Use of a sequential high throughput screening assay to identify novel inhibitors of the eukaryotic SRP-Sec61 targeting/translocation pathway

Lipid Scrambling Pathways in the Sec61 Translocon Complex

Cellular homeostasis depends on the rapid, ATP-independent translocation of newly synthesized lipids across the endoplasmic reticulum (ER) membrane. Lipid translocation is facilitated by membrane proteins known as scramblases, a few of which have recently been identified in the ER. Our previous structure of the translocon-associated protein (TRAP) bound to the Sec61 translocation channel revealed local membrane thinning, suggesting that the Sec61/TRAP complex might be involved in lipid scrambling. Using complementary fluorescence spectroscopy assays, we detected nonselective scrambling by reconstituted translocon complexes. This activity was unaffected by Sec61 inhibitors that block its lateral gate, suggesting a second lipid scrambling pathway within the complex. Molecular dynamics simulations indicate that the trimeric TRAP subunit forms this alternative route, facilitating lipid translocation via a "credit card" mechanism, using a crevice lined with polar residues to shield lipid head groups from the hydrophobic membrane interior. Kinetic and thermodynamic analyses confirmed that local membrane thinning enhances scrambling efficiency and that both Sec61 and TRAP scramble phosphatidylcholine faster than phosphatidylethanolamine and phosphatidylserine, reflecting the intrinsic lipid flip-flop tendencies of these lipid species. As the Sec61 scrambling site lies in the lateral gate region, it is likely inaccessible during protein translocation, in line with our experiments on Sec61-inhibited samples. Hence, our findings suggest that the metazoan-specific trimeric TRAP bundle is a viable candidate for lipid scrambling activity that is insensitive to the functional state of the translocon.

Inhibition of TSH Receptor Expression by a Cyclotriazadisulfonamide as a Potential Treatment of Graves Hyperthyroidism

Graves hyperthyroidism (GH) is a condition in which autoantibodies chronically activate the thyrotropin (TSH) receptor (TSHR). TSHR is one of the few G protein-coupled receptors (GPCRs) predicted to have a signal peptide, making it a potential target for cyclotriazadisulfonamide (CADA) compounds. We sought to determine whether a small-molecule drug that selectively induces nascent protein degradation could decrease TSHR expression in vitro and in vivo at therapeutically relevant levels. We tested several CADA compounds for their ability to reduce TSHR surface expression in HEK 293 cells overexpressing human TSHR (HEK-TSHR cells) using flow cytometry. Inhibition of downstream cAMP production and thyroglobulin (Tg) secretion were measured in HEK-TSHR and human thyrocytes, respectively. Follow-up studies in VGD040-treated BALB/c mice assessed plasma levels of free T4 in response to TSH stimulation. Among a number of CADA analogues, VGD040 decreased TSHR at the surface of HEK-TSHR cells. VGD040 was found to be selective toward TSHR compared to similar glycoprotein hormone receptors. In human thyrocytes, reduction of TSHR surface expression by VGD040 decreased cyclic adenosine monophosphate production and Tg secretion. Most important, VGD040 decreased TH secretion in mice without apparent toxicity at the effective dose studied. VGD040 is an important new lead with potential for developing safe drug treatments for GH.

From Bacterial Toxin to Therapeutic Agent: The Unexpected Fate of Mycolactone

"Recognizing a surprising fact is the first step towards discovery." This famous quote from Louis Pasteur is particularly appropriate to describe what led us to study mycolactone, a lipid toxin produced by the human pathogen Mycobacterium ulcerans. M. ulcerans is the causative agent of Buruli ulcer, a neglected tropical disease manifesting as chronic, necrotic skin lesions with a "surprising" lack of inflammation and pain. Decades after its first description, mycolactone has become much more than a mycobacterial toxin. This uniquely potent inhibitor of the mammalian translocon (Sec61) helped reveal the central importance of Sec61 activity for immune cell functions, the spread of viral particles and, unexpectedly, the viability of certain cancer cells. We report in this review the main discoveries that marked our research into mycolactone, and the medical perspectives they opened up. The story of mycolactone is not over and the applications of Sec61 inhibition may go well beyond immunomodulation, viral infections, and oncology.

