Functional consequences of phenylalanine mutations in the predicted transmembrane domain of P-glycoprotein

Site-directed mutagenesis was used to investigate whether phenylalanine residues in predicted transmembrane sequences play essential roles in the function of human P-glycoprotein. Mutant cDNAs, in which codons for each of the 31 phenylalanine residues were changed to alanine, were expressed in mouse NIH 3T3 cells and analyzed with respect to their ability to confer resistance to various drugs. Mutation of either Phe-335 to Ala in transmembrane segment 6, or Phe-978 to Ala in transmembrane segment 12, drastically altered the drug resistance profile conferred by the mutant P-glycoprotein in transfected cells. Mutant Phe-335-->Ala conferred little resistance to vinblastine or actinomycin D but retained the ability to confer resistance to colchicine and adriamycin. The mutant also showed increased binding of azidopine, which could be inhibited by lower levels of vinblastine, relative to the wild-type enzyme. By contrast, mutant Phe-978-->Ala conferred little or no resistance to colchicine or adriamycin, while its ability to confer resistance to vinblastine or actinomycin D was retained. These results suggest that Phe-335 and Phe-978 play important roles in the recognition and transport of specific substrates by P-glycoprotein. Mutation of Phe-777 to Ala affected the biosynthesis of the transporter. Mutation of the other 28 phenylalanine residues yielded protein products with structural and functional characteristics that were indistinguishable from the wild-type enzyme.