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The tumor suppressor p53 (p53) is regulated by murine double minute 2 (Mdm2) and its homologous MdmX in maintaining the basal level of p53. Overexpressed Mdm2/MdmX inhibits cellular p53 activity, which is highly relevant to cancer occurrence. Coiled-coil domain-containing protein 106 (CCDC106) has been identified as a p53-interacting partner. However, the molecular mechanism of the p53/Mdm2/MdmX/CCDC106 interactions is still elusive. Here, we show that CCDC106 functions as a signaling regulator of the p53-Mdm2/MdmX axis. We identified that CCDC106 directly interacts with the p53 transactivation domain by competing with Mdm2 and MdmX. CCDC106 overexpression downregulates the cellular level of p53 and Mdm2/MdmX, and decreased p53 reversibly downregulates the cellular level of CCDC106. Our work provides a molecular mechanism by which CCDC106 regulates the cellular levels of p53 and Mdm2/MdmX.

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BACKGROUND: As a rare hexose with low calories and various physiological functions, d-allulose has drawn increasing attention. The current industrial production of d-allulose from d-fructose or d-glucose is achieved via epimerization based on the Izumoring strategy; however, the inherent reaction equilibrium during reversible reaction limits its high conversion yield. Although the conversion of d-fructose to d-allulose could be enhanced via phosphorylation-dephosphorylation mediated by metabolic engineering, biomass reduction and byproduct accumulation remain a largely unresolved issue. RESULTS: After modifying the glycolytic pathway of Escherichia coli and optimizing the whole-cell reaction condition, the engineered strain produced 7.57 ± 0.61 g L-1 d-allulose from 30 g L-1 d-glucose after 24 h of catalysis. By developing an ATP regeneration system for enhanced substrate phosphorylation, the cell growth inhibition was alleviated and d-allulose production increased by 55.3% to 11.76 ± 0.58 g L-1 (0.53 g g-1 ). Fine-tuning of carbon flux caused a 48% reduction in d-fructose accumulation to 1.47 ± 0.15 g L-1 . After implementing fed-batch co-substrate strategy, the d-allulose titer reached 15.80 ± 0.31 g L-1 (0.62 g g-1 ) with a d-glucose conversion rate of 84.8%. CONCLUSION: The present study reports a novel strategy for high-yield d-allulose production from low-cost substrate. © 2023 Society of Chemical Industry.

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4-Androstene-3,17-dione (4-AD) and 22-hydroxy-23,24-bisnorchol-4-ene-3-one (BA) are the most important and representative C19- and C22-steroidal materials. The optimalization of sterol production with mycobacterial phytosterol conversion has been investigated for decades. One of the major challenges is that current industrial mycobacterial strains accumulate unignorable impurities analogous to desired sterol intermediates, significantly hampering product extractions and refinements. Previously, we identified Mycobacterium neoaurum HGMS2 as an efficient 4-AD-producing strain (Wang et al. in Microb Cell Fact. 19:187, 2020). Recently, we have genetically modified the HGMS2 strain to remove its major impurities including ADD and 9OH-AD (Li et al. in Microb Cell Fact. 20:158, 2021). Unexpectedly, the modified mutants started to significantly accumulate BA compared with the HGMS2 strain. In this work, while we attempted to block BA occurrence during 4-AD accumulation in HGMS2 mutants, we identified a few loop pathways that regulated metabolic flux switching between 4-AD and BA accumulations and found that both the 4-AD and BA pathways shared a 9,10-secosteroidial route. One of the key enzymes in the loop pathways was Hsd4A1, which played an important role in determining 4-AD accumulation. The inactivation of the hsd4A1 gene significantly blocked the 4-AD metabolic pathway so that the phytosterol degradation pathway flowed to the BA metabolic pathway, suggesting that the BA metabolic pathway is a complementary pathway to the 4-AD pathway. Thus, knocking out the hsd4A1 gene essentially made the HGMS2 mutant (HGMS2Δhsd4A1) start to efficiently accumulate BA. After further knocking out the endogenous kstd and ksh genes, an HGMS2Δhsd4A1 mutant, HGMS2Δhsd4A1/Δkstd1, enhanced the phytosterol conversion rate to BA in 1.2-fold compared with the HGMS2Δhsd4A1 mutant in pilot-scale fermentation. The final BA yield increased to 38.3 g/L starting with 80 g/L of phytosterols. Furthermore, we knocked in exogenous active kstd or ksh genes to HGMS2Δhsd4A1/Δ kstd1 to construct DBA- and 9OH-BA-producing strains. The resultant DBA- and 9OH-BA-producing strains, HGMS2Δhsd4A1/kstd2 and HGMS2Δkstd1/Δhsd4A1/kshA1B1, efficiently converted phytosterols to DBA- and 9OH-BA with the rates of 42.5% and 40.3%, respectively, and their final yields reached 34.2 and 37.3 g/L, respectively, starting with 80 g/L phytosterols. Overall, our study not only provides efficient strains for the industrial production of BA, DBA and 9OH-BA but also provides insights into the metabolic engineering of the HGMS2 strain to produce other important steroidal compounds.

