Considerable evidence has accumulated indicating that the multidrug transporter or P-glycoprotein plays a role in the development of simultaneous resistance to multiple cytotoxic drugs in cancer cells. In recent years, various approaches such as mutational analyses and biochemical and pharmacological characterization have yielded significant information about the relationship of structure and function of P-glycoprotein. However, there is still considerable controversy about the mechanism of action of this efflux pump and its function in normal cells. This review summarizes current research on the structure-function analysis of P-glycoprotein, its mechanism of action, and facts and speculations about its normal physiological role.

ABSTRACT

Dengue virus type 2, a member of the family Flaviviridae, encodes a single polyprotein precursor consisting of 3391 amino acids residues that is processed to at least 10 mature proteins by host and viral proteases. The NS3 protein contains a domain commonly found in cellular serine proteinases that in cooperation with NS2B is involved in polyprotein processing. In addition, NS3 and NS5 proteins contain conserved motifs found in several RNA helicases and RNA-dependent RNA polymerases, respectively. Both enzymatic activities have been suggested to be involved in viral RNA replication. In this report, we demonstrate that the NS3 and NS5 proteins interact in vivo in dengue virus type 2-infected monkey kidney (CV-1) cells and in HeLa cells coinfected with recombinant vaccinia viruses encoding these proteins as shown by coimmunoprecipitations and immunoblotting methods. We also show by immunofluorescence, metabolic labeling, and two-dimensional peptide mapping that NS5 is a nuclear phosphoprotein and that phosphorylation occurs on serine residues at multiple sites. Furthermore, NS5 exists in differentially phosphorylated states in the nuclear and the cytoplasmic fractions, and only the cytoplasmic form of NS5 is found to coimmunoprecipitate with NS3, suggesting that differential phosphorylation may control the interaction between these proteins and its function in the viral RNA replicase.

Human P-glycoprotein (Pgp), a plasma membrane protein that confers multidrug resistance, functions as an ATP-dependent drug efflux pump. Pgp contains two ATP binding/utilization sites and exhibits ATPase activity that is stimulated in the presence of substrates and modulating agents. The mechanism of coupling of ATP hydrolysis to drug transport is not known. To understand the role of ATP hydrolysis in drug binding, it is necessary to develop methods for purifying and reconstituting Pgp that retains properties including stimulation of ATPase activity by known substrates to an extent similar to that in the native membrane. In this study, (His)6-tagged Pgp was expressed in Trichoplusia ni (High Five) cells using the recombinant baculovirus system and purified by metal affinity chromatography. Upon reconstitution into phospholipid vesicles, purified Pgp exhibited specific binding to analogues of substrates and ATP in affinity labeling experiments and displayed a high level of drug-stimulated ATPase activity (specific activity ranging from 4.5 to 6.5 micromol min-1 mg-1). The ATPase activity was inhibited by ADP in a competitive manner, and by vanadate and N-ethylmaleimide at low concentrations. Vanadate which is known to inhibit ATPase activity by trapping MgADP at the catalytic site inhibited photoaffinity labeling of Pgp with substrate analogues, [125I]iodoarylazidoprazosin and [3H]azidopine, only under ATP hydrolysis conditions. Because vanadate-trapped Pgp is known to resemble the ADP and phosphate-bound catalytic transition state, our findings indicate that ATP hydrolysis results in a conformation with reduced affinity for substrates. A catalytic transition conformation with reduced affinity would essentially result in substrate dissociation and supports a model for drug transport in which an ATP hydrolysis-induced conformational change leads to drug release toward the extracellular medium.

