An Outward-Facing Aromatic Amino Acid Is Crucial for Signaling between the Membrane-Spanning and Nucleotide-Binding Domains of Multidrug Resistance Protein 1 (MRP1; ABCC1)

The 190-kDa human MRP1 is an ATP-binding cassette multidrug and multiorganic anion efflux transporter. The 17 transmembrane helices of its three membrane-spanning domains, together with its two nucleotide binding domains (NBDs), form a stabilizing network of domain-domain interactions that ensure substrate binding in the cytoplasm is efficiently coupled to ATP binding and hydrolysis to effect solute efflux into the extracellular milieu. Here we show that Ala substitution of Phe(583) in an outward-facing loop between the two halves of the transporter essentially eliminates the binding of multiple organic anions by MRP1. Conservative substitutions with Trp and Tyr had little or no effect. The F583A mutation also caused a substantial increase in orthovanadate-induced trapping of azidoADP by the cytoplasmic NBDs of MRP1, although the binding of ATP was unaffected. These observations indicate that the loss of the aromatic side chain at position 583 impairs the release of ADP and thus effectively locks the transporter in a low-affinity solute binding state. Phe(583) is the first outward-facing amino acid in MRP1 found to be critical for its transport function. Our data provide evidence for long-range coupling, presumably via allosteric interaction, between this outward-facing region of MRP1 and both the solute binding and nucleotide binding regions of the transporter. Cryoelectron microscopy structural and homology models of MRP1 indicate that the orientation of the Phe(5)(83) side chain is altered by ATP binding but are currently unable to provide insights into the molecular mechanism by which this long-range signaling is propagated.