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Cellular protein-protein interactions are largely dependent on the activities of signaling proteins. Here, we present a technique to tune gene expression at translation level based on G418-inducible readthrough premature termination codon (PTC-on). To demonstrate how this PTC-on can control the expression level of a cellular signaling protein to regulate signal transduction, we settled a p53 PTC-on system in p53-null H1299 cells. After treating with G418, the cells expressed full-length p53 protein in a dose-dependent manner. We further demonstrated to use this PTC-on approach to dissect p53-dependent and p53-independent apoptosis in response to the DNA double strand breaks in H1299 cells. In principle, the PTC-on can be used as a general approach for exploring the functions of any other signaling proteins.

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Overexpressed Mdm2 and its 7homolog MdmX impair p53 activity in many cancers. Small molecules mimicking a p53 peptide can effectively inhibit Mdm2 but not MdmX. Here, we show a strategy for improving lead compounds for Mdm2 and MdmX inhibition based on the multivalency of the p53 peptide. Crystal structures of MdmX complexed with nutlin-3a, a strong Mdm2 inhibitor but a weak one for MdmX, reveal that nutlin-3a fits into the ligand binding pocket of MdmX mimicking the p53 peptide. However, due to distinct flexibility around the MdmX ligand binding pocket, the structures are missing many important intermolecular interactions that exist in the MdmX/p53 peptide and Mdm2/nultin-3a complexes. By targeting these flexible regions, we identify allosteric and additive fragments that enhance the binding affinity of nutlin-3a for MdmX, leading to potent Mdm2/MdmX inhibitors with anticancer activity. Our work provides a practical approach to drug design for signal transduction therapy.

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4-Androstene-3,17-dione (4-AD), 1,4-androstadiene-3,17-dione (ADD) and 9α-hydroxyl-4-androstene-3,17-dione (9OH-AD), which are important starting compounds for the synthesis of steroidal medicines, can be biosynthetically transformed from phytosterols by Mycobacterium strains. Genomic and metabolic analyses have revealed that currently available 4-AD-producing strains maintain the ability to convert 4-AD to ADD and 9OH-AD via 3-ketosteroid-1,2-dehydrogenase (KstD) and 3-ketosteroid-9α-hydroxylase (Ksh), not only lowering the production yield of 4-AD but also hampering its purification refinement. Additionally, these 4-AD industrial strains are excellent model strains to construct ADD- and 9OH-AD-producing strains. We recently found that Mycobacterium neoaurum HGMS2, a 4-AD-producing strain, harbored fewer kstd and ksh genes through whole-genomic and enzymatic analyses, compared with other strains (Wang et al. in Microbial Cell Fact 19:187, 2020). In this study, we attempted to construct an efficient 4-AD-producing strain by knocking out the kstd and ksh genes from the M. neoaurum HGMS2 strain. Next, we used kstd- and ksh-default HGMS2 mutants as templates to construct ADD- and 9OH-AD-producing strains by knocking in active kstd and ksh genes, respectively. We found that after knocking out its endogenous kstd and ksh genes, one of these knockout mutants, HGMS2Δkstd211 + ΔkshB122, showed a 20% increase in the rate of phytosterol to 4-AD conversion, compared relative to the wild-type strain and an increase in 4-AD yield to 38.3 g/L in pilot-scale fermentation. Furthermore, we obtained the ADD- and 9OH-AD-producing strains, HGMS2kstd2 + Δkstd211+ΔkshB122 and HGMS2kshA51 + Δkstd211+ΔkshA226, by knocking in heterogenous active kstd and ksh genes to selected HGMS2 mutants, respectively. During pilot-scale fermentation, the conversion rates of the ADD- and 9OH-AD-producing mutants transforming phytosterol were 42.5 and 40.3%, respectively, and their yields reached 34.2 and 37.3 g/L, respectively. Overall, our study provides efficient strains for the production of 4-AD, ADD and 9OH-AD for the pharmaceutical industry and provides insights into the metabolic engineering of the HGMS2 strain to produce other important steroidal compounds.

