalytically active and inactive filaments of human acetyl-CoA carboxylase 1 reveal the structural basis of its regulation.

The tubulin polymerization and Src kinase inhibitor tirbanibulin was superior to placebo ointment in clearing actinic keratoses on the face and scalp at 2 months. As is typical of the disorder, lesions recurred in 47% of patients at 1 year. Adverse events were related to local irritation.

Microtubules are dynamic tubulin polymers responsible for many cellular processes, including the capture and segregation of chromosomes during mitosis. In contrast to textbook models of tubulin self-assembly, we have recently demonstrated that microtubules elongate by addition of bent guanosine triphosphate tubulin to the tips of curving protofilaments. Here we explore this mechanism of microtubule growth using Brownian dynamics modeling and electron cryotomography. The previously described flaring shapes of growing microtubule tips are remarkably consistent under various assembly conditions, including different tubulin concentrations, the presence or absence of a polymerization catalyst or tubulin-binding drugs. Simulations indicate that development of substantial forces during microtubule growth and shortening requires a high activation energy barrier in lateral tubulin-tubulin interactions. Modeling offers a mechanism to explain kinetochore coupling to growing microtubule tips under assisting force, and it predicts a load-dependent acceleration of microtubule assembly, providing a role for the flared morphology of growing microtubule ends. Microtubules are dynamic tubulin polymers which elongate by addition of bent guanosine triphosphate tubulin to the tips of curving protofilaments. Here authors use Brownian dynamics modeling and electron cryotomography to show that the lateral activation energy barrier in tubulin-tubulin interactions is a key parameter for this process, controlling the development of high pulling forces.

Steroid hormone receptors are ligand-binding transcription factors essential for mammalian physiology. The androgen receptor (AR) binds androgens mediating gene expression for sexual, somatic and behavioural functions, and is involved in various conditions including androgen insensitivity syndrome and prostate cancer 1 . Here we identified functional mutations in the formin and actin nucleator DAAM2 in patients with androgen insensitivity syndrome. DAAM2 was enriched in the nucleus, where its localization correlated with that of the AR to form actin-dependent transcriptional droplets in response to dihydrotestosterone. DAAM2 AR droplets ranged from 0.02 to 0.06 µm 3 in size and associated with active RNA polymerase II. DAAM2 polymerized actin directly at the AR to promote droplet coalescence in a highly dynamic manner, and nuclear actin polymerization is required for prostate-specific antigen expression in cancer cells. Our data uncover signal-regulated nuclear actin assembly at a steroid hormone receptor necessary for transcription. Functional mutations identified in patients with androgen insensitivity syndrome, in the formin and actin nucleator DAAM2, uncover signal-regulated nuclear actin assembly at a steroid hormone receptor necessary for transcription.

Using virtual FragLites screening and a fragment-based drug discovery (FBDD) strategy, we designed and synthesized a series of 6-substituted-1-(3,4,5-trimethoxyphe-nyl)-1H-indole derivatives as potential tubulin polymerization inhibitors. Among them, compound 3g exhibited the best antiproliferative activity within this series and affected microtubule dynamics in a concentration-dependent manner. Further studies indicated that 3g induced G2/M cell-cycle arrest and triggered apoptosis in MCF-7 cells. In vivo, 3g achieved tumor growth inhibition rates of 23.3% and 44.2% at 20 mg/kg and 50 mg/kg, respectively, without evident systemic toxicity. These results suggest that 3g shows preliminary antitumor efficacy and may serve as a starting point for further mechanistic and structural studies. Further optimization and detailed pharmacokinetic and toxicity studies are merited to advance these inhibitors in preclinical development.

