Cryo-EM structure of TMEM63C suggests it functions as a monomer.

The TMEM63 family proteins (A, B, and C), calcium-permeable channels in animals that are preferentially activated by hypo-osmolality, have been implicated in various physiological functions. Deficiency of these channels would cause many diseases including hearing loss. However, their structures and physiological roles are not yet well understood. In this study, we determine the cryo-electron microscopy (cryo-EM) structure of the mouse TMEM63C at 3.56 Å, and revealed structural differences compared to TMEM63A, TMEM63B, and the plant orthologues OSCAs. Further structural guided mutagenesis and calcium imaging demonstrated the important roles of the coupling of TM0 and TM6 in channel activity. Additionally, we confirm that TMEM63C exists primarily as a monomer under physiological conditions, in contrast, TMEM63B is a mix of monomer and dimer in cells, suggesting that oligomerization is a regulatory mechanism for TMEM63 proteins.

Insights into 4-hydroxyphenylpyruvate dioxygenase-inhibitor interactions from comparative structural biology.

4-Hydroxyphenylpyruvate dioxygenase (HPPD) plays a key role in tyrosine metabolism and has been identified as a promising target for herbicide and drug discovery. The structures of HPPD complexed with different types of inhibitors have been determined previously. We summarize the structures of HPPD complexed with structurally diverse molecules, including inhibitors, natural products, substrates, and catalytic intermediates; from these structures, the detailed inhibitory mechanisms of different inhibitors were analyzed and compared, and the key structural factors determining the slow-binding behavior of inhibitors were identified. Further, we propose four subpockets that accommodate different inhibitor substructures. We believe that these analyses will facilitate in-depth understanding of the enzymatic reaction mechanism and enable the design of new inhibitors with higher potency and selectivity.

Discovery of Subnanomolar Inhibitors of 4-Hydroxyphenylpyruvate Dioxygenase via Structure-Based Rational Design.

High-potency 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are usually featured by time-dependent inhibition. However, the molecular mechanism underlying time-dependent inhibition by HPPD inhibitors has not been fully elucidated. Here, based on the determination of the HPPD binding mode of natural products, the π-π sandwich stacking interaction was found to be a critical element determining time-dependent inhibition. This result implied that, for the time-dependent inhibitors, strengthening the π-π sandwich stacking interaction might improve their inhibitory efficacy. Consequently, modification with one methyl group on the bicyclic ring of quinazolindione inhibitors was achieved, thereby strengthening the stacking interaction and significantly improving the inhibitory efficacy. Further introduction of bulkier hydrophobic substituents with higher flexibility resulted in a series of HPPD inhibitors with outstanding subnanomolar potency. Exploration of the time-dependent inhibition mechanism and molecular design based on the exploration results are very successful cases of structure-based rational design and provide a guiding reference for future development of HPPD inhibitors.

The human pre-replication complex is an open complex.

In eukaryotes, DNA replication initiation requires assembly and activation of the minichromosome maintenance (MCM) 2-7 double hexamer (DH) to melt origin DNA strands. However, the mechanism for this initial melting is unknown. Here, we report a 2.59-Å cryo-electron microscopy structure of the human MCM-DH (hMCM-DH), also known as the pre-replication complex. In this structure, the hMCM-DH with a constricted central channel untwists and stretches the DNA strands such that almost a half turn of the bound duplex DNA is distorted with 1 base pair completely separated, generating an initial open structure (IOS) at the hexamer junction. Disturbing the IOS inhibits DH formation and replication initiation. Mapping of hMCM-DH footprints indicates that IOSs are distributed across the genome in large clusters aligning well with initiation zones designed for stochastic origin firing. This work unravels an intrinsic mechanism that couples DH formation with initial DNA melting to license replication initiation in human cells.

Allosteric enhancement of ORP1-mediated cholesterol transport by PI(4,5)P2/PI(3,4)P2.

Phosphatidylinositol phosphates (PIPs) and cholesterol are known to regulate the function of late endosomes and lysosomes (LELs), and ORP1L specifically localizes to LELs. Here, we show in vitro that ORP1 is a PI(4,5)P2- or PI(3,4)P2-dependent cholesterol transporter, but cannot transport any PIPs. In cells, both ORP1L and PI(3,4)P2 are required for the efficient removal of cholesterol from LELs. Structures of the lipid-binding domain of ORP1 (ORP1-ORD) in complex with cholesterol or PI(4,5)P2 display open conformations essential for ORP function. PI(4,5)P2/PI(3,4)P2 can facilitate ORP1-mediated cholesterol transport by promoting membrane targeting and cholesterol extraction. Thus, our work unveils a distinct mechanism by which PIPs may allosterically enhance OSBP/ORPs-mediated transport of major lipid species such as cholesterol.