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Mycobacterium neoaurum strains can transform phytosterols to 4-androstene-3,17-dione (4-AD), a key intermediate for the synthesis of advanced steroidal medicines. In this work, we presented the complete genome sequence of the M. neoaurum strain HGMS2, which transforms ß-sitosterol to 4-AD. Through genome annotation, a phytosterol-degrading pathway in HGMS2 was predicted and further shown to form a 9,10-secosteroid intermediate by five groups of enzymes. These five groups of enzymes included three cholesterol oxidases (ChoM; group 1: ChoM1, ChoM2 and Hsd), two monooxygenases (Mon; group 2: Mon164 and Mon197), a set of enzymes for side-chain degradation (group 3), one 3-ketosteroid-1,2-dehydrogenase (KstD; group 4: KstD211) and three 3-ketosteroid-9a-hydroxylases (Ksh; group 5: KshA226, KshA395 and KshB122). A gene cluster encoding Mon164, KstD211, KshA226, KshB122 and fatty acid ß-oxidoreductases constituted one integrated metabolic pathway, while genes encoding other key enzymes were sporadically distributed. All key enzymes except those from group 3 were prepared as recombinant proteins and their activities were evaluated, and the proteins exhibited distinct activities compared with enzymes identified from other bacterial species. Importantly, we found that the KstD211 and KshA395 enzymes in the HGMS2 strain retained weak activities and caused the occurrence of two major impurities, i.e., 1,4-androstene-3,17-dione (ADD) and 9-hydroxyl-4-androstene-3,17-dione (9OH-AD) during ß-sitosterol fermentation. The concurrence of these two 4-AD analogs not only lowered 4-AD production yield but also hampered 4-AD purification. HGMS2 has the least number of genes encoding KstD and Ksh enzymes compared with current industrial strains. Therefore, HGMS2 could be a potent strain by which the 4-AD production yield could be enhanced by disabling the KstD211 and KshA395 enzymes. Our work also provides new insight into the engineering of the HGMS2 strain to produce ADD and 9OH-AD for industrial application.

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PURPOSE: Phosphatidylinositol 3-kinase (PI3K) plays an important role in tumorigenesis by cross-talking with several signaling pathways. p55PIK is a unique regulatory subunit of PI3K and contains an extra 24-residue N-terminal domain (N24). This study aimed to explore the interaction of p55PIK with p53 and the role of p55PIK in regulating p53-dependent apoptosis in cancer cells. MATERIALS AND METHODS: The expression of p55PIK was detected in cancer cells, and the interaction of p55PIK with p53 was examined by immunoprecipitation and pull-down assay. The expression of p53-dependent apoptosis-related genes was detected by PCR. RESULTS: N24 domain of p55PIK interacted with DNA-specific binding domain (DBD) of p53. The increase or decrease of p55PIK expression led to the change of the expression of p53 and p53-regulated genes in cancer cells. Moreover, N24 peptide led to the change of the expression of p53-regulated genes. Moreover, a membrane-permeable N24 peptide enhanced p53-dependent apoptosis induced by methyl methanesulfonate. CONCLUSION: Our results reveal a novel mechanism that regulates p53-dependent apoptosis in cancer cells via p55PIK-p53 interaction.

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To effective capture and universal enrichment of His-tagged protein, polyacrylic acid (PAA) brushes were used to encapsulate Fe3O4 nanoparticles, connect NTA, and Ni2+ to prepare magnetic beads. These materials provide many advantages, such as excellent stability, tuneable particle size, and a surface for further functionalisation with biomolecules. His-tagged green fluorescence protein (GFP) was separated efficiently, and the binding capacity of Fe3O4/MPS@PAA/NTA-Ni2+ was 93.4 mg/g. Compared with High-Affinity Ni-NTA Resin and Ni-NTA Magnetic Agarose Beads, Fe3O4/MPS@PAA/NTA-Ni2+ nanocomposites exhibited higher separation efficiency and binding capacity towards His-tagged GFP. Moreover, the selectivity and recyclability of them for the target proteins were maintained well after six cycles. This study would widen the application of PAA in constructing multifunctional nanocomposites for biomedical fields.