Hydrogen sulfide (H₂S) is an important signaling molecule in plants. Our previous report suggested that H₂S signaling affects the actin cytoskeleton and root hair growth. However, the underlying mechanisms of its effects are not understood. S-Sulfhydration of proteins is regulated directly by H₂S, which converts the thiol groups of cysteine (Cys) residues to persulfides and alters protein function. In this work, we studied the effects of S-sulfhydration on actin dynamics in Arabidopsis (Arabidopsis thaliana). We generated transgenic plants overexpressing the H₂S biosynthesis-related genes l-CYSTEINE DESULFHYDRASE (LCD) and d-CYSTEINE DESULFHYDRASE in the O-acetylserine(thiol)lyase isoform a1 (oasa1) mutant and Columbia-0 backgrounds. The H₂S content increased significantly in overexpressing LCD/oasa1 plants. The density of filamentous actin (F-actin) bundles and the F-actin/globular actin ratio decreased in overexpressing LCD/oasa1 plants. S-Sulfhydration also was enhanced in overexpressing LCD/oasa1 plants. An analysis of actin dynamics suggested that S-sulfhydration inhibited actin polymerization. We also found that ACTIN2 (ACT2) was S-sulfhydrated at Cys-287. Cys-287 is adjacent to the D-loop, which acts as a central region for hydrophobic and electrostatic interactions and stabilizes F-actin filaments. Overaccumulation of H₂S caused the depolymerization of F-actin bundles and inhibited root hair growth. Introduction of ACT2 carrying a Cys-287-to-Ser mutation into an act2-1 mutant partially suppressed H₂S-dependent inhibition of root hair growth. We conclude that H₂S regulates actin dynamics and affects root hair growth.

Post-translational modification of tubulin serves as a powerful means for rapidly adjusting the functional diversity of microtubules. Acetylation of the ε-amino group of K40 in α-tubulin is one such modification that is highly conserved in ciliated organisms. Recently, αTAT1, a Gcn5-related N -acetyltransferase, was identified as an α-tubulin acetyltransferase in Tetrahymena and C . elegans . Here we generate mice with a targeted deletion of Atat1 to determine its function in mammals. Remarkably, we observe a loss of detectable K40 α-tubulin acetylation in these mice across multiple tissues and in cellular structures such as cilia and axons where acetylation is normally enriched. Mice are viable and develop normally, however, the absence of Atat1 impacts upon sperm motility and male mouse fertility, and increases microtubule stability. Thus, αTAT1 has a conserved function as the major α-tubulin acetyltransferase in ciliated organisms and has an important role in regulating subcellular specialization of subsets of microtubules. Acetylation of tubulin is proposed to be an important mechanism for the regulation of microtubule stability and diversity. Kalebic et al . generate mice lacking α-tubulin acetyltransferase activity, and reveal that an apparent absence of detectable tubulin acetylation is associated with impaired sperm motility.

SET domain protein 3 (SETD3) is an actin-specific methyltransferase, a rare post-translational modification with limited known biological functions. Till now, the function of SETD3 in cerebral ischemia-reperfusion (I/R)-induced injury remains unknown. Here, we show that the protein level of SETD3 is decreased in rat neurons after cerebral I/R injury. SETD3 promotes neuronal survival after both glucose and oxygen deprivation/reoxygenation (OGD/R) and cerebral I/R injury, and knockdown of SETD3 increases OGD/R-induced neuronal death. We further show that OGD/R-induced downregulation of SETD3 leads to the decrease of cellular ATP level, the reduction of mitochondrial electric potential and the increase of ROS production, thereby promoting mitochondrial dysfunction. We found that SETD3 reduction-induced mitochondrial dysfunction is mediated by the suppression of actin polymerization after OGD/R. Furthermore, we demonstrate that I/R-induced upregulation of PTEN leads to the downregulation of SETD3, and suppressing PTEN protects against ischemic neuronal death through downregulation of SETD3 and enhancement of actin polymerization. Together, this study provides the first evidence suggesting that I/R-induced downregulation of SETD3 mediates PTEN upregulation-induced ischemic neuronal death through downregulation of SETD3 and subsequent suppression of actin polymerization. Thus, upregulating SETD3 is a potential approach for the development of ischemic stroke therapy.

Voxelotor (Oxbryta™) is a haemoglobin S polymerization inhibitor that has been developed for the treatment of sickle cell disease. In November 2019, voxelotor received its first global approval in the USA for the treatment of sickle cell disease in adults and paediatric patients aged ≥ 12 years. The drug was granted accelerated approval based on the results of the phase III HOPE trial. Phase III clinical development of voxelotor for sickle cell disease is ongoing worldwide. Voxelotor also has Orphan Drug designation and Priority Medicine status in Europe for the treatment of sickle cell disease. This article summarizes the milestones in the development of voxelotor leading to this first approval as a disease-modifying agent for sickle cell disease.