Received for publication November 8, 2005.
Revised January 5, 2006.
Accepted for publication January 12, 2006.

Mutational analysis of polar amino acid residues within predicted transmembrane helices 10 and 16 of Multidrug Resistance Protein 1 (ABCC1): Effect on substrate specificity

Abstract

Human Multidrug Resistance Protein1 (MRP1) has a total of 17 transmembrane (TM) helices arranged in three domains, MSD0, MSD1 and MSD2 with a 5+6+6 TM configuration. Photolabeling studies indicate that TMs 10 and 11 in MSD1 and 16 and 17 in MSD2 contribute to the substrate binding pocket of the protein. Previous mutational analyses of charged and polar amino acids in predicted TM helices 11, 16 and 17 support this suggestion. Mutation of Trp⁵⁵³ in TM10 also affects substrate specificity. To extend this analysis, we mutated six additional polar residues within TM10 and the remaining uncharacterized polar residue in TM16, Asn¹²⁰⁸. Although mutation of Asn¹²⁰⁸ was without effect, two of six mutations in TM10, T550A and T556A modulated the drug resistance profile of MRP1 without affecting transport of leukotriene C4, estradiol-17-D-glucuronide (E₂17G) and glutathione. Mutation T550A increased vincristine resistance but decreased doxorubicin resistance, while mutation T556A decreased resistance to VP-16 and doxorubicin. Although conservative mutation of Tyr⁵⁶⁸ in TM10 to Phe or Trp had no apparent effect on substrate specificity, substitution with Ala decreased the affinity of MRP1 for E₂17G without affecting drug resistance or the transport of other substrates tested. These analyses confirm that several amino acids in TM10 selectively alter the substrate specificity of MRP1 suggesting that they interact directly with certain substrates. The location of these and other functionally important residues in TM helices 11, 16 and 17 is discussed in the context of an energy-minimized model of the membrane spanning domains of MRP1.

Key words: ABC transporters, MRP, multi-drug resistance, site-directed mutagenesis

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