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Withdrawal: Xiyao Cheng, Yongqi Huang, and Zhengding Su. Invisible State of MdmX and Design of its Inhibitors. The FASEB Journal. 34:s1. doi: 10.1096/fasebj.2020.34.s1.09193. The above abstract, published online on April 20, 2020, in Wiley Online Library (wileyonlinelibrary.com), has been withdrawn by agreement between the authors, FASEB, and Wiley Periodicals, Inc. The abstract has been withdrawn at the request of the authors.

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Withdrawal: Xiyao Cheng, Yongqi Huang, and Zhengding Su. A Controllable Protein Translation Strategy for Exploring Cellular Signal Transduction Based on Reading through Premature Termination Codons. The FASEB Journal. 34:s1. doi: 10.1096/fasebj.2020.34.s1.09195. The above abstract, published online on April 19, 2020, in Wiley Online Library (wileyonlinelibrary.com), has been withdrawn by agreement between the authors, FASEB, and Wiley Periodicals, Inc. The abstract has been withdrawn at the request of the authors.

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Polyamines are positively charged small molecules ubiquitously existing in all living organisms, and they are considered as one kind of the most ancient cellular components. The most common polyamines are spermidine, spermine, and their precursor putrescine generated from ornithine. Polyamines play critical roles in cells by stabilizing chromatin structure, regulating DNA replication, modulating gene expression, etc., and they also affect the structure and function of proteins. A few studies have investigated the impact of polyamines on protein structure and function previously, but no reports have focused on a protein-based biological module with a dedicated function. In this report, we investigated the impact of polyamines (putrescine, spermidine, and spermine) on the cyanobacterial KaiABC circadian oscillator. Using an established in vitro reconstitution system, we noticed that polyamines could disrupt the robustness of the KaiABC oscillator by inducing the denaturation of the Kai proteins (KaiA, KaiB, and KaiC). Further experiments showed that the denaturation was likely due to the induced change of the thermal stability of the clock proteins. Our study revealed an intriguing role of polyamines as a component in complex cellular environments and would be of great importance for elucidating the biological function of polyamines in future.

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The type 3 secretion system (T3SS) found as cell-surface appendages of many pathogenic Gram-negative bacteria, although nonessential for bacterial survival, is an important therapeutic target for drug discovery and development aimed at inhibiting bacterial virulence without inducing antibiotic resistance. We designed a fluorescence-polarization-based assay for high-throughput screening as a mechanistically well-defined general strategy for antibiotic discovery targeting the T3SS and made a serendipitous discovery of a subset of tanshinones-natural herbal compounds in traditional Chinese medicine widely used for the treatment of cardiovascular and cerebrovascular diseases-as effective inhibitors of the biogenesis of the T3SS needle of multi-drug-resistant Pseudomonas aeruginosa. By inhibiting the T3SS needle assembly and, thus, cytotoxicity and pathogenicity, selected tanshinones reduced the secretion of bacterial virulence factors toxic to macrophages in vitro, and rescued experimental animals challenged with lethal doses of Pseudomonas aeruginosa in a murine model of acute pneumonia. As first-in-class inhibitors with a demonstrable safety profile in humans, tanshinones may be used directly to alleviate Pseudomonas-aeruginosa-associated pulmonary infections without inducing antibiotic resistance. Since the T3SS is highly conserved among Gram-negative bacteria, this antivirulence strategy may be applicable to the discovery and development of novel classes of antibiotics refractory to existing resistance mechanisms for the treatment of many bacterial infections.

