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The Inability of the Mouse mdr2 Gene to Confer Multidrug Resistance Is Linked to Reduced Drug Binding to the Protein¹

Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3G 1Y6

The mouse mdr gene family is composed of three members designated mdr1, mdr2, and mdr3. A full-length mdr2 complementary DNA clone has been introduced in an amplifiable eukaryotic expression vector (pEMC2b1) which directs amplification and overexpression of a bicistronic mdr2-dihydrofolate reductase mRNA after stepwise methotrexate selection of transfected mutant dihydrofolate reductase Chinese hamster ovary DUK cells. Independent cell clones expressing low to high amounts of mdr2 cellular mRNA and Mdr2 protein in their membrane fraction could be obtained by this selection procedure. Comparison of drug survival characteristics of cell clones expressing similar amounts of either Mdr1 or Mdr2 proteins revealed that Mdr1 but not Mdr2 could confer readily detectable levels of colchicine or vinblastine resistance. Labeling experiments using membrane-enriched fractions and a photoactivatable analogue of ATP showed that the Mdr2 protein was properly inserted in the membrane of transfected cells and could bind this ligand with an apparent affinity similar to that of Mdr1. However, labeling studies with the photoactivatable drug analogue lodoarylazidoprazosin showed considerably reduced binding of this ligand to Mdr2 as compared to Mdr1. Our findings demonstrate that Mdr2 cannot confer drug resistance and suggest that this inability is linked to reduced drug binding to the Mdr2 protein.

¹ This work was supported by grants from the Medical Research Council of Canada (P. G.). P. G. is supported by a fellowship from the Natural Sciences and Engineering Research Council of Canada and is an International Research Scholar of the Howard Hughes Medical Institute.

² To whom requests for reprints should be addressed, at Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3G 1Y6.

Received 5/31/94. Accepted 7/19/94.

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