Nucleotides and transported substrates modulate different steps of the ATPase catalytic cycle of MRP1 multidrug transporter

Omnipresent Maxwell's demons orchestrate information management in living cells

The development of synthetic biology calls for accurate understanding of the critical functions that allow construction and operation of a living cell. Besides coding for ubiquitous structures, minimal genomes encode a wealth of functions that dissipate energy in an unanticipated way. Analysis of these functions shows that they are meant to manage information under conditions when discrimination of substrates in a noisy background is preferred over a simple recognition process. We show here that many of these functions, including transporters and the ribosome construction machinery, behave as would behave a material implementation of the information-managing agent theorized by Maxwell almost 150 years ago and commonly known as Maxwell's demon (MxD). A core gene set encoding these functions belongs to the minimal genome required to allow the construction of an autonomous cell. These MxDs allow the cell to perform computations in an energy-efficient way that is vastly better than our contemporary computers.

Allosteric coupling between the intracellular coupling helix 4 and regulatory sites of the first nucleotide-binding domain of CFTR

Cystic fibrosis is caused by mutations in CFTR (cystic fibrosis transmembrane conductance regulator), leading to folding and processing defects and to chloride channel gating misfunction. CFTR is regulated by ATP binding to its cytoplasmic nucleotide-binding domains, NBD1 and NBD2, and by phosphorylation of the NBD1 regulatory insert (RI) and the regulatory extension (RE)/R region. These regulatory effects are transmitted to the rest of the channel via NBD interactions with intracellular domain coupling helices (CL), particularly CL4. Using a sensitive method for detecting inter-residue correlations between chemical shift changes in NMR spectra, an allosteric network was revealed within NBD1, with a construct lacking RI. The CL4-binding site couples to the RI-deletion site and the C-terminal residues of NBD1 that precede the R region in full-length CFTR. Titration of CL4 peptide into NBD1 perturbs the conformational ensemble in these sites with similar titration patterns observed in F508del, the major CF-causing mutant, and in suppressor mutants F494N, V510D and Q637R NBD1, as well as in a CL4-NBD1 fusion construct. Reciprocally, the C-terminal mutation, Q637R, perturbs dynamics in these three sites. This allosteric network suggests a mechanism synthesizing diverse regulatory NBD1 interactions and provides biophysical evidence for the allosteric coupling required for CFTR function.

Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1)

MRP1 (ABCC1) is a peculiar member of the ABC transporter superfamily for several aspects. This protein has an unusually broad substrate specificity and is capable of transporting not only a wide variety of neutral hydrophobic compounds, like the MDR1/P-glycoprotein, but also facilitating the extrusion of numerous glutathione, glucuronate, and sulfate conjugates. The transport mechanism of MRP1 is also complex; a composite substrate-binding site permits both cooperativity and competition between various substrates. This versatility and the ubiquitous tissue distribution make this transporter suitable for contributing to various physiological functions, including defense against xenobiotics and endogenous toxic metabolites, leukotriene-mediated inflammatory responses, as well as protection from the toxic effect of oxidative stress. In this paper, we give an overview of the considerable amount of knowledge which has accumulated since the discovery of MRP1 in 1992. We place special emphasis on the structural features essential for function, our recent understanding of the transport mechanism, and the numerous assignments of this transporter.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

Probing ATP-dependent conformational changes in the multidrug resistance protein 1 (MRP1/ABCC1) in live tumor cells with a novel recombinant single-chain Fv antibody targeted to the extracellular N-terminus

The multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-driven transporter that mediates the cellular extrusion of various chemotherapeutic agents. We have previously isolated a novel recombinant single-chain Fv antibody (A5scFv), which specifically targets the extracellular N-terminus of the human MRP1 expressed on the surface of live tumor cells. Fusion of A5scFv to Pseudomonas exotoxin revealed an immunotoxin that bound to the immobilized MRP1-derived peptide upon ELISA, but surprisingly failed to recognize MRP1 on the surface of live tumor cells. As these results suggested that the N-terminus of MRP1 has a limited accessibility to the extracellular space, we used the A5scFv antibody to probe for putative conformational changes that might occur in viable tumor cells upon ATP binding. A5scFv recognized viable MRP1-expressing cells with intact ATP pools, whereas ATP depletion resulted in the loss of A5scFv reactivity. Consistently, restoration of cellular ATP levels resulted in resumption of A5scFv binding to MRP1 in live tumor cells. Flow cytometric analysis confirmed that ATP-depleted cells accumulated significantly higher levels of the established substrate calcein AM, whereas after restoration of cellular ATP pools, cells displayed a much lower level of calcein AM accumulation. Moreover, pretreatment of MRP1-expressing cells with the membrane fluidizer benzyl alcohol resulted in a dramatic increase in A5scFv reactivity, suggesting that membrane fluidization results in the exposure of the N-terminus of MRP1 to the extracellular milieu. These results constitute the first extracellular probing of the putative conformational changes that MRP1 adopts in viable tumor cells upon ATP binding. Furthermore, although ATP binding occurs in the cytosolic nucleotide binding domains of MRP1, significant conformational changes are apparently propagated to the N-terminus residing at the extracellular compartment.

The Role of the Conserved Glycines of ATP-binding Cassette Signature Motifs of MRP1 in the Communication between the Substrate-binding Site and the Catalytic Centers

A key element of the structural model of ABC-ATP-ases is the interaction of the two ABC domains. They complement each other's active sites in a way that the ABC signature motif (LSGGQ) of one subunit interacts with the gamma-phosphate of the ATP, bound at the Walker motifs of the opposite subunit. In the present study, the conserved glycines in the fourth position of the LSGGQ motifs of human MRP1 were substituted for aspartic acids (G771D and G1433D), the mutants were expressed in Sf9 insect cells, and the nucleotideas well as the transported substrate-protein interactions were studied. We found that these transport- and ATPase-incompetent mutants showed no nucleotide trapping under any of the conditions examined. However, when measuring the effect of nucleotide and transported substrates on the vanadate-induced cleavage reactions, we found that the effect of substrates on the cleavage reactions was significantly different in the mutant MRP1 proteins than in the wild type. Although the transported substrates (e.g. etoposide + oxidized glutathione) stimulated the formation of the posthydrolytic complex in the wild type, this reaction was inhibited in the signature mutants. Our study also revealed that a similar mutation in the ABC signature of either ABC unit resulted in the same effect. We suggest that the conserved glycine residues in both LSGGQ segments are part of the conformational network, which is responsible for the accelerated hydrolytic activity upon interaction of the protein with its transported substrates. This intramolecular communication between the substrate-binding site and the catalytic centers is assumed to be a general feature of the molecular mechanism of ABC transporters.