Molecular insights into the mechanism of 4-hydroxyphenylpyruvate dioxygenase inhibition: enzyme kinetics, X-ray crystallography and computational simulations.

Slow-binding inhibitors with long residence time on the target often display superior efficacy in vivo. Rationally designing inhibitors with low off-target rates is restricted by a limited understanding of the structural basis of slow-binding inhibition kinetics in enzyme-drug interactions. 4-Hydroxyphenylpyruvate dioxygenase (HPPD) is an important target for drug and herbicide development. Although the time-dependent behavior of HPPD inhibitors has been studied for decades, its structural basis and mechanism remain unclear. Herein, we report a detailed experimental and computational study that explores structures for illustrating the slow-binding inhibition kinetics of HPPD. We observed the conformational change of Phe428 at the C-terminal α-helix in the inhibitor-bound structures and further identified that the inhibition kinetics of drugs are related to steric hindrance of Phe428. These detailed structural and mechanistic insights illustrate that steric hindrance is highly associated with the time-dependent behavior of HPPD inhibitors. These findings may enable rational design of new potent HPPD-targeted drugs or herbicides with longer target residence time and improved properties. DATABASE: Structure data are available in the PDB under the accession numbers 5CTO (released), 5DHW (released), and 5YWG (released).

ORP2 Delivers Cholesterol to the Plasma Membrane in Exchange for Phosphatidylinositol 4, 5-Bisphosphate (PI(4,5)P2).

Cholesterol is highly enriched at the plasma membrane (PM), and lipid transfer proteins may deliver cholesterol to the PM in a nonvesicular manner. Here, through a mini-screen, we identified the oxysterol binding protein (OSBP)-related protein 2 (ORP2) as a novel mediator of selective cholesterol delivery to the PM. Interestingly, ORP2-mediated enrichment of PM cholesterol was coupled with the removal of phosphatidylinositol 4, 5-bisphosphate (PI(4,5)P2) from the PM. ORP2 overexpression or deficiency impacted the levels of PM cholesterol and PI(4,5)P2, and ORP2 efficiently transferred both cholesterol and PI(4,5)P2in vitro. We determined the structure of ORP2 in complex with PI(4,5)P2 at 2.7 Å resolution. ORP2 formed a stable tetramer in the presence of PI(4,5)P2, and tetramerization was required for ORP2 to transfer PI(4,5)P2. Our results identify a novel pathway for cholesterol delivery to the PM and establish ORP2 as a key regulator of both cholesterol and PI(4,5)P2 of the PM.

ORP5 and ORP8 bind phosphatidylinositol-4, 5-biphosphate (PtdIns(4,5)P 2) and regulate its level at the plasma membrane.

ORP5 and ORP8, members of the oxysterol-binding protein (OSBP)-related proteins (ORP) family, are endoplasmic reticulum membrane proteins implicated in lipid trafficking. ORP5 and ORP8 are reported to localize to endoplasmic reticulum-plasma membrane junctions via binding to phosphatidylinositol-4-phosphate (PtdIns(4)P), and act as a PtdIns(4)P/phosphatidylserine counter exchanger between the endoplasmic reticulum and plasma membrane. Here we provide evidence that the pleckstrin homology domain of ORP5/8 via PtdIns(4,5)P 2, and not PtdIns(4)P binding mediates the recruitment of ORP5/8 to endoplasmic reticulum-plasma membrane contact sites. The OSBP-related domain of ORP8 can extract and transport multiple phosphoinositides in vitro, and knocking down both ORP5 and ORP8 in cells increases the plasma membrane level of PtdIns(4,5)P 2 with little effect on PtdIns(4)P. Overall, our data show, for the first time, that phosphoinositides other than PtdIns(4)P can also serve as co-exchangers for the transport of cargo lipids by ORPs.ORP5/8 are endoplasmic reticulum (ER) membrane proteins implicated in lipid trafficking that localize to ER-plasma membrane (PM) contacts and maintain membrane homeostasis. Here the authors show that PtdIns(4,5)P 2 plays a critical role in the targeting and function of ORP5/8 at the PM.