Structural insights into TRAP association with ribosome-Sec61 complex and translocon inhibition by a CADA derivative

During cotranslational translocation, the signal peptide of a nascent chain binds Sec61 translocon to initiate protein transport through the endoplasmic reticulum (ER) membrane. Our cryo-electron microscopy structure of ribosome-Sec61 shows binding of an ordered heterotetrameric translocon-associated protein (TRAP) complex, in which TRAP-γ is anchored at two adjacent positions of 28S ribosomal RNA and interacts with ribosomal protein L38 and Sec61α/γ. Four transmembrane helices (TMHs) of TRAP-γ cluster with one C-terminal helix of each α, β, and δ subunits. The seven TMH bundle helps position a crescent-shaped trimeric TRAP-α/β/δ core in the ER lumen, facing the Sec61 channel. Further, our in vitro assay establishes the cyclotriazadisulfonamide derivative CK147 as a translocon inhibitor. A structure of ribosome-Sec61-CK147 reveals CK147 binding the channel and interacting with the plug helix from the lumenal side. The CK147 resistance mutations surround the inhibitor. These structures help in understanding the TRAP functions and provide a new Sec61 site for designing translocon inhibitors.

Synthesis, Biological Evaluation and Docking Studies of Ring-Opened Analogues of Ipomoeassin F

The plant-derived macrocyclic resin glycoside ipomoeassin F (Ipom-F) binds to Sec61α and significantly disrupts multiple aspects of Sec61-mediated protein biogenesis at the endoplasmic reticulum, ultimately leading to cell death. However, extensive assessment of Ipom-F as a molecular tool and a therapeutic lead is hampered by its limited production scale, largely caused by intramolecular assembly of the macrocyclic ring. Here, using in vitro and/or in cellula biological assays to explore the first series of ring-opened analogues for the ipomoeassins, and indeed all resin glycosides, we provide clear evidence that macrocyclic integrity is not required for the cytotoxic inhibition of Sec61-dependent protein translocation by Ipom-F. Furthermore, our modeling suggests that open-chain analogues of Ipom-F can interact with multiple sites on the Sec61α subunit, most likely located at a previously identified binding site for mycolactone and/or the so-called lateral gate. Subsequent in silico-aided design led to the discovery of the stereochemically simplified analogue 3 as a potent, alternative lead compound that could be synthesized much more efficiently than Ipom-F and will accelerate future ipomoeassin research in chemical biology and drug discovery. Our work may also inspire further exploration of ring-opened analogues of other resin glycosides.

Inhibitors of the Sec61 Complex and Novel High Throughput Screening Strategies to Target the Protein Translocation Pathway

Proteins targeted to the secretory pathway start their intracellular journey by being transported across biological membranes such as the endoplasmic reticulum (ER). A central component in this protein translocation process across the ER is the Sec61 translocon complex, which is only intracellularly expressed and does not have any enzymatic activity. In addition, Sec61 translocon complexes are difficult to purify and to reconstitute. Screening for small molecule inhibitors impairing its function has thus been notoriously difficult. However, such translocation inhibitors may not only be valuable tools for cell biology, but may also represent novel anticancer drugs, given that cancer cells heavily depend on efficient protein translocation into the ER to support their fast growth. In this review, different inhibitors of protein translocation will be discussed, and their specific mode of action will be compared. In addition, recently published screening strategies for small molecule inhibitors targeting the whole SRP-Sec61 targeting/translocation pathway will be summarized. Of note, slightly modified assays may be used in the future to screen for substances affecting SecYEG, the bacterial ortholog of the Sec61 complex, in order to identify novel antibiotic drugs.