Nucleotide binding to the human multidrug resistance protein 3, MRP3

Transport of Alzheimer's associated amyloid-β catalyzed by P-glycoprotein

P-glycoprotein (P-gp) is a critical membrane transporter in the blood brain barrier (BBB) and is implicated in Alzheimer’s disease (AD). However, previous studies on the ability of P-gp to directly transport the Alzheimer’s associated amyloid-β (Aβ) protein have produced contradictory results. Here we use molecular dynamics (MD) simulations, transport substrate accumulation studies in cell culture, and biochemical activity assays to show that P-gp actively transports Aβ. We observed transport of Aβ40 and Aβ42 monomers by P-gp in explicit MD simulations of a putative catalytic cycle. In in vitro assays with P-gp overexpressing cells, we observed enhanced accumulation of fluorescently labeled Aβ42 in the presence of Tariquidar, a potent P-gp inhibitor. We also showed that Aβ42 stimulated the ATP hydrolysis activity of isolated P-gp in nanodiscs. Our findings expand the substrate profile of P-gp, and suggest that P-gp may contribute to the onset and progression of AD.

NMR Spectroscopy to Study MAP Kinase Binding to MAP Kinase Phosphatases

NMR spectroscopy and other solution methods are increasingly being used to obtain novel insights into the mechanisms by which MAPK regulatory proteins bind and direct the activity of MAPKs. Here, we describe how interactions between the MAPK p38α and its regulatory proteins are studied using NMR spectroscopy, isothermal titration calorimetry, and small angle X-ray scattering (SAXS).

In silico identified targeted inhibitors of P-glycoprotein overcome multidrug resistance in human cancer cells in culture

Failure of cancer chemotherapies is often linked to the over expression of ABC efflux transporters like the multidrug resistance P-glycoprotein (P-gp). P-gp expression in cells leads to the elimination of a variety of chemically unrelated, mostly cytotoxic compounds. Administration of chemotherapeutics during therapy frequently selects for cells that over express P-gp and are therefore capable of robustly exporting diverse compounds, including chemotherapeutics, from the cells. P-gp thus confers multidrug resistance to a majority of drugs currently available for the treatment of cancers and diseases like HIV/AIDS. The search for P-gp inhibitors for use as co-therapeutics to combat multidrug resistances has had little success to date. In a previous study (Brewer et al., Mol Pharmacol 86: 716–726, 2014), we described how ultrahigh throughput computational searches led to the identification of four drug-like molecules that specifically interfere with the energy harvesting steps of substrate transport and inhibit P-gp catalyzed ATP hydrolysis in vitro. In the present study, we demonstrate that three of these compounds reversed P-gp-mediated multidrug resistance of cultured prostate cancer cells to restore sensitivity comparable to naïve prostate cancer cells to the chemotherapeutic drug, paclitaxel. Potentiation concentrations of the inhibitors were <3 μmol/L. The inhibitors did not exhibit significant toxicity to noncancerous cells at concentrations where they reversed multidrug resistance in cancerous cells. Our results indicate that these compounds with novel mechanisms of P-gp inhibition are excellent leads for the development of co-therapeutics for the treatment of multidrug resistances.

In silico screening for inhibitors of p-glycoprotein that target the nucleotide binding domains

Multidrug resistances and the failure of chemotherapies are often caused by the expression or overexpression of ATP-binding cassette transporter proteins such as the multidrug resistance protein, P-glycoprotein (P-gp). P-gp is expressed in the plasma membrane of many cell types and protects cells from accumulation of toxins. P-gp uses ATP hydrolysis to catalyze the transport of a broad range of mostly hydrophobic compounds across the plasma membrane and out of the cell. During cancer chemotherapy, the administration of therapeutics often selects for cells which overexpress P-gp, thereby creating populations of cancer cells resistant to a variety of chemically unrelated chemotherapeutics. The present study describes extremely high-throughput, massively parallel in silico ligand docking studies aimed at identifying reversible inhibitors of ATP hydrolysis that target the nucleotide-binding domains of P-gp. We used a structural model of human P-gp that we obtained from molecular dynamics experiments as the protein target for ligand docking. We employed a novel approach of subtractive docking experiments that identified ligands that bound predominantly to the nucleotide-binding domains but not the drug-binding domains of P-gp. Four compounds were found that inhibit ATP hydrolysis by P-gp. Using electron spin resonance spectroscopy, we showed that at least three of these compounds affected nucleotide binding to the transporter. These studies represent a successful proof of principle demonstrating the potential of targeted approaches for identifying specific inhibitors of P-gp.

