Crystal structure of alpha-momorcharin in 80% acetonitrile--water mixture

Divide, conquer and reconstruct: How to solve the 3D structure of recalcitrant Micro-Exon Gene (MEG) protein from Schistosoma mansoni

Micro-Exon Genes are a widespread class of genes known for their high variability, widespread in the genome of parasitic trematodes such as Schistosoma mansoni. In this study, we present a strategy that allowed us to solve the structures of three alternatively spliced isoforms from the Schistoma mansoni MEG 2.1 family for the first time. All isoforms are hydrophobic, intrinsically disordered, and recalcitrant to be expressed in high yield in heterologous hosts. We resorted to the chemical synthesis of shorter pieces, before reconstructing the entire sequence. Here, we show that isoform 1 partially folds in a-helix in the presence of trifluoroethanol while isoform 2 features two rigid elbows, that maintain the peptide as disordered, preventing any structuring. Finally, isoform 3 is dominated by the signal peptide, which folds into a-helix. We demonstrated that combining biophysical techniques, like circular dichroism and nuclear magnetic resonance at natural abundance, with in silico molecular dynamics simulation for isoform 1 only, was the key to solve the structure of MEG 2.1. Our results provide a crucial piece to the puzzle of this elusive and highly variable class of proteins.

Atomic-resolution structures of type I ribosome inactivating protein alpha-momorcharin with different substrate analogs

Alpha-momorcharin (Alpha-MMC) from the seed of bitter melon is a type I ribosome inactivating protein (RIP) that removes a specific adenine from 28S rRNA and inhibits protein biosynthesis. Here, we report seven crystal complex structures of alpha-MMC with different substrate analogs (adenine, AMP, cAMP, dAMP, ADP, GMP, and xanthosine) at 1.08 Å to 1.52 Å resolution. These structures reveal that not only adenine, but also guanine and their analogs can effectively bind to alpha-MMC. The side chain of Tyr93 adopts two conformations, serving as a switch to open and close the substrate binding pocket of alpha-MMC. Although adenine, AMP, GMP, and guanine are located in a similar active site in different RIPs, residues involved in the interaction between RIPs and substrate analogs are slightly different. Complex structures of alpha-MMC with different substrate analogs solved in this study provide useful information on its enzymatic mechanisms and may enable the development of new inhibitors to treat the poisoning of alpha-MMC.

In-silico analysis of ligand-receptor binding patterns of α-MMC, TCS and MAP30 protein to LRP1 receptor

Alpha-momorcharin (α-MMC), trichosanthin (TCS), and momordica anti-HIV protein of 30 kD (MAP30) are potential anti-tumor drug candidates but have cytotoxicity to normal cells. The binding of these proteins to LRP1 receptor and the subsequent endocytosis are essential to their cytotoxicity, but this binding process remains largely unknown. This study, in-silico analysis of the binding patterns, was conducted via the protein-protein docking software, ZDOCK 3.0.2 package, to better understand the binding process. Specifically, α-MMC, TCS and MAP30 were selected and bound to binding subunits CR56 and CR17 of LRP1. After docking, the 10 best docking solutions are retained based on the default ZDOCK scores and used for structural assessment. Our results showed that, α-MMC bound to LRP1 stably at the amino acid residues 1-20, at which 8 residues formed 21 hydrogen bonds with 15 residues of CR56 and 10 residues formed 15 hydrogen bonds with 12 residues of CR17. In contrast, TCS and MAP30 bound mainly to LRP1 at the residues 1-57/79-150 and residues 58-102, respectively, which were functional domains of TCS and MAP30. Since residues 1-20 are outside the functional domain of α-MMC, α-MMC is considered more suitable to attenuate by mutating the receptor binding site. Thus, our analysis lays the foundation for future genetic engineering work on α-MMC, and makes important contributions to its potential clinical use in cancer treatment.

Curcin C, a novel type I ribosome-inactivating protein from the post-germinating cotyledons of Jatropha curcas

A novel type I ribosome-inactivating protein (RIP), designated as curcin C, was purified from Jatropha curcas, an important feedback source of bio-fuel. Molecular mass and isoelectric point of curcin C were 31.398 kDa and 7.12 as detected by MALTI-TOF assay and capillary electrophoresis assay, respectively. N-terminal sequence and LC-MS/MS analyses confirmed that curcin C is a type I RIP having high homology, but not the exactly the same with curcin, another type 1 RIP isolated from the endosperm of J. curcas. It exhibited N-glycosidase activity and in vitro translation inhibition activity. Moreover, curcin C displayed a strong selectively anti-tumor activity on human cancer cells. Its cytotoxicity against osteosarcoma cell line U20S is even higher than that of Paclitaxel with IC₅₀ of 0.019 μM. Purification and identification of curcin C not only suggested its potential in natural anticancer drug development, but also provide chance to understanding different cytotoxic action among different RIPs.