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Protein engineering based on structure homology holds the potential to engineer steroid-transforming enzymes on demand. Based on the genome sequencing analysis of industrial Mycobacterium strain HGMS2 to produce 4-androstene-3,17-dione (4-AD), three hypothetical proteins were predicted as putative Δ5-3-ketosteroid isomerases (KSIs) to catalyze an intramolecular proton transfer involving the transformation of 5-androstene-3,17-dione (5-AD) into 4-AD, which were defined as mKSI228, mKSI291 and mKSI753. Activity assays indicated that mKSI228 and mKSI291 exhibited weak activity, as low as 0.7% and 1.5%, respectively, of a well-studied and highly active KSI from Pseudomonas putida KSI (pKSI), while mKSI753 had no activity similar to Mycobacterium tuberculosis KSI (mtKSI). Although the 3D structures of the putative mKSIs were homologous to pKSI, their amino acid sequences were significantly different from those of pKSI and tKSI. Thus, by use of these two KSIs as homology models, we were able to convert the low-active mKSI291 into a high-active active KSI by site-directed mutagenesis. On the other hand, an X-ray crystallographic structure of mKSI291 identified a water molecule in its active site. This unique water molecule might function as a bridge to connect Ser-OH, Tyr57-OH and C3O of the intermediate form a hydrogen-bonding network that was responsible for its weak activity, compared with that of mtKSI. Our results not only demonstrated the use of a protein engineering approach to understanding KSI catalytic mechanism, but also provided an example for engineering the catalytic active sites and gaining a functional enzyme based on homologous structures.

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Cyclization of the polypeptide backbone has proven to be a powerful strategy for enhancing protein stability for fundamental research and pharmaceutical application. The use of such an approach is restricted by how well a targeted polypeptide can be efficiently ligated. Recently, an Asx-specific peptide ligase identified from a tropical cyclotide-producing plant and named butelase 1 exhibited excellent cyclization kinetics that cannot be matched by other known ligases, including intein, PATG, PCY1, and sortase A. In this work, we aimed to examine whether butelase 1 facilitated protein conformational stability for structural investigation. First, we successfully expressed recombinant butelase 1 (rBTase) in the yeast Pichia pastoris. Next, rBTase was shown to be highly efficient in the cyclization of the p53-binding domain (N-terminal domain) of murine double minute X (N-MdmX), an important target for designing anticancer drugs. The cyclized N-MdmX (cMdmX) exhibited increased conformational stability and improved interaction with the ligand compared with those of noncyclized N-MdmX. Importantly, the thermal melting process was completely reversible, contrary to noncyclized N-MdmX, and the melting temperature ( Tm) of cMdmX was increased to 47 from 43 °C. This stable conformation of cMdmX was further confirmed by 15N-1H heteronuclear single-quantum coherence nuclear magnetic resonance (NMR) spectroscopy. The complex of cMdmX and the ligand was tested for protein crystallization, and several promising findings were revealed. Therefore, our work not only provides a recombinant version of butelase 1 but also suggests a conventional approach for preparing stable protein samples for both protein crystallization and NMR structural investigation.

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The oncoprotein MdmX (mouse double minute X) is highly homologous to Mdm2 (mouse double minute 2) in terms of their amino acid sequences and three-dimensional conformations, but Mdm2 inhibitors exhibit very weak affinity for MdmX, providing an excellent model for exploring how protein conformation distinguishes and alters inhibitor binding. The intrinsic conformation flexibility of proteins plays pivotal roles in determining and predicting the binding properties and the design of inhibitors. Although the molecular dynamics simulation approach enables us to understand protein-ligand interactions, the mechanism underlying how a flexible binding pocket adapts an inhibitor has been less explored experimentally. In this work, we have investigated how the intrinsic flexible regions of the N-terminal domain of MdmX (N-MdmX) affect the affinity of the Mdm2 inhibitor nutlin-3a using protein engineering. Guided by heteronuclear nuclear Overhauser effect measurements, we identified the flexible regions that affect inhibitor binding affinity around the ligand-binding pocket on N-MdmX. A disulfide engineering mutant, N-MdmXC25-C110/C76-C88, which incorporated two staples to rigidify the ligand-binding pocket, allowed an affinity for nutlin-3a higher than that of wild-type N-MdmX (Kd ∼ 0.48 vs Kd ∼ 20.3 µM). Therefore, this mutant provides not only an effective protein model for screening and designing of MdmX inhibitors but also a valuable clue for enhancing the intermolecular interactions of the pharmacophores of a ligand with pronounced flexible regions. In addition, our results revealed an allosteric ligand-binding mechanism of N-MdmX in which the ligand initially interacts with a compact core, followed by augmenting intermolecular interactions with intrinsic flexible regions. This strategy should also be applicable to many other protein targets to accelerate drug discovery.

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