Sav1866 from Staphylococcus aureus and P-glycoprotein: similarities and differences in ATPase activity assessed with detergents as allocrites

The ATP-binding cassette exporters Sav1866 from Staphylococcus aureus and P-glycoprotein are known to share a certain sequence similarity and disposition for cationic allocrites. Conversely, the two ATPases react very differently to neutral detergents that have previously been shown to be inhibitory allocrites for P-glycoprotein. To gain insight into the functional differences of the two proteins, we compared their basal and detergent-stimulated ATPase activity. P-Glycoprotein was investigated in NIH-MDR1-G185 plasma membrane vesicles and Sav1866 in lipid vesicles exhibiting a membrane packing density and a surface potential similar to those of the plasma membrane vesicles. Under basal conditions, Sav1866 revealed a lower catalytic efficiency and concomitantly a more pronounced sodium chloride and pH dependence than P-glycoprotein. As expected, the cationic allocrites (alkyltrimethylammonium chlorides) induced similar bell-shaped activity curves as a function of concentration for both exporters, suggesting stimulation upon binding of the first and inhibition upon binding of the second allocrite molecule. However, the neutral allocrites (n-alkyl-β-d-maltosides and n-ethylene glycol monododecyl ethers) reduced P-glycoprotein's ATPase activity at concentrations well below their critical micelle concentration (CMC) but strongly enhanced Sav1866's ATPase activity even at concentrations above their CMC. The lack of ATPase inhibition at high concentrations of neutral of detergents could be explained by their comparatively low binding affinity for the transmembrane domains of Sav1866, which seems to prevent binding of a second inhibitory molecule. The high ATPase activity in the presence of hydrophobic, long chain detergents moreover revealed that Sav1866, despite its lower basal catalytic efficiency, is a more efficient floppase for lipidlike amphiphiles than P-glycoprotein.

Substrate translocation and stimulated ATP hydrolysis of human ABC transporter MRP3 show positive cooperativity and are half-coupled

ABC transporters are involved in countless processes from lipid excretion over cellular detoxification to multidrug resistance of cancer cells. The latter is especially conferred by the ABCC subfamily also called multidrug resistance-associated proteins (MRPs) that excrete a variety of amphipathics including anticancer drugs by ATP-dependent transport. As the mechanisms of substrate translocation and ATP hydrolysis are still unclear for MRPs, we investigated the kinetics of both processes with focus on cooperativity and coupling between ATPase activity and substrate transport using purified MRP3 in proteoliposomes. Although the ATP-dependent uptake of amphipathics and the hydrophilic 5(6)-carboxy-2'-7'-dichlorofluorescein (CDCF) into the lumen of proteoliposomes showed affinity constants similar to those reported for cell-based assays, the maximal uptake rates were up to 250 times higher. Moreover, all substrates showed cooperative interactions of two subunits. Upon stimulation with amphipathics, ATPase activity of MRP3 increased from 80nmol/(mgmin) to 180nmol/(mgmin) showing positive cooperativity with a Hill coefficient of 2. While Hill coefficient and maximal ATPase activity were found to be substrate independent, the affinity constants are characteristic for a given substrate and correspond to the value for transport. Therefore, cooperative interactions of the two nucleotide binding domains (NBDs) in MRP3 are mediated by substrate binding to the transmembrane domains (TMDs). In contrast to amphipathic substrates, CDCF did not stimulate ATPase activity despite being transported in an ATP-dependent manner. This indicates that ATP hydrolysis and substrate translocation are half-coupled in MRP3 as CDCF shuttles on a basal TMD activity resulting from the basal ATPase activity.

Probing the ATP hydrolysis cycle of the ABC multidrug transporter LmrA by pulsed EPR spectroscopy

Members of the ATP binding cassette (ABC) transporter superfamily translocate various types of molecules across the membrane at the expense of ATP. This requires cycling through a number of catalytic states. Here, we report conformational changes throughout the catalytic cycle of LmrA, a homodimeric multidrug ABC transporter from L. lactis. Using site-directed spin labeling and pulsed electron-electron double resonance (PELDOR/DEER) spectroscopy, we have probed the reorientation of the nucleotide binding domains and transmembrane helix 6 which is of particular relevance to drug binding and part of the dimerization interface. Our data show that LmrA samples a very large conformational space in its apo state, which is significantly reduced upon nucleotide binding. ATP binding but not hydrolysis is required to trigger this conformational change, which results in a relatively fixed orientation of both the nucleotide binding domains and transmembrane helices 6. This orientation is maintained throughout the ATP hydrolysis cycle until the protein cycles back to its apo state. Our data present strong evidence that switching between two dynamically and structurally distinct states is required for substrate translocation.

Structural basis of p38α regulation by hematopoietic tyrosine phosphatase

MAP kinases regulate essential cellular events, including cell growth, differentiation and inflammation. The solution structure of a complete MAPK-MAPK-regulatory protein complex, p38α-HePTP, was determined, enabling a comprehensive investigation of the molecular basis of specificity and fidelity in MAPK regulation. Structure determination was achieved by combining NMR spectroscopy and small-angle X-ray scattering data with a new ensemble calculation-refinement procedure. We identified 25 residues outside of the HePTP kinase interaction motif necessary for p38α recognition. The complex adopts an extended conformation in solution and rarely samples the conformation necessary for kinase deactivation. Complex formation also does not affect the N-terminal lobe, the activation loop of p38α or the catalytic domain of HePTP. Together, these results show how the downstream tyrosine phosphatase HePTP regulates p38α and provide for fundamentally new insights into MAPK regulation and specificity.