Biological activities of ribosome-inactivating proteins and their possible applications as antimicrobial, anticancer, and anti-pest agents and in neuroscience research

Ribosome-inactivating proteins (RIPs) are enzymes which depurinate ribosomal RNA (rRNA), thus impeding the process of translation resulting in inhibition of protein synthesis. They are produced by various organisms including plants, fungi and bacteria. RIPs from plants are linked to plant defense due to their antiviral, antifungal, antibacterial, and insecticidal activities in which they can be applied in agriculture to combat microbial pathogens and pests. Their anticancer, antiviral, embryotoxic, and abortifacient properties may find medicinal applications. Besides, conjugation of RIPs with antibodies or other carriers to form immunotoxins has been found useful to research in neuroscience and anticancer therapy.

Preferential cytotoxicity of the type I ribosome inactivating protein alpha-momorcharin on human nasopharyngeal carcinoma cells under normoxia and hypoxia

All primary nasopharyngeal carcinoma (NPC) tumors contain hypoxic regions which are implicated in decreased local control and increased distant metastases, as well as resistance to chemotherapy in advanced NPC patients. One of the promising therapeutic approaches for NPC is to use drugs that can target hypoxic factors in tumors. In the present investigation, the type I ribosome inactivating protein α-momorcharin (α-MMC), isolated from seeds of the bitter gourd Momordica charantia, reduced cell viability and inhibited clonogenic formation of human NPC CNE2 and HONE1 cells under normoxia and cobalt chloride-induced hypoxia. By comparison, α-MMC exhibited only slight cytotoxicity on human nasopharyngeal epithelial NP69 cells under normoxia. Interestingly, α-MMC suppressed the expression levels of hypoxia-inducible factor 1-alpha (HIF1α) and vascular endothelial growth factor (VEGF) in hypoxic NPC, as well as the growth of human umbilical vein endothelial cells. Further study disclosed that α-MMC targeted endoplasmic reticulum and down-regulated unfolded protein response (UPR) in NPC cells. Moreover, α-MMC induced apoptosis in NPC cells in a dose- and time-dependent manner. It initiated mitochondrial- and death receptor-mediated apoptotic signaling in CNE2 cells, but there was hardly any effect on HONE1 cells. In addition, α-MMC brought about G0/G1 phase cell cycle arrest in CNE2 cells and S phase arrest in HONE1 cells. Collectively, α-MMC preferentially exhibited inhibitory effect on normoxic and hypoxic NPC cells partly by blocking survival signaling (e.g. HIF1α, VEGF and UPR), and triggering apoptotic pathways mediated by mitochondria or death receptor. These observations indicate the potential utility of α-MMC for prophylaxis and therapy of NPC.

PEGylation alleviates the non-specific toxicities of Alpha-Momorcharin and preserves its antitumor efficacy in vivo

Alpha-Momorcharin (α-MMC) is a ribosome inactivating protein from Momordica charantia with anti-tumor activity. Previously, we had observed that modification of α-MMC with polyethylene glycol (PEG) could reduce toxicity, but it also reduces its anti-tumor activity in vitro. This study aims to investigate whether the metabolism-extended properties of α-MMC resulting from PEGylation could preserve its anti-tumor efficacy in vivo through pharmacokinetics and antitumor experiments. The pharmacokinetics experiments were conducted in rats using the TCA (Trichloroacetic Acid) method. Antitumor activity in vivo was investigated in murine mammary carcinoma (EMT-6) and human mammary carcinoma (MDA-MB-231) transplanted tumor mouse models. The results showed that PEGylation increased the plasma half-life of α-MMC in rats from 6.2-7.5 h to 52-87 h. When administered at 1 mg/kg, α-MMC-PEG and α-MMC showed similar anti-tumor activities in vivo, with a T/C% of 38.56% for α-MMC versus 35.43% for α-MMC-PEG in the EMT-6 tumor model and 36.30% for α-MMC versus 39.88% for α-MMC-PEG in the MDA-MB-231 tumor model (p > 0.05). Importantly, at the dose of 3 mg/kg, all the animals treated with α-MMC died while the animals treated with α-MMC-PEG exhibited only moderate toxic reactions, and α-MMC-PEG exhibited improved anti-tumor efficacy with a T/C% (relative tumor growth rate) of 25.18% and 21.07% in the EMT-6 and MDA-MB-231 tumor models, respectively. The present study demonstrates that PEGylation extends the half-life of α-MMC and alleviates non-specific toxicity, thereby preserving its antitumor efficacy in vivo, and a higher lever of dosage can be used to achieve better therapeutic efficacy.