The Structure of the Multidrug Resistance Protein 1 (MRP1/ABCC1)

Abstract

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-binding cassette (ABC) polytopic membrane transporter of considerable clinical importance that confers multidrug resistance on tumor cells by reducing drug accumulation by active efflux. MRP1 is also an efficient transporter of conjugated organic anions. Like other ABC proteins, including the drug resistance conferring 170-kDa P-glycoprotein (ABCB1), the 190-kDa MRP1 has a core structure consisting of two membrane-spanning domains (MSDs), each followed by a nucleotide binding domain (NBD). However, unlike P-glycoprotein and most other ABC superfamily members, MRP1 contains a third MSD with five predicted transmembrane segments with an extracytosolic NH 2 terminus. Moreover, the two nucleotide-binding domains of MRP1 are considerably more divergent than those of P-glycoprotein. In the present study, the first structural details of MRP1 purified from drug-resistant lung cancer cells have been obtained by electron microscopy of negatively stained single particles and two-dimensional crystals formed after reconstitution of purified protein with lipids. The crystals display p2 symmetry with a single dimer of MRP1 in the unit cell. The overall dimensions of the MRP1 monomer are ϳ80 ؋ 100 Å. The MRP1 monomer shows some pseudo-2-fold symmetry in projection, and in some orientations of the detergent-solubilized particles, displays a stain filled depression (putative pore) appearing toward the center of the molecule, presumably to enable transport of substrates. These data represent the first structural information of this transporter to ϳ22-Å resolution and provide direct structural evidence for a dimeric association of the transporter in a reconstituted lipid bilayer.The 190-kDa multidrug resistance protein MRP1 1 (ABCC1) is a polytopic membrane transport protein that belongs to the ATP-binding cassette (ABC) superfamily and has been detected in many different drug-resistant cell lines and tumor tissues since it was first cloned in 1992 (1-6). When overexpressed in tumor cells, MRP1 confers multidrug resistance by reducing intracellular drug concentrations in an ATP-dependent manner (6 -8). In this respect, MRP1 is similar to another ABC transporter, the well characterized 170-kDa P-glycoprotein (Pgp) (ABCB1) (9, 10). ABC proteins play important physiological and protective functions in bacteria, yeast, plants, and mammals and are capable of transporting a wide variety of molecules across biological membranes. Known substrates for ABC transporters include ions, phospholipids, steroids, polysaccharides, amino acids, peptides, and in the case of several MRPrelated proteins, anionic conjugated endo-and xenobiotics (6, 10 -12). In addition to MRP1 and P-gp, other examples of clinically important human ABC proteins include the cystic fibrosis transmembrane conductance regulator CFTR (13), and the sulfonylurea receptor (SUR), which is part of an ATPsensitive potassium channel involved in insulin secretion (14).The amino acid sequence of MRP1 predicts that it contains a cor...

Three-dimensional Structure of Transporter Associated with Antigen Processing (TAP) Obtained by Single Particle Image Analysis

“…The projection structure of TAP at low resolution is broadly comparable with that of other ABC transporters, P-glycoprotein, and MRP-1 (27,28). The first structural data for a eukaryotic halftransporter, TAP2, comprising a single transmembrane domain associated with a single nucleotide binding domain is also shown.…”

“…Single particle image analysis has successfully been applied to molecules of comparable size to TAP (23-26). Furthermore, low resolution electron microscopic structures of the ABC transporters P-glycoprotein and MRP1 have been determined using a combination of single particle and crystallographic analysis (27,28). Recently a structure for MsbA, a prokaryotic half-ABC transporter, has been published, allowing the packing of the six transmembrane helices in the TMD and the intracellular domains to be visualized for the first time (29).…”

Structure-Function Analysis of Peroxisomal ATP-binding Cassette Transporters Using Chimeric Dimers

“…If tetramers of ABCD1 and ABCD2 exist, we have to remind ourselves that the functionality and the stability of ABC transporters strictly depend on the formation of dimers. MRP1, a full-length ABC transporter, was found to exist in the plasma membrane as a dimer (39). Interestingly, the structure analysis of this dimer revealed a single "side-by-side" organization of the two monomers.…”

The MRP family of drug efflux pumps

“…Images of MRP1 obtained from single particle image analysis and electron microscopy of two-dimensional crystals suggest that the reconstituted protein is dimeric (Rosenberg et al, 2001). However, the resolution of the currently available images is too low to permit inferences concerning how the pump binds and transports substrates.…”