Human P-glycoprotein (P-gp) is a cell surface drug efflux pump that contains two nucleotide binding domains (NBDs). Mutations were made in each of the Walker B consensus motifs of the NBDs at positions D555N and D1200N, thought to be involved in Mg(2+) binding. Although the mutant and wild-type P-gps were expressed equivalently at the cell surface and bound the drug analogue [(125)I]iodoarylazidoprazosin ([(125)I]IAAP) comparably, neither of the mutant proteins was able to transport fluorescent substrates nor had detectable basal nor drug-stimulated ATPase activities. The wild-type and D1200N P-gps were labeled comparably with [alpha-(32)P]-8-azido-ATP at a subsaturating concentration of 2.5 microM, whereas labeling of the D555N mutant was severely impaired. Mild trypsin digestion, to cleave the protein into two halves, demonstrated that the N-half of the wild-type and D1200N proteins was labeled preferentially with [alpha-(32)P]-8-azido-ATP. [alpha-(32)P]-8-Azido-ATP labeling at 4 degrees C was inhibited in a concentration-dependent manner by ATP with half-maximal inhibition at approximately 10-20 microM for the P-gp-D1200N mutant and wild-type P-gp. A chimeric protein containing two N-half NBDs was found to be functional for transport and was also asymmetric with respect to [alpha-(32)P]-8-azido-ATP labeling, suggesting that the context of the ATP site rather than its exact sequence is an important determinant for ATP binding. By use of [alpha-(32)P]-8-azido-ATP and vanadate trapping, it was determined that the C-half of wild-type P-gp was labeled preferentially under hydrolysis conditions; however, the N-half was still capable of being labeled with [alpha-(32)P]-8-azido-ATP. Neither mutant was labeled under vanadate trapping conditions, indicating loss of ATP hydrolysis activity in the mutants. In confirmation of the lack of ATP hydrolysis, no inhibition of [(125)I]IAAP labeling was observed in the mutants in the presence of vanadate. Taken together, these data suggest that the two NBDs are asymmetric and intimately linked and that a conformational change in the protein may occur upon ATP hydrolysis. Furthermore, these data are consistent with a model in which binding of ATP to one site affects ATP hydrolysis at the second site.

P-glycoprotein (P-gp), the product of human MDR1 gene, which functions as an ATP-dependent drug efflux pump, is N-linked glycosylated at asparagine residues 91, 94, and 99 located within the first extracellular loop. We report here the biochemical characterization of glycosylation-deficient (Gly(-)) P-gp using a vaccinia virus based transient expression system. The staining of HeLa cells expressing Gly(-) P-gp (91, 94, and 99N-->Q), with P-gp specific monoclonal antibodies, MRK-16, UIC2 and 4E3 revealed a 40 to 50% lower cell-surface expression of mutant P-gp compared to the wild-type protein. The transport function of Gly(-) P-gp, assessed using a variety of fluorescent compounds indicated that the substrate specificity of the pump was not affected by the lack of glycosylation. Additional mutants, Gly(-) D (91, 94, 99N-->D) and Gly(-) Delta (91, 94, 99 N deleted) were generated to verify that the reduced cell surface expression, as well as total expression, were not a result of the glutamine substitutions. Gly(-) D and Gly(-) Delta Pgps were also expressed to the same level as the Gly(-) mutant protein. (35)S-Methionine/cysteine pulse-chase studies revealed a reduced incorporation of (35)S-methionine/cysteine in full length Gly(-) P-gp compared to wild-type protein, but the half-life ( approximately 3 hr) of mutant P-gp was essentially unaltered. Since treatment with proteasome inhibitors (MG-132, lactacystin) increased only the intracellular level of nascent, mutant P-gp, the decreased incorporation of (35)S-methionine/cysteine in Gly(-) P-gp appears to be due to degradation of improperly folded mutant protein by the proteasome and endoplasmic reticulum-associated proteases. These results demonstrate that the unglycosylated protein, although expressed at lower levels at the cell surface, is functional and suitable for structural studies.

No abstract

The FDA approved HIV-1 protease inhibitors, ritonavir, saquinavir, and indinavir, are very effective in inhibiting HIV-1 replication, but their long-term efficacy is unknown. Since in vivo efficacy depends on access of these drugs to intracellular sites where HIV-1 replicates, we determined whether these protease inhibitors are recognized by the MDR1 multidrug transporter (P-glycoprotein, or P-gp), thereby reducing their intracellular accumulation. In vitro studies in isolated membrane preparations from insect cells infected with MDR1-expressing recombinant baculovirus showed that these inhibitors significantly stimulated P-gp-specific ATPase activity and that this stimulation was inhibited by SDZ PSC 833, a potent inhibitor of P-gp. Furthermore, photoaffinity labeling of P-gp with the substrate analogue [125I]iodoarylazidoprazosin (IAAP) was inhibited by all three inhibitors. Cell-based approaches to evaluate the ability of these protease inhibitors to compete for transport of known P-gp substrates showed that all three HIV-1 protease inhibitors were capable of inhibiting the transport of some of the known P-gp substrates but their effects were generally weaker than other documented P-gp modulators such as verapamil or cyclosporin A. Inhibition of HIV-1 replication by all three protease inhibitors was reduced but could be restored by MDR1 inhibitors in cells expressing MDR1. These results indicate that the HIV-1 protease inhibitors are substrates of the human multidrug transporter, suggesting that cells in patients that express the MDR1 transporter will be relatively resistant to the anti-viral effects of the HIV-1 protease inhibitors, and that absorption, excretion, and distribution of these inhibitors in the body may be affected by the multidrug transporter.