The structure of the multidrug resistance protein 1 (MRP1/ABCC1) - Crystallization and single-particle analysis

Multidrug resistance protein 1 (MRP1/ABCC1) is an ATP-binding cassette (ABC ) polytopic membrane transporter of considerable clinical importance that c onfers 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 consi sting of two membrane-spanning domains (MSDs), each followed by a nucleotid e binding domain (NBD). However, unlike P-glycoprotein and most other ABC s uperfamily members, MRP1 contains a third MSD with five predicted transmemb rane segments with an extracytosolic NH2 terminus. Moreover, the two nucleo tide-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 electr on microscopy of negatively stained single particles and two-dimensional cr ystals formed after reconstitution of purified protein with lipids. The cry stals display p2 symmetry with a single dimer of MRP1 in the unit cell. The overall dimensions of the MRP1 monomer are similar to 80 x 100 Angstrom. T he MRP1 monomer shows some pseudo-2-fold symmetry in projection, and in som e orientations of the detergent-solubilized particles, displays a stain fil led depression (putative pore) appearing toward the center of the molecule, presumably to enable transport of substrates. These data represent the fir st structural information of this transporter to similar to 22-Angstrom res olution and provide direct structural evidence for a dimeric association of the transporter in a reconstituted lipid bilayer.