Objectives: It has recently been suggested that substrate recognition patterns for human P-glycoprotein encoded by mdr1 consist of two electron donor groups with a spatial separation of 2.5 +/- 0.3 A (type I units) or three electron donor groups with a spatial separation of the two outer groups of 4.6 +/- 0.6 A (type II units) [Seelig 1998]. Since P-gp and the multidrug resistance-associated protein (MRP1) have overlapping substrate specificity, we screened the chemical structures of 21 compounds, previously tested as MRP1 substrates, for electron donor units. In addition, we searched the putative transmembrane domains (TMD 1-12) of P-gp and (TMD 6-17) of MRP1 for amino acid side chains having the potential to interact with the respective substrates.
Methods: The three-dimensional structures of potential MRP1 substrates were modeled with a force-field approach and were then screened for electron donor units. Helical wheel projections of the 12 putative transmembrane domains of P-gp (1-12) and MRP (6-17) were analyzed for their content of amino acid residues with hydrogen bonding side chains, charged amino acid residues, and amino acid residues with pi-electron systems.
Results: MRP1 recognizes compounds with type I and type II units. At least one electrically neutral together with either one negatively charged type I unit or two electrically neutral type I units are required for the compound to be bound and transported. Transport increases with increasing number of electron donor units. Compounds which carry exclusively electrically neutral type I units (P-gp substrates) are transported only weakly by MRP1, and compounds with cationic type I units (P-gp substrates) are not transported at all. An analysis of the putative transmembrane alpha-helices of MRP1 and P-gp reveals that the amino acid residues with hydrogen-bond donor side chains are arranged preferentially on one side of the helix and amino acid residues with inert (non-hydrogen-bonding) side chains on the other side. In the case of MRP1, the hydrogen-bonding face also contains several cationic residues whereas, in the case of P-gp, it contains clusters of amino acid residues with beta-electron systems.
Conclusions: We propose that P-gp and MRP1 recognize type I or type II units in chemical compounds having diverse structures, and that these transporters bind their substrates via hydrogen bond formation. Furthermore, we propose that transport of anionic substrates by MRP1 is facilitated by cationic amino acid residues present in the transmembrane helices of MRP1, whereas the transport of cationic substrates by P-gp is facilitated by a beta-electron slide guide.